Roofing
Roof purlins: types, spacing, spans, bracing, and field checks
Roof purlins are secondary framing members that support roofing between the main frames or trusses. Check their span, spacing, bracing, and connections together before ordering steel or pricing a reroof.
Direct answer
Roof purlins are secondary structural members that carry roof panels or decking between rafters, trusses, or building frames, usually running parallel to the ridge. Common types are cold-formed steel C and Z sections and structural wood members. Their size and spacing depend on the roof loads, support span, material, bracing, and connections; there is no universal purlin spacing or span that fits every metal roof.
On this page
What are roof purlins, and where do they fit?How do C, Z, and wood purlins differ?What is the difference between purlin spacing and span?Which codes and design standards apply in 2026?How do you turn roof pressure into purlin load?Why do bracing and roof attachments change capacity?How should laps, bolts, and bearing be checked?What changes when you reroof or add equipment?How do you inspect existing purlins before pricing repairs?What belongs in a purlin replacement or retrofit estimate?Roof purlin field checklistWhat should the handoff record contain?Which mistakes cause trouble before sheeting?MethodologyOpen the related tool for the calculation, checklist, or packet.
FieldOS by AnvilfieldOffline-first FieldOS for job photos, proof packets, property memory, calculations, teams, billing, and owner-ready reports.Key takeaways
- Purlin spacing sets the panel support distance; purlin span is the distance between supports along the member.
- A section depth or gauge does not establish capacity without its material properties, bracing, span arrangement, and connections.
- Replacing roof panels can change the restraint that keeps existing purlins stable.
- Record member locations, thicknesses, laps, braces, and added loads before requesting a retrofit design.
What are roof purlins, and where do they fit?
Roof purlins carry the roof covering between the main supports. In a typical metal building, the rigid-frame rafters run up the roof slope, while purlins cross those rafters and run along the building. Roof panels span from one purlin line to the next. Gravity load travels from the panels through their attachments into the purlins, then through the rafters, columns, and foundations. Wind uplift reverses the direction of force through that attachment chain.
Start an inspection by identifying that arrangement from below, using approved access. A wall member supporting siding is a girt. The member at the roof and wall junction may be an eave strut with its own loading and connection details. A small roof batten or retrofit rail may resemble a purlin but have a different support arrangement. Name the part by its structural job and drawing mark, not its appearance alone.
The word also appears in wood construction. A wood purlin can support roofing between trusses, while a purlin beneath conventional rafters supports the rafters themselves. Those are different load paths. Sketch what bears on what before reaching for a span table, particularly when drawings use terminology from another building system.
How do C, Z, and wood purlins differ?
A cold-formed steel purlin gets its shape by bending sheet steel into a section with a web, flanges, and usually stiffening lips. A C section has both flanges projecting to the same side. A Z section has flanges projecting to opposite sides. Many Z profiles have unequal flange widths so adjacent pieces can nest at a designed lap. The lap and its fasteners help establish continuity across supports.
Do not order a substitute by depth alone. Record the section designation, base-metal thickness, flange and lip dimensions, specified yield strength, coating, hole pattern, and part mark. Two members that both measure 8 inches deep can have different capacities and different positions in the same roof. Gauge is an incomplete purchasing description; use the manufacturer's stated thickness and material designation.
Wood purlins need species, grade, actual dimensions, orientation, moisture conditions, and connection details. An on-edge member and the same board laid flat have different bending stiffness. The American Wood Council's National Design Specification covers member and connection design; an unlabeled board and a nominal lumber size do not supply that design information.
| Member | What identifies it | What to verify before substitution |
|---|---|---|
| C or channel purlin | Flanges project to one side of the web | Approved span arrangement, splice or end connection, thickness, and bracing |
| Z purlin | Flanges project to opposite sides; specified profiles nest | Lap length, nesting direction, bolt pattern, and support location |
| Structural wood purlin | Graded lumber or an identified engineered wood product | Species or product, grade, orientation, bearing, moisture, and uplift connection |
| Retrofit sub-purlin | Rail or notched member above an existing roof | Attachment into verified structure, tested assembly, and effect on the existing framing |
What is the difference between purlin spacing and span?
Spacing is the distance between adjacent purlin lines, normally measured across the roof slope. Span is the distance along a purlin between its supports. A roof can have purlins at 4-foot centers crossing frames 20 feet apart. In that arrangement, a panel crosses 4 feet between supports, while each purlin crosses a 20-foot frame bay. Neither dimension proves that the other member is adequate.
