How Do You Choose Purlins and Girts for a Steel Structure Building? A Complete Secondary Framing and Procurement Guide for B2B Buyers

1. Why Secondary Framing Deserves More Attention Than It Gets

When overseas buyers request quotations for a steel structure warehouse or workshop, the conversation tends to concentrate on the primary frame: clear span, bay spacing, steel grade of the H-sections, and the quoted tonnage. Purlins and girts — the secondary framing — are often treated as a line item to be accepted at face value. This is a costly habit. Depending on the building configuration, secondary framing can represent roughly 10–20% of total steel tonnage (a typical hypothetical range that varies significantly by span, bay spacing and cladding type), and it directly controls how the roof and walls perform for the life of the building.

The consequences of treating secondary framing casually are practical and immediate. Roof sheeting can only span as far as the purlins beneath it allow; wall cladding is only as tight and stable as the girts behind it; and the primary frames themselves rely on these smaller members for lateral restraint of their compression flanges. A purlin schedule that is vague, or a quote that omits section sizes, thicknesses and coating classes entirely, makes it impossible to compare offers on equal terms. Two quotations showing the same total tonnage can conceal very different structural systems — continuous lapped Z purlins on one side, simple-span C sections on the other — with different behavior under wind uplift and different erection requirements.

This guide is written for importers, contractors, factory owners and farm operators procuring steel buildings internationally. By the end of it, the reader should be able to read a purlin schedule line by line, compare competing quotations section by section rather than by headline price, and ask suppliers the specific questions — steel grade, coating mass, lap system, uplift design assumptions — that separate a complete offer from an incomplete one.

2. What Purlins, Girts and Eave Struts Actually Do

Secondary framing consists of three principal member types, each with a distinct role. Roof purlins span between the rafters of adjacent frames and carry the roof sheeting, its insulation, and the loads applied to the roof surface — dead load, live or maintenance load, snow where relevant, and wind uplift. Wall girts span horizontally between the columns of the frames and support the wall cladding; because exterior walls face wind pressure on the windward side and suction on the leeward and side walls, girts must be designed for bending in both directions, not just for cladding weight. Eave struts sit at the junction of roof and wall, tying the tops of the columns together, stiffening each frame line, and carrying the gutter loads and the accumulated edge-of-roof forces.

Beyond these, secondary systems include fly bracing — small angled members connecting purlins to the tension flanges of rafters, used to stabilize the bottom flange in uplift situations — and roof plane bracing (rod or angle cross-bracing) which, together with the eave struts, transfers longitudinal wind and crane forces through the building to the foundations.

The load path of the whole building runs through these members in sequence: wind and gravity loads act on the sheeting; the sheeting transfers them to purlins and girts; purlins and girts transfer them to the frames at each connection point; the frames deliver them to the foundations. Any weakness in one link — an undersized purlin, a girt fastener pulled through the sheet — becomes a visible failure at the cladding, even when the primary frame is generously designed.

Secondary members also perform a structural service for the primary frame itself. A purlin bolted to the top flange of a rafter restrains that flange laterally, allowing the rafter to develop its designed bending capacity. In uplift, the bottom flange goes into compression and requires its own restraint, typically from fly bracing. Removing or thinning secondary framing therefore does not just weaken the roof — it can undermine the stability assumptions of the frames.

3. Z Purlins vs C Purlins: Choosing the Right Section

Cold-formed purlins and girts are manufactured by folding flat steel coil at room temperature into two dominant shapes: the Z section, whose flanges point in opposite directions, and the C section (a channel), whose flanges point the same way. The difference in shape produces a difference in how each is used, and a competent quotation should show the right section in the right place.

Z sections are commonly installed lapped continuously over the supports: because the flanges face opposite ways, one Z can nest inside the back-to-back facing surface of the next over a frame, creating a doubled section at the support where bending demand is highest. This makes Z purlins the natural choice for long continuous runs over multiple frames. C sections, with parallel flanges, cannot lap in the same way; they are normally used as simple-span members sitting on each frame individually, or as single cut-to-length pieces.

The shape difference also matters in logistics and detailing. Z sections nest point-to-point when stacked, packing efficiently into containers and reducing shipping volume. C sections stack face-to-face and are well suited to short members. In practice, C sections are the sensible choice for eave struts (where a closed, torsionally stiffer shape at the eave is convenient), door jambs and lintels, short spans, and any cut piece around openings; Z sections are the sensible choice for continuous purlin and girt runs over several frames.

