A purlin is a horizontal structural member that spans between rafters, columns, or trusses to support the roof decking, panels, or sheathing. In a metal building, purlins are typically cold-formed steel Z- or C-sections at 4–6 ft on-centre. In a heavy-timber roof, purlins are sawn or glulam beams. Their job: convert distributed roof loads into concentrated reactions at the primary frame.

What is the difference between a purlin and a rafter?

Rafters are the sloped primary roof members spanning from ridge to eave (or from column to column in a single-slope frame); they carry purlins and ultimately the roof load down to the supporting walls or columns. Purlins are the secondary horizontal members that span perpendicular to the rafters between two adjacent rafters, supporting the roof panels themselves. Rafters are deeper and more widely spaced; purlins are shallower and at 4–6 ft o.c.

What is the difference between a purlin and a girt?

Both are cold-formed C or Z secondary members; the only difference is orientation. Purlins are roof members oriented horizontally with the web vertical, supporting roof panels under gravity loads. Girts are wall members oriented horizontally with the web horizontal (laid on its side), supporting wall panels under lateral wind pressure and suction. Same product family — the manufacturer's tables list both spans.

How far apart should purlins be?

Standard metal-building spacing is 5 ft 0 in on-centre for 24 / 26 GA standing-seam roofing. Tighter spacing — 4 ft 0 in — is common in high-snow regions or when 22 GA panels need a smaller span. Wider spacing — 6 ft 0 in — is the practical maximum for most through-fastened panels. Sawn-lumber purlins on cathedral ceilings sit at 24 in o.c. to match standard rafter / joist layout.

How do I size a purlin?

Compute the design load: roof dead + live (or snow) + wind uplift × tributary spacing = w (plf). Then enter the manufacturer's allowable-span table for the target purlin section (e.g. 8" × 14 GA Z) — the table lists max simple-span and max continuous-span lengths for each combination of load and deflection limit. Pick the lightest section whose allowable span ≥ the actual span. For preliminary sizing, the rule-of-thumb 8" depth handles 25 ft spans at 5 ft o.c. for typical office snow loads.

A purlin is a horizontal structural member that spans between rafters, columns, or trusses to support the roof decking, panels, or sheathing. In a metal building, purlins are typically cold-formed steel Z- or C-sections at 4–6 ft on-centre. In a heavy-timber roof, purlins are sawn or glulam beams. Their job: convert distributed roof loads into concentrated reactions at the primary frame.

What is the difference between a purlin and a rafter?

Rafters are the sloped primary roof members spanning from ridge to eave (or from column to column in a single-slope frame); they carry purlins and ultimately the roof load down to the supporting walls or columns. Purlins are the secondary horizontal members that span perpendicular to the rafters between two adjacent rafters, supporting the roof panels themselves. Rafters are deeper and more widely spaced; purlins are shallower and at 4–6 ft o.c.

What is the difference between a purlin and a girt?

Both are cold-formed C or Z secondary members; the only difference is orientation. Purlins are roof members oriented horizontally with the web vertical, supporting roof panels under gravity loads. Girts are wall members oriented horizontally with the web horizontal (laid on its side), supporting wall panels under lateral wind pressure and suction. Same product family — the manufacturer's tables list both spans.

How far apart should purlins be?

Standard metal-building spacing is 5 ft 0 in on-centre for 24 / 26 GA standing-seam roofing. Tighter spacing — 4 ft 0 in — is common in high-snow regions or when 22 GA panels need a smaller span. Wider spacing — 6 ft 0 in — is the practical maximum for most through-fastened panels. Sawn-lumber purlins on cathedral ceilings sit at 24 in o.c. to match standard rafter / joist layout.

How do I size a purlin?

Compute the design load: roof dead + live (or snow) + wind uplift × tributary spacing = w (plf). Then enter the manufacturer's allowable-span table for the target purlin section (e.g. 8" × 14 GA Z) — the table lists max simple-span and max continuous-span lengths for each combination of load and deflection limit. Pick the lightest section whose allowable span ≥ the actual span. For preliminary sizing, the rule-of-thumb 8" depth handles 25 ft spans at 5 ft o.c. for typical office snow loads.