Check the roof panel and the purlin separately. Panel tables depend on profile, metal thickness, span arrangement, loading direction, fastening, and the listed assembly. Purlin calculations depend on the member properties, restraint, support arrangement, and load combinations. A panel approved for the support spacing can still sit on inadequate purlins. A strong purlin cannot make an undersized panel span farther.
Ask whether a drawing dimension is measured along the sloping surface or on horizontal projection. Roof snow pressures and panel-normal wind pressures do not automatically use the same area basis. At roof edges, corners, openings, and drift locations, the design may call for different supports or attachments. Keep those zones identified on the layout rather than extending an interior-bay detail across the entire roof.
Before selecting a table, identify simple spans, continuous runs, laps, and cantilevers. A continuous Z system carries moments through its support regions. Cutting a lap or substituting isolated simple-span pieces changes the analysis even if the visible support spacing stays the same.
Which codes and design standards apply in 2026?
This guide uses the 2024 International Building Code, or IBC, as its model-code reference and ASCE/SEI 7-22 for the loading framework. A permit is governed by the edition and amendments adopted by the authority having jurisdiction. Confirm those before applying a table. For an existing building, also establish the adopted existing-building code and the alteration provisions that apply to the proposed work.
Cold-formed steel requires an edition check. The 2024 IBC references AISI S100-16, reaffirmed in 2020, with Supplement 2-20. The Steel Deck Institute now lists ANSI/SDI AISI S100-24 as the current published specification. Publication of the newer standard does not automatically replace the permit's referenced edition. ICC-ES report ESR-5429, Table 8, provides a useful code-edition cross-check; its product-specific fastener capacities are not generic purlin capacities.
For wood, use the NDS edition and matching Supplement required by the project; the 2024 IBC references the 2024 NDS. The Metal Building Manufacturers Association's 2024 Roof Framing Design Guide explains purlin systems, but identifies AISI S100-16 as the basis of its calculations. Keep that distinction visible when using its examples. This article performs no electrical design and applies no National Electrical Code sizing provisions.
| Reference | Role in this guide | Field decision |
|---|---|---|
| 2024 IBC and the adopted existing-building code | Structural requirements and project code basis | Write the actual adopted editions and amendments on the survey or submittal |
| ASCE/SEI 7-22 | Loads, combinations, and relevant wind and snow conditions | Obtain project pressures and their design basis from the responsible designer |
| AISI S100-16(2020) with S2-20; current publication S100-24 | Cold-formed steel member and connection design | Use the edition required or expressly accepted for the project |
| 2024 NDS and matching Supplement | Wood member properties, adjustments, and connections | Match species, grade, service conditions, and the adopted design basis |
| 29 CFR 1926.758 | Systems-engineered metal building erection | Use the applicable erection and fall-protection plan; a roof load check does not authorize access |
How do you turn roof pressure into purlin load?
Anvilfield load analysis gives a 20 ft simply supported purlin at 4 ft spacing a 2,400 lb downward load under 30 psf uniform service pressure. The reaction is 1,200 lb at each support and the maximum bending moment is 6,000 lb-ft. These are demand values; the designer selects the member and verifies its capacity.
The table compares 3 ft, 4 ft and 5 ft interior-line spacing at the same span and pressure. Increasing spacing from 4 ft to 5 ft raises tributary load and bending moment by 25 percent and increases the panel support span. Use the actual tributary width at boundary lines.
The Anvilfield stiffness calculation gives 1.24 in of deflection for the 4 ft spacing case. It exceeds the case's 1.00 in project limit. The member check must address the actual effective stiffness, bracing, loads and project deflection criteria.
Under the separate 20 psf net uplift case, line load is 80 plf upward and each support reaction is 800 lb upward. Check the complete path through panels, clips, fasteners, purlins and frame connections on a consistent allowable-stress or strength-design basis.
| Purlin spacing | Tributary area | Downward line load | Reaction at each end | Maximum moment |
|---|---|---|---|---|
| 3 ft | 60 sq ft | 90 plf | 900 lb | 4,500 lb-ft |
| 4 ft | 80 sq ft | 120 plf | 1,200 lb | 6,000 lb-ft |
| 5 ft | 100 sq ft | 150 plf | 1,500 lb | 7,500 lb-ft |
Anvilfield analysis: 20 ft simple span and 30 psf downward service pressure; calculation inputs are in Methodology.