Neither section is universally better. A buyer's task is to check that the quoted system matches the spans: long 6–9 m bays detailed as simple-span C purlins, for example, deserve a question about why continuity was not used, while a short-span shed detailed with lapped Z purlins on every frame may be carrying detailing cost without benefit.

4. Section Depth, Thickness and Steel Grade: Reading the Numbers

A purlin schedule usually reads as a code such as Z200×2.0, or a similar local format: the first number is the section depth in millimeters, the second the base steel thickness. Cold-formed purlins and girts for typical industrial buildings fall roughly in the 150–300 mm depth range and roughly 1.5–3.5 mm thickness range, with the exact combination set by span, spacing, loads and steel grade.

Depth and thickness buy different things. Increasing depth raises the section's bending capacity and stiffness steeply — deeper sections carry more load over the same span with modest weight increase, which is why deeper purlins often appear on wider bays. Increasing thickness also raises capacity and, importantly, improves resistance to local buckling: very thin flat elements of a cold-formed section can wrinkle locally under compression before the section as a whole is stressed to its limit. The designer's job is to balance the two against weight and cost.

Steel grade interacts with both. Higher-yield steel (Q345 versus Q235, for example) allows a lighter or shallower section for the same load, but higher yield strength also changes how the thin-walled section behaves: local buckling tends to govern earlier relative to yield capacity, and ductility is reduced, so the design calculations must properly account for the grade rather than simply swapping labels. Suppliers such as Jidian list cold-formed C/Z purlins on H-section frames with Q235/Q345 steel in their public catalog — which is typical of the industry — but a catalog line is not a design. What the buyer should confirm in writing for every quote is threefold: the yield strength class actually used for each member (Q235 and Q345 must not be left ambiguous), the base steel standard the grade refers to, and the thickness tolerance class of the supplied coil, because a nominal 2.0 mm section supplied at the bottom of a wide tolerance band has measurably less capacity than one supplied at nominal thickness. These three items cost nothing to state and should appear on the material schedule, not only in conversation.

5. Galvanized, Galvalume and Pre-Painted Coatings for Purlins and Girts

Purlins and girts are thin sections, so coating loss matters proportionally far more to them than to a heavy H-section. Most cold-formed secondary framing is supplied as hot-dip galvanized coil, and coating mass is expressed in grams of zinc per square meter of total surface. Z275 is a common baseline class — 275 g/m² total, roughly 20 microns per face — suitable for ordinary inland environments. In coastal air, high-humidity tropical climates, or around industrial emissions, heavier classes such as Z450 or Z600 give a proportionally longer service life before rust reaches the base steel; the right class is a function of environment and expected maintenance access, not a universal default.

Alternatives exist. Zinc-aluminium alloy substrates of the Galvalume type offer strong atmospheric corrosion resistance and are frequently used under pre-painted systems. Pre-painted (color-coated) material is most relevant for exposed girts and anywhere the secondary steel is visible in the finished building, where appearance and an additional organic barrier are wanted.

Three practical points deserve attention in any specification. First, cut edges and scratches: a galvanized coating protects adjacent bare steel sacrificially, which is why field drilling and site cutting of galvanized purlins is generally acceptable, whereas cutting pre-painted material exposes an edge with no zinc adjacent to it and touches up become necessary. Second, every site-cut end and drilled hole removes coating locally; fastener type and any touch-up method should be agreed before erection. Third, environments differ sharply — a coastal poultry shed and an inland dry-store warehouse should not carry the same coating class. For a fuller treatment of matching coating systems to environment, including coastal, industrial and tropical conditions, see the guide to anti-corrosion coating systems for coastal, industrial, and tropical steel structures. A quotation that names the coating class (Z275, Z450, Z600) is comparable; one that says only "galvanized" is not.

6. Purlin Spacing and Span: How the Numbers Connect

Purlin design is governed by two independent dimensions that buyers frequently conflate. The first is spacing — the distance between adjacent purlins up the slope of the roof. This is set primarily by the capability of the roof sheeting profile: a deep trapezoidal or standing-seam profile can carry cladding loads across a wider gap than a shallow profile, so stronger sheeting permits wider spacing. Typical spacings for metal roof sheeting fall roughly between 1.0 and 1.8 m (a hypothetical typical range; the actual figure depends on the profile, fastening type and the governing load case).

The second dimension is span — the distance a purlin travels between frames, which equals the frame bay spacing. Most industrial catalogs, including Jidian's light steel warehouse line, are built around 6–9 m modular bays, so a purlin in such a building spans 6–9 m between supports (or more if continuity and lapping are used to create multi-span action).