Reference · Structural · MBMA · AISI S100 · Cold-formed steel

Purlins & Girts — Spans, Spacing & Section Reference

Purlins are horizontal roof members spanning between rafters; girts are the same shape rotated 90° on a wall. Both are cold-formed steel Z- or C-sections in pre-engineered metal buildings (PEMB) or sawn / engineered lumber in residential framing. This page covers spans, spacing, the C-vs-Z trade-off, the purlin-vs-rafter and purlin-vs-girt distinction, and a worked metal-building purlin example. Reviewed by a licensed PE.

Purlin calculator and spacing tool

For preliminary purlin selection, the embedded MBMA / manufacturer load tables below give allowable spans by gauge and depth. For full continuous-Z analysis with lap-splice continuity factors, use the AISI cold-formed steel design spreadsheet from the manufacturer (Cornerstone, Varco-Pruden, Nucor). The site truss solver computes reactions for the primary rafter / truss that carries the purlin loads; the moment-of-inertia tool gives I_x for any C / Z section if you need to check deflection from first principles.

→ Truss solver · → Moment of inertia (all shapes) · → ASCE 7 hazard tool (snow / wind loads)

Purlin design formulas

Eq. 01 — Distributed line load on a purlin SI

w = (D + L \text{ or } S) \cdot s_{purlin}

·: D = roof dead load (psf)
·: L or S = roof live or snow load (psf, take whichever is larger)
·: s_purlin = purlin spacing (ft) — converts psf → plf

Eq. 02 — Maximum bending moment (simple span) SI

M_{max} = \frac{w \, L^2}{8}

·: w = uniform line load (plf)
·: L = clear span (ft)
·: For continuous-span Z with lap, M_max ≈ wL²/12 at supports — manufacturer tables capture this

Eq. 03 — Bending stress check SI

\sigma = \frac{M \, c}{I_x} \leq F_b

·: M = bending moment from Eq. 02
·: c = distance from neutral axis to extreme fibre
·: I_x = section moment of inertia (Eurocode mm⁴, AISC in⁴)
·: F_b = allowable bending stress (cold-formed: 0.6·Fy ASD, φ·Fy LRFD)

Eq. 04 — Deflection (simple span) SI

\delta_{max} = \frac{5 \, w \, L^4}{384 \, E \, I_x}

·: L/240 limit for roof live load
·: L/180 limit for total load on roof
·: L/120 limit for wall girts under wind load

Standards governing purlin and girt design

Document	Scope
AISI S100-16 (R2020)	North American Specification for Cold-Formed Steel Structural Members — Z, C, hat, eave-strut sections
MBMA Metal Building Systems Manual (2018)	Industry-standard layout, loads, secondary framing details
ASCE 7-22	Loads — roof live, snow, wind pressure / suction on purlin tributaries
IBC 2021 §1604.3	Deflection limits — L/240 roof live, L/180 roof total, L/120 girt wind
AISI S200 series	Cold-formed steel framing standards (S202 — code of standard practice; S240 — light-frame)
ANSI/AWC NDS-2018	Sawn-lumber and glulam purlin design (residential and heavy-timber)
EN 1993-1-3 (Eurocode 3)	Cold-formed steel design — European C / Z purlin practice

Reference: typical Z-purlin allowable spans (MBMA)

Section	Gauge	I_x (in⁴)	Max simple span — 30 psf load (ft)	Max simple span — 50 psf load (ft)
6Z2.5	16 GA	5.1	17	13
8Z2.5	16 GA	11.4	22	17
8Z2.5	14 GA	13.4	25	19
10Z2.5	14 GA	23.6	30	23
10Z3	14 GA	26.0	32	25
12Z2.5	14 GA	40.5	34	27
12Z3.5	12 GA	56.4	40	32

Values assume 5 ft purlin spacing and L/180 deflection limit. For continuous Z with lap splice, allowable spans are roughly 25 % longer. Always confirm with manufacturer load tables for the specific gauge and finish.