Why do bracing and roof attachments change capacity?
A thin steel section can twist or buckle before its material reaches a simple bending-stress limit. The roof assembly restrains that behavior through some combination of panels, clips, bridging, discrete braces, and anchorage. The MBMA framing guide treats these as interacting parts of the roof system. A catalog section property alone leaves out the restraint conditions that make the member work.
The flange in compression changes with bending direction. Under gravity, a simple span has its upper flange in compression at midspan. Under uplift, the lower flange can be in compression there. Continuous members also have negative-moment regions at supports. A brace arrangement adequate for one condition requires a separate check for the other conditions.
Standing seam clips permit roof movement, and their restraint differs from a directly screwed roof. Changing the panel profile, clip, insulation stack, or attachment pattern can change purlin behavior. Obtain the system designer's review before switching roof types. Likewise, keep purlin restraint distinct from the building's wind-bracing system: an installed brace needs its specified connections and load path, not merely the right appearance.
How should laps, bolts, and bearing be checked?
Start with the erection drawing and part marks. Record which member belongs in each bay, which flange faces uphill, where the lap starts and ends, and which holes are used. Lap length is a design detail, not a standard percentage that a field crew can choose. An extra hole or a shortened lap can change the intended force transfer.
At supports, check the specified seat or clip, bearing arrangement, fastener type, hole condition, and any stiffener. The web can be vulnerable to concentrated bearing forces even when the member's midspan bending check passes. Photograph crushed webs, elongated holes, missing fasteners, and bent clips with a location mark. Send discrepancies for a repair detail before sheeting hides them.
Do not torch new holes, trim stiffening lips, weld reinforcement, or exchange bolts simply to make pieces fit. Confirm the approved correction and required coating repair. For roof screws, identify the actual supporting steel thickness and the specified screw; a screw that penetrates the member is not automatically adequate against pull-out, pull-over, or combined actions. ICC-ES product reports illustrate why those connection limits are checked separately.
What changes when you reroof or add equipment?
A reroof changes more than dead weight. Removing through-fastened panels can remove restraint that existing purlins depend on. The completed replacement assembly and the temporary open-roof condition both need consideration. Agree on the removal sequence, temporary stability measures, weather protection, and material staging before the first bay is stripped.
A metal-over-metal retrofit adds sub-purlins or another engineered support assembly. Roof Hugger's details show the importance of connecting the new system into the existing structural purlins. A screw attached only to the old sheet is a different connection. Verify the existing frame positions and thicknesses, the new attachment details, and any additional framing at high-demand roof zones. Verify the original building's capacity under the proposed retrofit loads and restraint conditions.
Equipment, solar supports, suspended piping, and ceilings can introduce concentrated or eccentric loads. A building-wide average weight can hide a large reaction on one purlin. Record each attachment location, reaction, and direction. MBMA's reinforcing guidance specifically cautions against hanging utilities from a purlin's stiffening lip because that attachment can twist the section. Have the responsible designer detail the support and any load distribution.
Keep the proposed roof geometry and drainage in the review. New curbs, changes in elevation, or neighboring construction can affect snow accumulation. State what will remain, what will be removed, and what will be added, with weights and locations. The engineer can then assess the changed condition instead of guessing from a roofing product name.
How do you inspect existing purlins before pricing repairs?
Ask for the original manufacturer, building order number, erection drawings, structural calculations, and records of alterations. MBMA's retrofit guidance recommends pursuing those records before relying on field measurement alone. Bay lengths and purlin thicknesses can vary within one building. A measurement taken near a door is not a reliable substitute for a roof-wide member schedule.
Assign locations before photographs. Use frame lines and purlin rows, then identify the member, connection, and observation in each image. Separate surface staining from measured section loss. If thickness is needed, agree on the measurement method and coating allowance with the reviewer. A visual rust grade does not establish remaining structural capacity.
Plan access for the condition being investigated. Do not load or walk a suspect roof to prove that it holds. For systems-engineered metal building erection, OSHA 1926.758(g) requires written approval from a qualified person before purlins or girts serve as fall-arrest anchorage. Paragraph (h) restricts walking or working on purlins to installing safety systems after permanent bridging is installed and fall protection is provided. Those provisions are not permission for routine roof travel.
| Record | Useful field evidence | Decision it supports |
|---|---|---|
| Geometry and identity | Frame bay, purlin row, part mark, span, spacing, and orientation | Match the survey to the original framing schedule |
| Member condition | Dimensioned distortion, corrosion location, measured thickness where specified | Determine whether analysis or an engineered repair is needed |
| Connections and restraint | Lap measurements, fastener condition, braces, and anchorage | Identify departures from the approved system |
| Added loads | Equipment and hanger locations with supported weights or reactions | Define concentrated loads and support changes |
| Roof assembly | Panel profile, clips or screws, insulation, and proposed replacement | Assess changes to attachment and purlin restraint |
What belongs in a purlin replacement or retrofit estimate?