The two dimensions combine through tributary area. Each purlin carries the roof load applied over a strip of roof roughly one spacing wide on either side of it. Consider a simple hypothetical illustration: if purlins are spaced at 1.5 m and the design gravity load on the roof is 1.0 kN/m², each purlin carries 1.5 kN per meter of its length. Over a 7 m simple span, that is a distributed line load the section must be checked against — and doubling the spacing nearly doubles the line load, which is why sheeting capability and purlin size cannot be chosen independently.

Two local effects modify the neat picture. Snow drift accumulates against parapets, taller adjacent walls and roof valleys, loading specific purlin zones far beyond the uniform value; and wind uplift concentrates at roof corners and edges, where suction is highest. In both cases, the affected zones may need closer spacing or heavier sections than the general field of the roof — a detail a serious supplier reflects in the purlin schedule. For the snow side of this question in depth, see the guide to designing steel structure buildings for heavy snow loads and extreme cold.

7. Lapped Continuous Z Purlins vs Simple-Span C Purlins

The economics of long purlin runs come from continuity. A simple-span purlin carries its maximum bending moment at mid-span, and the section must be sized for that peak. A continuous purlin passing over several supports, lapped and bolted to its neighbor on each side, redistributes the moment: the doubled section at each support takes the peak demand there, and the mid-span moment is materially reduced. The result is that, for the same span and load, a lapped continuous Z system can use a lighter or thinner section than a simple-span system — which is precisely why it is the standard arrangement for 6–9 m bays.

The lap itself is a designed detail, not an overlap by habit. Lap length is related to the span and the section depth and is positioned so the doubled section sits where the moment peaks at the support. Connection within the lap is by a bolt group — typically two or more bolts through the webs — that must transfer shear and moment between the two members. Sleeve systems, where a short separate sleeve piece joins adjacent purlins instead of direct lapping, are an alternative used with some section ranges.

What goes wrong in practice is predictable. Laps left only partially bolted (or bolts omitted entirely on the hidden faces) fail to develop continuity, and the roof behaves as a series of simple spans with a section sized for something better. Lap lengths shorter than designed shift the critical moment into a single-thickness region. Mixing systems on one roof — lapped Z on some runs, simple C on others — creates inconsistent deflection and hidden weak lines. None of this is visible after the sheeting goes on.

For the buyer, the practical conclusion is that the structural system must be stated in the quotation. Two quotes for the same building, one assuming lapped continuous Z purlins and one assuming simple-span C purlins, will show different tonnages, different prices and different behavior — and comparing them by weight or price alone is meaningless. A packaging note also favors Z sections: they nest point-to-point in stacks, so a container load of continuous-run purlins occupies noticeably less volume than the same tonnage of C sections, which matters on international shipments.

8. Wind Uplift and Gravity Loads: What Actually Governs Design

Many buyers assume purlins are sized by gravity — sheeting weight, snow, maintenance loads. In many real buildings, particularly wide lightweight-roof structures in windy regions, the governing case is the opposite: wind suction lifting the roof upward. A roof plane in wind experiences negative pressure, and that suction is not uniform — it is highest at corners and edges, where localized pressure coefficients can be several times the field value. In those zones the uplift load on a purlin can exceed the entire gravity load in the opposite direction.

Uplift reverses the stress pattern in the section. Under gravity, the top flange is in compression and is restrained by the sheeting screwed to it; the bottom flange is in tension and needs no restraint. Under uplift, the bottom flange goes into compression — and it has nothing attached to it. Unrestrained, a thin cold-formed bottom flange can buckle laterally at a fraction of the section's theoretical capacity. This is why fly bracing to rafter tension flanges and purlin bridging (intermediate ties between parallel purlins) exist, and why their presence or absence in a quote is a structural question, not a detail. Uplift also governs the fasteners: sheeting screws must resist pull-out from the purlin and pull-through of the washer against the sheet, and in corner zones the fastener pattern usually tightens regardless of what the field of the roof needs.

Gravity loads still matter, and one category is chronically under-declared: suspended services. Sprinkler mains and branch lines, lighting, ventilation ducting and cable trays all hang from purlins, often at concentrated points rather than spread evenly. A purlin designed for code minimum live load but not for a declared sprinkler grid or heavy cable tray runs may be adequate on paper and deficient in reality. The buyer's obligation is to declare these loads to the designer before fabrication, not after.