Pick the roof or wall load Roof: dead (panel + purlin self-weight + insulation), live (20 psf reducible per IBC 1607.13.2), snow (per ASCE 7), wind uplift. Walls (girts): wind pressure / suction per ASCE 7 Chapter 30. Combine per ASCE 7 §2.3 (LRFD) or §2.4 (ASD).
Choose purlin spacing Drives both the purlin design load (= w_psf × tributary spacing) and the roof-panel span. Standard metal-building spacings: 4 ft 0 in (typical PEMB), 5 ft 0 in (most common), 6 ft 0 in (max for 22 GA standing-seam). Sawn-lumber purlins on residential cathedral ceilings sit at 24 in o.c.
Pick the purlin section Cold-formed steel C-section (channel) or Z-section (zee) — Z is preferred because adjacent purlins lap at supports for continuity. MBMA tables and manufacturer catalogues (Varco-Pruden, Nucor, Cornerstone) give allowable spans by gauge (12, 14, 16, 18 GA) and depth (8, 10, 12 in). Sawn-lumber 2×6 or 2×8 for residential.
Check span and deflection For continuous Z-purlins at typical spans use the manufacturer's simple-span value × 1.25 to capture the lap continuity, or compute per AISI S100. Live-load deflection limit L/240 (roof), L/180 for total load on roof; L/120 for wall girts under wind load.
Detail the connections and bracing Purlins bolt or weld to rafter / web flange clips with two A325 bolts per side. Add sag rods (5/8" diameter) at mid-span for spans > 25 ft to control deflection in the weak axis. Wall girts lap at columns and tie back with strut purlins at door / window jambs.

Worked example — 8Z14 purlin in a metal warehouse

A pre-engineered metal warehouse: 30 ft × 100 ft × 18 ft eave height in Denver. Roof: standing-seam 26 GA panels, R-19 batt insulation, snow load Pf = 30 psf, dead load 4 psf. Pick a Z-purlin at 5 ft on-centre.

Design load per purlin: w = (4 + 30) psf × 5 ft = 170 plf.
Span: 30 ft clear between rafter frames → continuous Z with lap.
Try 8Z2.5 × 14 GA: I_x = 13.4 in⁴; manufacturer table simple-span at 30 psf roof load = 25 ft, continuous-span ≈ 31 ft.
Compare: 30 ft actual ≤ 31 ft allowable continuous — accept.
Check deflection: Live-load deflection at L/180 = 30 × 12 / 180 = 2.0 in; manufacturer load chart confirms 8Z2.5 × 14 GA at 5 ft o.c. limits deflection to ~1.6 in at this load..
Specify: 8" × 2½" × 14 GA Z-purlin, 5 ft o.c., continuous over rafters with 2 ft lap each side.

Comparison — Z-purlin vs. C-purlin vs. wood purlin

Aspect	Z-section (cold-formed)	C-section (cold-formed)	Sawn-lumber
Continuity at supports	Excellent — laps in nesting orientation	Limited — symmetric flanges don\'t nest	Simple-span only
Span efficiency	~25 % longer with continuous lap	Baseline simple-span	Lowest (heavier per length)
Cost (per linear ft)	Lowest — high steel utilisation	Slightly higher	Variable; cheap in light residential
Eave detailing	Slightly trickier (Z-shape)	Simpler (channel face out)	Trim with metal flashing
Best for	Long-span PEMB roofs	Walls (girts), short spans	Residential, heavy timber

Variants and related queries

Girt and purlin — same product, different orientation

"Girt and purlin" is the umbrella phrase for cold-formed secondary framing in metal buildings. Purlins are roof members (web vertical, supporting roof panels under gravity); girts are wall members (web horizontal, supporting wall panels under wind pressure / suction). Manufacturer catalogues list both in the same load table — you read the column for the load case that applies.

Rafter and purlin — primary vs. secondary

Rafters are the sloped primary roof members carrying loads from ridge to eave (or column to column in monoslope frames). They are deeper and more heavily spaced — typical 25 ft on-centre in PEMB. Purlins span between adjacent rafters, perpendicular to them, supporting the actual roof panels. The hierarchy: panels → purlins → rafters → columns → foundations.