Price the survey and design coordination before promising a member count. Separate access equipment, interior protection, temporary stability work, selective roof removal, members, connections, bracing, corrosion treatment, insulation repairs, and final roofing. State how concealed conditions will be measured and priced. A linear-foot steel allowance alone will miss much of the work in an occupied building.
Anvilfield takeoff analysis gives an 80 ft long roof plane with 48 ft between its boundary support lines and 4 ft line spacing 13 support lines and 1,040 linear ft. Identify eave and ridge members separately in the purchase schedule and measure openings and added framing.
The case's 26 laps add 52 ft, bringing the steel quantity to 1,092 ft before fabrication allowances. Order by the approved piece list, overlap lengths and hole pattern. Tie labor to connections, access and temporary support as well as footage.
For a service territory built around metal-roof inspections, PorchRocket's Ground Control delivers lists of commercial buildings selected by roof material and size within 48 hours after payment. Qualify the owner and arrange access before assigning a framing survey; the list identifies buildings to contact, while site records and inspection establish the work. Hidden purlin condition still requires field investigation.
Roof purlin field checklist
Use this checklist before panel installation or before releasing a retrofit scope. Mark unresolved items with a responsible person and a required response date. A photograph closes an observation only when the needed review or correction is also recorded.
Want this checklist to run itself on every job — with photo proof and a signed record crews can hand the customer? That's FieldOS.
What should the handoff record contain?
Deliver a marked roof plan, member schedule, connection and bracing photographs, and a list of deviations with their dispositions. Include measured values and units rather than statements such as 'purlins look good.' A record entry can read: 'Frame B-C, row P6: 20-foot bay; lower lip locally bent beside added hanger; photo P6-04; hanger load requested; structural review open.' That reports an observation without claiming acceptance.
Keep design responsibility visible. The engineer's analysis, manufacturer's assembly information, contractor's installation record, and inspector's observations serve different purposes. Identify who resolved the load question, who supplied the repair detail, and who checked the completed installation. Record the drawing revision used for each decision.
For future maintenance, retain an accessible roof plan that identifies repaired bays and approved equipment attachment locations. An owner should be able to locate a repair after the crew leaves and before the next trade adds a hanger or curb. That continuity makes the next inspection specific instead of starting the investigation again.
Which mistakes cause trouble before sheeting?
The first is treating a familiar dimension as approval: 'We always use 5-foot spacing' or 'an 8-inch Z will carry it.' Replace that shortcut with the approved member and panel schedules. The second is treating a missing brace as cleanup work. Resolve it while the connections are visible and before loading the roof.
Another mistake is copying an old load table without reading its edition, load basis, or restraint notes. Check whether loads are total or superimposed, whether self-weight is included, and whether the table applies to gravity or uplift. A result in pounds per linear foot cannot be compared directly with a pressure in pounds per square foot.
Finally, avoid turning a surface repair into an unsupported structural claim. Paint can address a coating condition; it does not document restored steel thickness. A replacement roof panel does not establish the adequacy of an existing purlin. Close the specific structural question with the responsible reviewer before covering the evidence.
Methodology
Anvilfield's simple-beam model uses a level roof strip, 20 ft span and equal spacing on both sides of an interior member. The 30 psf downward service-pressure parameter includes assigned dead load and purlin self-weight; the separate net uplift parameter is 20 psf. Tributary area is span x spacing, line load is pressure x spacing, end reaction is wL/2 and maximum moment is wL squared/8.
Deflection uses 5wL to the fourth power/(384EI), w = 10 lb/in, L = 240 in, E = 29,000,000 psi and effective I = 12 in to the fourth power. The case criterion is L/240 = 1 in. The quantity model uses 48/4 + 1 support lines, each 80 ft long, plus 26 laps at 2 ft. These model parameters define the calculation, not an approved member, local design load, lap requirement or universal spacing limit.