Because uplift is zone-based, a proper design produces a zoned purlin layout — heavier sections or closer fastening at corners and edges, standard sections in the field. A quotation showing a single uniform section across the whole roof, for a building in a typhoon-exposed coastal location, is worth a direct question. The engineering background is covered further in the guide to wind load design for steel structure buildings in coastal regions.

9. Wall Girts: Orientation, Spacing and Cladding Support

Wall girts perform the same load-bearing role on the vertical faces of a steel structure building that purlins perform on the roof, but the choice of orientation is a genuine design decision with consequences for cladding, insulation and wind behavior. The two standard layouts are by-span girts, which run horizontally between portal frames, and by-height girts, which run vertically from eave to base and must be supported at intermediate points by struts or sag rods.

By-span girts span between frames in the horizontal direction. Because frame spacing is typically 6–9 m in a light steel frame metal structure warehouse, these girts are usually cold-formed C or Z sections acting in bending over a relatively long span, often lapped in continuous fashion in the same way roof purlins are. By-height girts span the much shorter eave-to-base distance, so the members themselves can be smaller, but they need a horizontal tie at intermediate heights to control their slenderness under wind suction that pulls the cladding inward.

The choice follows the cladding system and the wind regime. Single-skin profiled sheeting with a separate liner can work with either orientation; sandwich panels, however, impose point loads at their fastener positions and generally prefer by-height girts with closer spacing, because the panel edge bearing must be supported at regular intervals. Where the wall carries high wind suction — open terrain, cyclone-prone coastlines in Southeast Asia or the Middle East — by-height layouts often perform better because the inward-acting load path is short and direct. A supplier's catalog, such as Jidian's, lists building enclosure systems alongside the frames themselves, which reflects how tightly cladding support and cladding choice are coupled; the interactions are covered further in roofing and cladding systems for steel structure buildings.

Wall wind loading is not uniform. Pressure and suction on walls increase toward the corners and near the eaves, where flow separation occurs, so codes divide walls into zones with higher design pressures in corner regions. Girts in those corner zones — typically the first bay or so from each building corner — may need closer spacing or heavier sections than girts in the central field of the wall. A buyer reviewing a girt schedule should therefore expect to see zone-dependent sections, not one uniform size throughout.

Openings change the picture locally. Every door, window or louver removes a girt or cuts one short, which concentrates load on the remaining members and on the jambs. Framed openings — additional columns, headers and extra girts trimmed into the opening edges — are standard practice, but they must be explicitly listed. A quotation that prices "wall girts by area" without opening framing will surface as a variation order later. When comparing offers, confirm that the number of openings, their sizes and their locations in the wall are stated in the specification and reflected in the girt schedule.

10. Eave Struts, Fly Bracing and Frame Stability

Secondary framing does more than carry cladding. Several of its members participate directly in the stability of the portal frames, and understanding this participation explains why some "minor" items in a quotation cannot be deleted without consequences.

The eave strut is the clearest example of a member with triple duty. Positioned along the eave line where roof meets wall, it first supports the gutter and the lower edge of the roof sheeting. Second, it stiffens the frame: by connecting the eaves of adjacent frames, it restrains the bottom flange of the rafter at the eave, a location where the rafter's compression flange needs lateral support to develop its full bending capacity. Third, it acts as a tie member in the roof bracing system, transferring horizontal wind loads from the gable end walls into the braced bays and eventually to the foundations. A member serving three functions cannot be evaluated by any one of them alone; deleting it or downsizing it "because the gutter only needs light support" removes restraint the rafter was assumed to have.

Fly bracing — sometimes called rafter or ridge bracing — performs a similar role higher up. These angled members run between the top of the rafter and the adjacent purlin line (or between rafters near the ridge), tying the rafter's compression flange to the braced purlin plane at intermediate points along the span. Without them, the purlins alone restrain the rafter only where purlins happen to be, and only if the purlin-to-cleat connection is complete. Fly bracing shortens the unrestrained length of the rafter flange, which is why frame design and purlin spacing interact: a wider purlin spacing generally means the restraint points are farther apart and the bracing must compensate.

This participation has a practical sequencing implication for erection. The frames are not self-supporting in their final stability condition until the eave struts, bracing and a defined set of purlin connections are installed. An erection sequence that loads cladding or crane girders onto frames before the restraint system is complete asks the frames to work in a condition the design did not assume. Buyers commissioning a fully bolted system — which suppliers such as Jidian list as standard for their warehouse line — should ask for the erection sequence to identify at what stage the stability system is complete, and hold cladding and heavy loading until that point.