Strut purlin and eave-strut purlin

A strut purlin is a heavier purlin section (often a back-to-back C or hot-rolled angle) used at the eave or at locations where the roof framing must transfer in-plane horizontal load (wind drag, seismic) to the bracing system. The eave strut at the wall-roof interface combines roof purlin and wall girt functions — it resists both gravity and lateral loads.

Purlin spacing and roof panel span

Purlin spacing is governed by the allowable span of the roof panel: 5 ft 0 in for 24 / 26 GA standing-seam, 4 ft 0 in for 22 GA in high-snow zones, 2 ft 0 in for through-fastened R-panel residential roofs. The metal-building manufacturer typically picks spacing to balance panel span with purlin self-weight; the architect rarely overrides this.

Wall purlin / wall girt / lateral bracing

Girts on the wall serve the same load-transfer role as purlins on the roof but face different loads. Wind pressure and suction (ASCE 7 Chapter 30) on the wall panel transfers to the girts, then to the wind columns or rafter sidewalls. Long girt spans (> 25 ft) typically need a sag-rod or strut girt at mid-height to control deflection in the weak axis.

Frequently asked questions

What is a purlin?: A purlin is a horizontal structural member that spans between rafters, columns, or trusses to support the roof decking, panels, or sheathing. In a metal building, purlins are typically cold-formed steel Z- or C-sections at 4–6 ft on-centre. In a heavy-timber roof, purlins are sawn or glulam beams. Their job: convert distributed roof loads into concentrated reactions at the primary frame.
What is the difference between a purlin and a rafter?: Rafters are the sloped primary roof members spanning from ridge to eave (or from column to column in a single-slope frame); they carry purlins and ultimately the roof load down to the supporting walls or columns. Purlins are the secondary horizontal members that span perpendicular to the rafters between two adjacent rafters, supporting the roof panels themselves. Rafters are deeper and more widely spaced; purlins are shallower and at 4–6 ft o.c.
What is the difference between a purlin and a girt?: Both are cold-formed C or Z secondary members; the only difference is orientation. Purlins are roof members oriented horizontally with the web vertical, supporting roof panels under gravity loads. Girts are wall members oriented horizontally with the web horizontal (laid on its side), supporting wall panels under lateral wind pressure and suction. Same product family — the manufacturer's tables list both spans.
How far apart should purlins be?: Standard metal-building spacing is 5 ft 0 in on-centre for 24 / 26 GA standing-seam roofing. Tighter spacing — 4 ft 0 in — is common in high-snow regions or when 22 GA panels need a smaller span. Wider spacing — 6 ft 0 in — is the practical maximum for most through-fastened panels. Sawn-lumber purlins on cathedral ceilings sit at 24 in o.c. to match standard rafter / joist layout.
How do I size a purlin?: Compute the design load: roof dead + live (or snow) + wind uplift × tributary spacing = w (plf). Then enter the manufacturer's allowable-span table for the target purlin section (e.g. 8" × 14 GA Z) — the table lists max simple-span and max continuous-span lengths for each combination of load and deflection limit. Pick the lightest section whose allowable span ≥ the actual span. For preliminary sizing, the rule-of-thumb 8" depth handles 25 ft spans at 5 ft o.c. for typical office snow loads.

Historic source — purlin terminology

The principal beams of the roof rest upon the walls. Above them are placed the rafters (cantherii), which support the purlins (templa), and on these the roofing battens (asseres) carry the tiles. So the roof transfers its weight downward through this sequence of members.

Vitruvius — De Architectura → Book IV, Chapter 2, sect. 5 (c. 30 BC)

Related calculators and references

Sources and further reading

AISI S100-16 (R2020) — North American Specification for the Design of Cold-Formed Steel Structural Members.
MBMA — Metal Building Systems Manual, 2018 edition.
ASCE 7-22 — Chapters 7 (snow), 26–30 (wind), §1607 (live).
IBC 2021 §1604.3 and Table 1604.3 — deflection limits.
Yu, W-W.; LaBoube, R. A. Cold-Formed Steel Design, 5th edition. Wiley, 2020.
Vitruvius. De Architectura, Book IV. c. 30 BC — earliest written purlin terminology.