Roof
Purlins
Horizontal members spanning across the roof between the primary frames. They support the roof panels and transfer roof loads back to the main steel frames.
Component guide
Purlins and Girts
Purlins and girts are the horizontal members between the main frames and the panels. They rarely appear in a sales conversation and they routinely appear in a dispute, because this is one of the easiest places for two quotes to differ without either being wrong. This guide explains what they are, what they do, and what a buyer can reasonably check.
If a steel building's main frames are the backbone, purlins and girts are the ribs. Butler's parts documentation uses exactly that comparison, and it is the right one: the frames carry the building, the secondary framing connects everything together and gives the panels something to fasten to.
This page is deliberately not a design guide. Member sizes, gauges, spacings, spans, loads, connections and bridging are outputs of an engineered building design for your site and your loads, and nothing here should be used as a substitute for that. What follows is what a buyer needs in order to read a proposal, ask a useful question, and notice when two proposals are not describing the same building.
Walls
Girts
Horizontal members running across the walls. They support the wall panels and help handle wind pressure and impact.
This is not a design guide
Member sizes, gauges, spacing, spans, loads, connections and bracing are outputs of an engineered building design for your site. Nothing on this page should be used in place of that design, and no spacing or size is recommended here.
Purlin or girt: the distinction in one paragraph
Both are horizontal steel members that span between the primary frames. The difference is location and therefore job. Butler describes purlins as horizontal members spanning across the roof, supporting the roof panels and transferring weight — snow, or crews walking on the roof — back to the main steel frames. Girts run horizontally across the walls, support the wall panels, and help handle wind pressure and impact.
That is the whole distinction, and it is worth holding onto because the terms are used loosely in conversation. If somebody says "purlins" while pointing at a wall, they mean girts. On a proposal, both usually appear together under a heading such as secondary framing.
- Purlins — roof
- Span across the roof between frames, support roof panels, and transfer roof loads back to the primary frames.
- Girts — walls
- Run horizontally across the walls, support wall panels, and help resist wind pressure and impact.
- Eave struts
- The member at the junction where roof meets wall. Usually quoted within secondary framing rather than separately.
One bay, drawn open
Common secondary-framing shapes
C-shaped
Z-shaped
- Roof panels sit on purlins; purlins span between primary frames.
- Wall panels sit on girts; girts span between primary columns.
Why secondary framing matters more than its price suggests
Butler's documentation sets out what these members do beyond simply existing: distribute weight across the building, prevent panel sagging, improve the walkability of roof surfaces, support insulation systems and interior liner panels, and add strength against wind uplift and other external forces. It also notes the loads involved are of two kinds — dead loads such as the panels, insulation and the members themselves, and live loads such as snow, rain, wind or workers on the roof.
Read that list again with a buyer's eye and you will notice something: three of those five functions belong to other line items on your quote. The insulation system is banded to the purlins. The roof panel is fastened to them. The liner panel hangs from them. Change the secondary framing and you have quietly changed the roof assembly, the insulation options and sometimes the interior finish.
This is also why substitution is a genuine risk rather than a theoretical one. Butler lists mixing generic purlins with pre-engineered systems among the common mistakes it sees, on the basis that they will not line up correctly, and notes its own members are supplied factory pre-punched so alignment is fixed before delivery. Whatever brand of building you buy, the underlying point holds: secondary framing in a pre-engineered building is part of a designed system, not a commodity length of steel.
C and Z shapes, and why you see both
Cold-formed secondary framing is usually shaped as a C section or a Z section — Butler notes you will often see both — and the two are not interchangeable. Butler's own guidance is explicit on this point: C and Z members each have their own structural characteristics and installation purpose.
As a buyer you do not need to know which the engineer should choose. You do need to know that if one proposal quotes Z members and another quotes C members, they are describing different framing, and that if a supplier offers to substitute one for the other after the design is fixed, that is an engineering change rather than a swap.
Materials vary too, and the variation is a legitimate cost difference rather than a trick. Butler describes its own purlin and girt designs as made from high-tensile galvanised steel and cites a coating mass of 120 gsm conforming to ASTM A653M for its products. That is a manufacturer statement about its own components; it is not an industry minimum and should not be read as one. If corrosion resistance matters on your site, ask each supplier what coating their secondary framing carries and get the answer in writing.
Why spacing and configuration vary between proposals
Spacing is the number buyers most want a rule for, and it is precisely the number that cannot be given as one. Purlin and girt spacing is an output of the building's engineering: the loads the building is designed for, the span between frames, the panel selected, and the code requirements applying at your site all feed into it. Two suppliers designing the same building to different assumed loads will produce different secondary framing, and both may be correct for the assumptions they used.