The takeaway for procurement is that eave struts, fly bracing and the purlin lines they connect to form one stability system. They should be specified, priced and checked as a package, not as a list of miscellaneous steel.

11. Connections, Cleats and Site Tolerances

Purlins and girts are only as effective as their connections. In a bolted secondary framing system, the load path runs from the sheeting, through the purlin, through a cleat, into the frame. Each link in that path has requirements a buyer can and should specify.

The cleat is a small angle or plate shop-welded (or bolted) to the frame, with holes to receive the purlin bolts. Its design matters because the cleat must carry the purlin reaction, resist the uplift that pulls the purlin away from the frame, and provide lateral restraint to the purlin's bottom flange. Cleat angles that are too small, or cleats with too few bolts, are a common point of under-specification precisely because the component looks trivial next to the frames. The specification should state the number and grade of bolts per purlin end, not just "cleats supplied."

Bolt grades and hole types follow a simple logic. Standard holes give a nominally snug fit and are the default for purlin-to-cleat bolting. Oversize holes — typically 2 mm larger than the bolt — and slotted holes make site adjustment easier where frames or purlins arrive with small misalignments. The trade-off is slip: a connection relying on bearing with oversize holes allows slightly more movement before the bolts engage. Buyers should confirm which hole types are quoted where, because oversize and slotted holes everywhere may ease erection but reduce the precision of the restraint system that fly bracing and eave struts depend on.

One requirement deserves emphasis: purlin-to-cleat bolting must be complete before the purlins are loaded. An unbolted purlin resting on a cleat has essentially no uplift capacity and unreliable lateral restraint. Roof sheeting, insulation bundles and service loads placed before bolting is finished load the purlins in a condition the design never covered. This is an erection discipline issue that should be written into the works contract and checked on site, not left to habit. The broader choice between bolted and shop-welded approaches across the structure is discussed in bolted vs welded connections in steel structure buildings.

Site tolerances close the loop. Cold-formed purlins are dimensionally precise, but frames erected a few millimeters out of line accumulate: frame spacing errors change purlin span lengths, eave height errors show up as ripples in the sheeting lines, and out-of-plumb columns distort girt lines. Because profiled sheeting and sandwich panels are stiff, they will not absorb large tolerances — the sheet fastener lines telegraph every irregularity. Specifying erection tolerances (frame spacing, eave level, plumb) before steelwork starts, and confirming the supplier's stated tolerances for the cold-formed sections themselves, protects the appearance and the weathertightness of the finished envelope. Fully bolted systems help here for a different reason: bolted site connections allow shimming and adjustment that welded fixes do not, so small tolerance deviations can be corrected instead of accepted.

12. Thermal Bridging, Condensation and Insulation Interaction

Purlins and girts are continuous steel members penetrating the building envelope, and steel conducts heat hundreds of times better than the insulation surrounding it. Every purlin therefore acts as a thermal bridge: a short-circuit that carries heat from the warm interior side to the cold exterior side, bypassing the insulation blanket laid between or over the purlins.

The practical consequence appears at the purlin line itself. In a cold climate or a cold night, the purlin's interior-facing surface runs colder than the adjacent insulated surface. If that surface temperature falls below the dew point of the indoor air, moisture condenses on the purlin and the fastener heads around it — which is why condensation in steel buildings characteristically appears as damp streaks and drip lines following the purlin grid rather than as uniform wetness. The same mechanism affects wall girts behind lined walls. The mechanics and countermeasures are covered in depth in condensation control in steel structure buildings; the point for secondary framing procurement is that purlin selection is part of the condensation strategy, not separate from it.

The primary countermeasure at the purlin is to break the metal-to-metal path between the purlin and the outer sheet. Thermal spacer blocks or thermal break battens — low-conductivity components placed between the purlin flange and the sheeting rail — raise the outer sheet's temperature and reduce the heat lost through the fastener and purlin lines. These items are small, inexpensive and frequently omitted from quotations unless explicitly specified. A buyer comparing offers should check whether thermal breaks are included and what material and thickness they are.

The second countermeasure set works on the moisture side. A vapor control layer on the warm side of the insulation limits the moisture reaching the cold purlin surfaces in the first place, and roof space or cavity ventilation removes whatever moisture does get through. Neither layer substitutes for the thermal break at the purlin; they work together.

