Inputs

Change any value — results update instantly.

Purlin span L

m

Clear span between frames

Purlin spacing s

m

Tributary width of one purlin

Roof dead load G

kN/m²

Sheet, insulation, fittings

Live / snow load Q

kN/m²

Use your local code value

Wind uplift Wu

kN/m²

Peak roof-zone pressure

Sheet thickness t

mm

Standard gauges only

Steel yield strength f

N/mm²

Q235 cold-formed, GB 50018

Young's modulus E

N/mm²

Steel — normally unchanged

Deflection limit

L /

200 for roofs, 150 for walls

Lightest passing section —

Allowable span—

Utilisation—

Uplift check—

Unit mass—

Preliminary sizing · simply supported single span · GB 50018 cold-formed steel

All 19 sections checked

Same inputs as above. Pass means the section carries both the gravity load and the wind uplift at your span.

Model	Type	kg/m	Wx cm³	Allowable span m	Utilisation	Check

Section properties

C and Z purlin section properties

All 19 sections we roll, with the exact values used in the calculation above. Mass is in kg per metre, Ix and Iy are the second moments of area, and Wx is the section modulus about the strong axis.

C purlin sections

Model	Height mm	Thickness mm	Mass kg/m	Ix cm⁴	Wx cm³	Iy cm⁴
C80	80	2.0	2.98	38.57	9.64	10.07
C80	80	2.5	3.73	47.60	11.90	12.65
C80	80	3.0	4.47	56.39	14.10	15.26
C100	100	2.0	3.77	76.51	15.30	20.06
C100	100	2.5	4.71	94.66	18.93	25.17
C100	100	3.0	5.65	112.42	22.48	30.32
C120	120	2.0	4.08	117.12	19.52	21.42
C120	120	2.5	5.10	145.20	24.20	26.89
C120	120	3.0	6.12	172.80	28.80	32.40
C140	140	2.0	4.40	168.14	24.02	22.58
C140	140	2.5	5.50	208.74	29.82	28.36
C140	140	3.0	6.59	248.78	35.54	34.19
C160	160	2.0	5.02	255.32	31.91	35.67
C160	160	2.5	6.28	317.29	39.66	44.77
C160	160	3.0	7.54	378.54	47.32	53.96
C180	180	2.0	5.34	336.25	37.36	37.05
C180	180	2.5	6.67	418.21	46.47	46.52
C180	180	3.0	8.01	499.34	55.48	56.07
C200	200	2.0	5.65	430.78	43.08	38.27
C200	200	2.5	7.07	536.12	53.61	48.06
C200	200	3.0	8.48	640.54	64.05	57.95
C220	220	2.0	6.28	587.24	53.39	56.21
C220	220	2.5	7.85	731.18	66.47	70.56
C220	220	3.0	9.42	873.97	79.45	85.04
C250	250	2.0	6.75	793.37	63.47	58.35
C250	250	2.5	8.44	988.43	79.07	73.27
C250	250	3.0	10.13	1182.18	94.57	88.32

Z purlin sections

Model	Height mm	Thickness mm	Mass kg/m	Ix cm⁴	Wx cm³	Iy cm⁴
Z100	100	2.0	3.85	78.86	15.46	42.55
Z100	100	2.5	4.81	97.57	19.13	53.55
Z100	100	3.0	5.77	115.88	22.73	64.71
Z120	120	2.0	4.16	120.54	19.72	42.56
Z120	120	2.5	5.20	149.43	24.45	53.57
Z120	120	3.0	6.24	177.84	29.10	64.73
Z140	140	2.0	4.47	172.82	24.27	42.57
Z140	140	2.5	5.59	214.54	30.13	53.58
Z140	140	3.0	6.71	255.69	35.91	64.74
Z150	150	2.0	4.99	228.34	29.89	69.54
Z150	150	2.5	6.24	283.62	37.13	87.46
Z150	150	3.0	7.49	338.19	44.27	105.61
Z160	160	2.0	5.15	265.17	32.56	69.55
Z160	160	2.5	6.44	329.53	40.46	87.47
Z160	160	3.0	7.72	393.12	48.27	105.62
Z180	180	2.0	5.46	348.76	38.10	69.56
Z180	180	2.5	6.83	433.76	47.39	87.48
Z180	180	3.0	8.20	517.89	56.58	105.64
Z200	200	2.0	5.78	446.27	43.92	69.56
Z200	200	2.5	7.22	555.39	54.66	87.50
Z200	200	3.0	8.67	663.54	65.31	105.66
Z250	250	2.0	6.91	823.87	65.03	103.74
Z250	250	2.5	8.63	1026.40	81.02	130.42
Z250	250	3.0	10.36	1227.56	96.90	157.40
Z280	280	2.0	7.38	1076.55	75.93	103.76
Z280	280	2.5	9.22	1341.81	94.65	130.43
Z280	280	3.0	11.07	1605.54	113.25	157.42
Z300	300	2.0	7.69	1268.33	83.54	103.76
Z300	300	2.5	9.62	1581.26	104.15	130.44
Z300	300	3.0	11.54	1892.53	124.66	157.43

Cold-formed from Q235 galvanised strip to GB 50018. Sections outside this table are quoted on request.

Method

How this is calculated

LoadsLine load q = (G + Q) × s and uplift qu = Wu × s, both in kN/m; the purlin self-weight is added to q for every section.

StrengthAllowable span from M = qL²/8 with the section modulus Wx, using f = 205 N/mm² for Q235 cold-formed steel.

DeflectionAllowable span from 5qL⁴/384EI ≤ L/limit, using the moment of inertia Ix.

UpliftRequired modulus Wreq = qu·L²/8/f is compared with Wx — it catches thin sections that pass on gravity but fail on wind suction.

ResultBoth spans are calculated and the smaller one governs; the recommendation is the lightest mass that passes both checks.

What this tool does not do

This is a preliminary sizing tool, not a structural design. It assumes a simply supported single span, uniform load and a straight purlin. It does not cover multi-span continuity, lapped or sleeved joints, local wind-pressure coefficients by roof zone, seismic action, suspended services, or crane loads. Final design must be verified by a structural engineer against the code that applies to your project.

Engineering Support

Need it engineered, not estimated?

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Purlin Span Calculator: C & Z Purlins by Span and Load

Inputs

All 19 sections checked

C and Z purlin section properties

C purlin sections

Z purlin sections

How this is calculated

What this tool does not do

Need it engineered, not estimated?

Request the calculation report