Butler's guidance frames the risk from the other direction: it lists over-spacing purlins to cut cost among the common mistakes it sees, on the basis that the spacing is engineered to carry specific loads and widening the gap can reduce strength and, in its case, affect warranties. That is a warning against changing an engineered spacing, not a licence for anyone to nominate a spacing from a web page.
The buyer's question is therefore not "what spacing should I have?" but "what loads was this building designed for, and who sealed that design?" Two proposals designed to the same loads by qualified engineers can be compared. Two proposals where one has quietly assumed lower loads cannot, and the difference will usually show up in the secondary framing before it shows up anywhere else.
How secondary framing meets panels, insulation and openings
Panels fasten to secondary framing, so the two decisions are linked. An exposed-fastener roof panel is screwed through the panel into the purlin below; a concealed-fastener standing seam roof is clipped or fastened at the seam. Which panel system you choose therefore has consequences for the framing it lands on, and a panel change late in the process is not always a like-for-like substitution.
Insulation is the other dependency. Blanket and banded systems are installed over, between or beneath purlins and girts, and Butler notes secondary framing supports insulation systems and interior liner panels. It also lists skipping vapour retarder protection where insulation runs around girts among the errors it sees — a reminder that the interaction between framing and insulation is where detailing goes wrong.
Openings interrupt everything. A roll-up door, a walk door or a window cuts through the girt line, and the framing around that opening has to be designed in rather than added later. If you are still deciding where doors go, say so before the building is engineered; moving a framed opening after the drawings are sealed is a change order, not a preference.
What secondary framing looks like on a proposal
On most quotes secondary framing is a single line, sometimes folded into the frame package, sometimes shown separately. That is normal. What is not acceptable is being unable to find out what is behind the line, because this is a common place for quotes to differ quietly: shape, gauge, coating, spacing and bracing all sit inside that one word.
You do not need to evaluate those values. You need them stated, consistently, across every proposal you are comparing, so that a lower price is visibly a lower price for the same thing rather than for less steel. Ask for the drawings that show the framing layout, and ask who sealed them.
The conversation to have with the supplier or engineer
Three questions do most of the work. What design loads was this building engineered for — wind, snow, seismic, collateral — and who sealed the drawings? Is the secondary framing part of the same engineered system as the frame? And if any part of the building will later carry something extra — a crane, a mezzanine, hanging services, solar — has that been included, because collateral load added after the fact is an engineering change.
Everything else on this page exists to help you understand those answers. None of it replaces them.
What to check in a building proposal
- Design loads
- The wind, snow, seismic and collateral loads the building was engineered for, stated on every proposal you compare.
- Sealed drawings
- Who seals the engineering, and whether secondary framing is covered by the same design as the primary frame.
- Shape and gauge
- Whether C or Z members are quoted and at what gauge — stated, not assumed, since this is a common quiet difference.
- Coating
- What corrosion protection the members carry, particularly on coastal, agricultural or humid sites.
- Framing layout
- A drawing showing the framing layout, so panel and insulation choices can be checked against it.
- Openings
- That every door, window and louvre you want is shown in the framing before the design is sealed.
- Future loads
- Whether cranes, mezzanines, hanging services or solar were included in the design assumptions.
Frequently asked questions
Location and job. Butler describes purlins as horizontal members spanning across the roof that support the roof panels and transfer loads such as snow or maintenance crews back to the main frames, and girts as running horizontally across the walls to support wall panels and help handle wind pressure and impact. Both are secondary framing; the roof member is a purlin and the wall member is a girt.
No. Butler's guidance states directly that each has its own structural characteristics and installation purpose. Which one belongs in your building is an engineering decision made as part of the building design, and a late substitution is an engineering change rather than a swap of equivalent parts.
We do not publish a spacing, and you should be wary of anyone who does without seeing your project. Spacing is an output of the engineered design: the design loads, the span between frames, the panel system and the applicable code all feed into it. Butler warns specifically against over-spacing purlins to cut cost, noting the spacing is engineered to carry specific loads. Ask what loads your building was designed for and who sealed the drawings.
Butler lists mixing generic purlins with pre-engineered systems among the common mistakes it sees, on the grounds that pre-punched holes, clips and panel integration will not match, and notes its own members are supplied factory pre-punched. The broader point applies to any manufacturer's system: secondary framing in a pre-engineered building is designed as part of that system, and substitution is a question for the building's engineer of record rather than a purchasing decision.