Insulation thickness decisions interact with purlin depth in a way buyers often discover late. If insulation is installed over the purlins in a single layer, the purlin depth sets a floor on achievable insulation thickness and creates compression points where the blanket is squeezed between purlin and sheet, degrading its performance locally. Deeper sections, spacer systems or a double-layer arrangement with a grid over the purlins relieve this. Before fixing a purlin depth on structural grounds alone, confirm the insulation build-up it must support — the insulation guide at steel building insulation and thermal performance explains the build-up options in detail. The order of decision should be: envelope performance target first, insulation system second, purlin depth and thermal breaks third.

13. How Purlin Choices Move Your Building Cost

Secondary framing is a modest share of total steel tonnage, but it is where several cost decisions concentrate, and where quotations diverge most in ways that are hard to compare.

The first lever is the trade-off between spacing and section size. Closer purlin spacing allows smaller, lighter sections; wider spacing demands deeper or thicker ones. Neither is inherently cheaper, because the saving in section weight from wider spacing is partly offset by the heavier section required, while closer spacing adds members, cleats and fasteners. The optimum depends on the load case — uplift zones on a large roof often pull the optimum tighter than gravity loads alone would suggest.

The second lever is the structural system. Continuous lapped Z systems over two or more spans use the lap to reduce mid-span bending moments, so for the same loads they can achieve a given strength and deflection limit with less total steel than simple-span C sections of the same depth. The trade is lap material, additional bolted splices and slightly more complex erection. On long buildings with regular frame spacing the continuous system is frequently lighter; the buyer's task is to confirm the quotes actually describe different systems before comparing their weights.

The third lever is coating class versus life-cycle cost. A higher zinc coating weight adds a small premium per tonne but extends the maintenance-free life of members that are expensive to access and replace once the building is clad. In humid coastal or industrial atmospheres the case for the higher class is stronger than in dry inland sites; the right answer is location-specific, but the comparison must be made on the same coating class for both quotes.

The fourth lever is subtle and financial rather than structural: the weight basis of the invoice. Cold-formed purlins can be priced on theoretical weight (calculated from nominal section dimensions and density) or actual weighed weight, and manufacturing tolerances mean the two differ. A quote on theoretical weight with loose tolerances can deliver less steel than the same nominal tonnage quoted on actual weight under tighter tolerances. Always establish which basis each quote uses, and how the invoiced tonnage will be verified.

Finally, make quotations comparable before comparing prices. Two offers are only comparable if they state the same structural system (simple span or lapped continuous), the same base steel grade, the same coating class, the same weight basis, and the same scope of ancillaries — bridging, sag rods, thermal breaks, cleats, fasteners. The estimation methods behind the tonnage figures are set out in steel quantity estimation for a steel structure building, and the tonnage feeds into the full budget picture described in steel building cost breakdown for B2B buyers. Normalizing these variables typically explains most of the spread between apparently competing prices.

14. Worked Example: Sizing Purlins for a Hypothetical 40 m x 80 m Warehouse

The following is a hypothetical illustration only. The loads, sections and weights are invented for teaching the arithmetic; they are not design values and must not be used as procurement specifications. Real sizing comes from the supplier's section property tables applied to the project's actual design loads.

Setting. Consider a warehouse 40 m wide and 80 m long, with portal frames at 8 m centers and purlins at 1.5 m spacing on the roof. Each interior purlin between two frames therefore carries a tributary area of 8.0 m × 1.5 m = 12 m² per purlin-bay.

Loads per purlin-bay.

Three observations follow from the table. First, dead load is small compared with the live and uplift cases, which is typical of light cladding. Second, and most importantly, the assumed corner-zone uplift of 14.4 kN per purlin-bay far exceeds the gravity combinations — uplift governs the design of purlins in the corner zones. Third, because uplift governs only in the zones near corners and eaves, a rational design uses heavier or closer-spaced members there and lighter ones in the central field; a single uniform section sized for the corner zones everywhere would be heavier than necessary.

Quantity sensitivity of spacing. Running along the 80 m length, the number of purlin runs is 80 m ÷ 1.5 m = 54 runs (rounded). At 40 m per run, that is 54 × 40 = 4,320 m of purlins. Assume a hypothetical section weighing 7.6 kg/m that satisfies the loads at 1.5 m spacing; total purlin weight is then 4,320 m × 7.6 kg/m ≈ 32,832 kg, about 32.8 t.

Now widen the spacing to 1.8 m: 80 ÷ 1.8 ≈ 45 runs, or 45 × 40 = 3,600 m. The wider spacing requires a stronger hypothetical section, say 8.4 kg/m; total weight is 3,600 × 8.4 ≈ 30,240 kg, about 30.2 t. In this hypothetical case the wider spacing saves roughly 2.6 t of purlin steel despite the heavier section — but it also reduces restraint points for the rafter, may increase sheeting fastener loads per fixing, and must still satisfy the corner-zone uplift, which in reality may force the heavier section back in the zones. The example shows the shape of the trade-off, not a universal result: which spacing wins depends on the actual section tables and the actual zone loads.