MBMA, ASCE, SDI, AWC and ICC-ES supply the system and edition references; federal 29 CFR 1926.758 supplies the erection-access rules. Sources were opened November 5, 2026. The saved AF-VR3 estimate script reproduces the results.
FAQ
Do all metal roofs need purlins?
No. Some metal roof systems require a continuous deck, while structural panels may span open framing. Use the panel manufacturer's approved support arrangement. Installing an architectural panel over open purlins without approval can leave it without the support it requires.
How far can a steel roof purlin span?
There is no universal span. Capacity depends on section geometry, thickness, steel strength, loads, bracing, continuity, and connections. Obtain the manufacturer's project schedule or an engineered design for the actual assembly rather than choosing a span from section depth alone.
Should roof purlins be spaced 4 feet or 5 feet apart?
Neither spacing is automatically correct. The panel and the purlin each need to pass their applicable checks. In Anvilfield's load analysis, moving from 4 to 5 feet increases the purlin's uniform tributary load by 25 percent and increases the panel support span.
Are Z purlins stronger than C purlins?
The letter does not establish strength. Z sections often suit nested continuous laps, but actual capacity depends on dimensions, thickness, material, loading, restraint, and connections. Compare approved systems under the same conditions without declaring a winner from the shape alone.
Can I use 2x4 or 2x6 wood purlins?
Only when that member and its connections meet the project design. Nominal size is insufficient: species, grade, actual dimensions, orientation, support span, loads, moisture conditions, and uplift attachment matter. A board installed flat cannot be assigned the on-edge capacity.
Do purlins run parallel to the ridge?
Usually, in a conventional metal building. They cross the primary rafters and support panels running up the slope. Check the drawings because other roof systems, especially timber framing and retrofit assemblies, can use different arrangements and terminology.
Can a purlin be used as a fall-arrest anchor?
Written approval is required for the erection use described here. For systems-engineered metal building erection, OSHA 1926.758(g) requires written approval from a qualified person before purlins or girts are used as fall-arrest anchorage. Use the project's approved access and fall-protection arrangements.
Can you coat rusty purlins instead of replacing them?
A coating repair and a structural repair answer different questions. Establish the condition and remaining section where corrosion may affect capacity, then follow the accepted treatment or repair detail. A fresh coating does not replace missing metal or establish the member's load capacity.
Does a standing seam reroof need a purlin review?
Changing from through-fastened roofing to standing seam can change purlin restraint as well as loads and attachments. Have the existing framing, proposed assembly, and temporary removal condition reviewed before treating the reroof as a panel-only replacement.
People also ask
Codes cited in this guide
This guide is written and reviewed against the published standards below. Always confirm the current adopted edition with the authority having jurisdiction.
Sources checked
- MBMA: Roof Framing Design Guide for Metal Building Systems, 2024 edition Chapters 1 through 3 explain panels, C and Z purlins, continuity, bracing, and load direction. Page 7 identifies the guide's AISI S100-16 calculation basis.
- MBMA: Retrofitting and Reinforcing Metal Building Systems, 2020 Pages 10 and 11 cover existing records, field measurements, coating allowance, and varying purlin thickness. Pages 26 through 28 address added purlins and concentrated loads. Used for investigation principles, not current code adoption.
- ASCE: ASCE/SEI 7-22 and current supplements and errata Publisher loading-standard reference, supplements, errata and hazard resources. Project pressures are supplied by the responsible designer.
- Steel Deck Institute: current editions of former AISI standards Lists ANSI/SDI AISI S100-24 as the current published cold-formed steel specification. A current publication is distinct from the edition adopted for a permit.
- American Wood Council: 2024 National Design Specification Confirms the 2024 NDS reference in the 2024 IBC and provides the wood member and connection design publication.
- ICC-ES: ESR-5429, code-edition matrix and screw connection limits Table 8 identifies AISI S100-16(2020) with S2-20 for the 2024 IBC. Connection tables distinguish fastener tension, pull-out, and pull-over; their capacities apply only to the reported products and conditions.
- U.S. Government Publishing Office: 29 CFR 1926.758 Official 2025 annual CFR text, cross-checked against OSHA's current regulation page on November 5, 2026. Paragraphs (g) and (h) govern purlin anchorage and walking or working surfaces during systems-engineered metal building erection.
- Roof Hugger: roof retrofit attachment considerations Manufacturer explanation of notched sub-purlins connecting to the existing structural purlins. Product-specific guidance does not establish the capacity of an uninspected existing building.