Yes, and this surprises buyers more than it should. Butler notes that secondary framing supports insulation systems and interior liner panels as well as the exterior panels. Banded and liner insulation systems are installed against the purlins, and the fabric or facing of some systems is attached across the underside of them, which means the framing partly determines what the inside of the building looks like.
Get the shape, gauge, coating and framing layout stated on each proposal, alongside the design loads. Secondary framing is often shown as one line, and a cheaper line can reflect a genuinely different design rather than a better price. Once the design loads and the framing description match, the comparison is meaningful.
Loads added after the building is designed are an engineering matter, not an accessory purchase. Anything that hangs from or bears on the structure — cranes, mezzanines, ductwork, sprinklers, solar — should be raised before the design is sealed so it can be included in the design assumptions. Retrofitting them is possible on some buildings and not on others, and only the engineer of record can say which yours is.
Verification and sources
Every material statement on this page maps to a published source, and each source record states what it does not cover. Where something was not verified it is recorded as not verified rather than estimated.
Definition — purlins · Tier 2
Purlins are horizontal steel members spanning across the roof that support the roof panels and transfer weight, such as snow or maintenance crews, back to the main steel frames.
Definition only. No span, spacing, size or load capacity is implied.
Butler MFG Parts — The Role of Purlins and GirtsDefinition — girts · Tier 2
Girts run horizontally across the walls, support the wall panels, and help handle wind pressure and impact.
Definition only. No wind pressure value or resistance claim is made.
Butler MFG Parts — The Role of Purlins and GirtsStructural role · Tier 2
Secondary framing distributes weight across the building, prevents panel sagging, improves roof walkability, supports insulation systems and interior liner panels, and adds strength against wind uplift.
Functional description from one manufacturer's technical content. Not a performance guarantee for any building.
Butler MFG Parts — The Role of Purlins and GirtsLoad types · Tier 2
Purlins and girts carry dead loads such as panels, insulation and the members themselves, and live loads such as snow, rain, wind or workers on the roof.
Load categories only. No load magnitude, combination or design value is stated.
Butler MFG Parts — The Role of Purlins and GirtsC and Z shapes · Tier 2
Secondary framing members are commonly shaped as C or Z sections, and the two are not interchangeable: each has its own structural characteristics and installation purpose.
Does not indicate which shape suits any project; that is an engineering decision.
Butler MFG Parts — The Role of Purlins and GirtsCoating — manufacturer specific · Tier 2
Butler states its purlins and girts are made from high-tensile galvanised steel with a coating mass of 120 gsm conforming to ASTM A653M.
Attributed to one manufacturer about its own components. Not an industry minimum and not applicable to other suppliers.
Butler MFG Parts — The Role of Purlins and GirtsSpacing risk · Tier 2
Over-spacing purlins to reduce cost is listed as a common mistake, on the basis that spacing is engineered to carry specific loads and widening it can reduce strength and affect warranties.
A warning against altering an engineered spacing. It authorises no spacing recommendation of any kind.
Butler MFG Parts — The Role of Purlins and GirtsGeneric substitution · Tier 2
Mixing generic purlins with pre-engineered systems is listed as a common mistake because pre-punched holes, clips and panel integration will not match; members are described as supplied factory pre-punched.
Manufacturer statement about its own system; presented as an illustration of system integration, not a universal prohibition.
Butler MFG Parts — The Role of Purlins and GirtsSecondary framing in metal roofing practice · Tier 3
Purlins are covered as a distinct element of steel-frame construction and metal roofing in published engineering and metals-industry guidance.
Used to corroborate that the topic is treated as an engineered element. No values are taken from it.
Kloeckner Metals — guide to purlins in metal roofing and steel frame constructionEngineering literature · Tier 3
Purlin design and behaviour are the subject of published structural engineering literature.
Corroborates that this is engineered design territory. No design guidance is reproduced.
STRUCTURE magazine — What About the Purlins?Metal building systems standards context · Tier 1
Metal building systems, including secondary framing, are covered by industry technical material published by the Metal Building Manufacturers Association.
Points to the industry body. No provision of any manual is quoted or applied.
Metal Building Manufacturers AssociationNot verified · Tier 4
Member sizes, gauges, spacings, spans, load capacities, connection details, fastener schedules and bridging requirements are not published on this page and were not verified for any building.
Records a deliberate absence. These belong to the engineer of record for a specific project.
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