What to request from the supplier. Because the numbers above are illustrative, the buyer's real task is verification. Ask for three documents: (1) the section property table for the proposed purlins, including section modulus and second moment of area for the lapped and single-span conditions; (2) a design load summary stating the dead, live, snow and wind uplift loads used, with the load combinations; and (3) a zone layout drawing showing where corner and eave uplift zones fall on the roof and which purlin runs receive heavier sections or closer spacing. With those three documents, any competent reviewing engineer can check the sizing independently.

15. What to Check Before You Order: A Supplier Comparison Framework

Comparing purlin and girt quotations is an exercise in making invisible assumptions visible. Most disputes trace back to an item one supplier included and another priced separately, or to two offers that used different bases. The framework below turns that into a structured check.

Start with the engineering documents. Every offer should include section property tables giving, at minimum, the section modulus and second moment of area of each proposed C and Z section — without these, the stated member sizes cannot be checked against the loads. Next, confirm the base steel standard and yield class: the supplier's stated grade (Q235 and Q345 are the grades listed in typical Chinese manufacturer catalogs such as Jidian's) must be mapped to a recognized standard with a certificate trail, not just a grade name. Then confirm the coating class and stated coating weight, the tolerance standards applied to the cold-formed sections, and the details of the lap or sleeve system if a continuous design is quoted — lap lengths, splice bolt counts and whether sleeves or lapped ends are used. Bridging and sag rods must be itemized, since they are small, essential and easy to omit. Finally, look at logistics and traceability: marking and bundle tagging compatible with container packing, mill certificates issued per batch, and the weight basis used on the invoice.

On the last two rows: manufacturer profiles commonly list certifications — Jidian, for example, lists ISO 9001, CE, SGS and BV alongside its catalog of cold-formed C/Z purlins on H-section frames with Q235/Q345 steel. A listed certificate establishes what the supplier claims; the buyer's verification step is to ask what each document actually covers — which factory, which product scope, which issue date — and to confirm mill certificates are issued per production batch rather than as a blanket document. Certificates about a factory are not certificates about the steel in your containers.

16. Common Specification Mistakes and How to Avoid Them

The recurring errors in purlin and girt procurement are predictable, which makes them preventable. Each item below states the consequence and the fix.

1. Ordering purlins by weight only. Cheapest per tonne is not cheapest per performance; a lighter-tonnage offer with weaker sections fails the deflection or uplift check. Fix: compare section properties and load summaries, then price.
2. Mixing structural systems between quotes. One supplier quotes simple-span C purlins, another lapped continuous Z purlins; the tonnages are not comparable. Fix: require every offer to state the system, and normalize before comparing.
3. Ignoring uplift zones. Uniform sections sized for gravity loads are inadequate in corner and eave zones. Fix: require a zone layout drawing and zone-specific sections.
4. Assuming coating class. "Galvanized" without a stated coating weight can mean very different protection levels. Fix: state the coating class and g/m² in the specification.
5. Forgetting service loads. Sprinklers, lighting, cable trays and HVAC hung from purlins are absent from the load summary and overstress members. Fix: declare all suspended services and their positions before sizing.
6. Skipping bridging and sag rods from the quote. Their omission is discovered on site as an extra or, worse, as unsupplied members. Fix: itemize them explicitly in the schedule.
7. Ignoring girt orientation for the chosen cladding. A wall layout that suits single-skin sheeting may be wrong for sandwich panels, forcing redesign after order. Fix: fix the cladding system first, then the girt layout.
8. Not declaring crane or suspended loads. Crane brackets and heavy point loads change the frame and the secondary framing around them. Fix: declare crane capacity, suspension points and loads at enquiry stage.
9. No tolerance requirements. Uncontrolled erection tolerances telegraph through to rippled sheeting lines and leaking laps. Fix: state erection and section tolerances in the contract.

A single specification sheet covering these nine points takes a buyer little time to prepare and removes most of the ambiguity that generates variation orders. It also has a side effect worth noting: suppliers respond differently to a specification they can price exactly, so the quality of the offers themselves improves.

17. Pre-Order Specification Checklist

Before approving a purlin and girt schedule, a buyer should be able to tick each of the following as personally confirmed, not assumed:

1. All design loads — dead, live, snow where relevant, wind pressure and uplift, and all suspended service loads — are declared in writing and included in the supplier's design summary.
2. The purlin structural system (simple-span or lapped continuous, C or Z) is stated identically in every quote being compared.
3. The base steel standard and yield class are named, with the mapping from catalog grades (e.g., Q235/Q345) to a verifiable standard.
4. The coating class and coating weight (g/m²) are specified, not left as "galvanized."
5. The tolerance standard for the cold-formed sections is identified in the offer.
6. Section property tables (section modulus, second moment of area) for all proposed sections have been received and reviewed.
7. Lap lengths, sleeve details and splice bolt counts for the continuous system, if quoted, are stated on drawings.
8. Bridging and sag rods are itemized with quantities and sections in the schedule.
9. The wind zone layout for the roof and walls is provided, showing corner and eave zones and the sections or spacings assigned to each.
10. Fasteners and cleats are specified: bolt grade, quantity per purlin end, and hole types at each connection.
11. All wall and roof openings are drawn and scheduled, with framed-opening steel and trimmed girts included in the schedule.
12. The girt orientation matches the selected cladding system, and cladding fastener spacings are compatible with the stated girt and purlin spacing.
13. The invoicing weight basis — theoretical or actual — is stated in writing, together with how invoiced tonnage will be verified.
14. Marking, bundle tagging and packing scheme for container shipping are agreed, with a packing list format.
15. The documentation set — per-batch mill certificates and the stated quality certifications with their scope — is defined as a condition of the order.

Items 1–9 protect the structure; items 10–15 protect the site works, the schedule and the invoice. A buyer who cannot complete this list is not yet ready to approve fabrication, because every unticked item is a decision that someone else will make by default.

18. FAQ

Are thicker purlins always better?

No. Thickness is one variable among several — depth, yield strength, spacing and the continuous-lap system all change capacity. A thicker section of the same depth may add weight and cost without solving a deflection or uplift problem that a deeper section or closer spacing would address more efficiently. Over-thick sections also add load to the frames and cost to the invoice. The right question is which section property the design actually needs, verified against the supplier's section tables and the project's load summary — not whether more material is safer by default.

Can purlins be replaced or upgraded after the building is erected?

Technically possible but practically difficult. Purlins support the sheeting, so replacing one usually requires temporarily stripping or supporting the cladding above it, and any change alters the load path assumed in the frame restraint system — purlins participate in rafter stability through the connections discussed earlier. Local reinforcement (additional bridging, sistered sections) is more common than full replacement. If a future upgrade is plausible, such as added rooftop equipment, it is far cheaper to declare it at design stage and size the affected purlins accordingly than to retrofit.

How are purlins and girts packed for container shipping?

Cold-formed sections are long, light and prone to handling distortion, so typical export packing bundles them in strapped bundles with timber or steel bearers, end protection, and per-bundle tags identifying section, length, quantity and destination position within the building. Bundles are sized to container internal dimensions, which constrains maximum member length. Because a mixed container holds several bundle types, the buyer should confirm the tagging scheme and packing list format before shipment, and check that unloading at site — crane or forklift capacity versus bundle weight — has been considered.

Do I need engineering approval to change purlin spacing on site?

Yes. Purlin spacing is part of the load path: it changes tributary loads on each purlin, the fastener loads in the sheeting, and the restraint spacing the frame design assumed for the rafter flanges. Widening spacing on site to save members, or shifting spacing to suit a cladding module, must be checked and approved by the design engineer against the original load summary before any change is made. Site improvisation here is one of the most common sources of sheeting failure under wind uplift, because the change is invisible once the cladding is on.

What information must I give a supplier for an accurate purlin quote?

At minimum: building dimensions and frame spacing; the cladding system for roof and walls, including insulation build-up; all design loads including suspended services and any crane or point loads; the number and size of openings; wind conditions at the site; the coating class and steel grade required; the structural system preference; and the packing and delivery requirements. The more of the checklist in the previous chapter the enquiry answers in advance, the closer the offer is to a final price — and the fewer variation orders the project will generate after the order is placed.

19. Related Steel Building Topics

Purlin and girt decisions connect to the wider envelope and structure; the following guides cover the adjacent topics in the same depth.

• roofing and cladding systems for steel structure buildings
• portal frame warehouse structures
• bolted vs welded connections in steel structure buildings
• condensation control in steel structure buildings
• steel building insulation and thermal performance
• steel quantity estimation for a steel structure building
• steel building cost breakdown for B2B buyers
• choosing the right steel grade for a steel structure building