Preliminary screen for steel roof purlin span using purlin spacing, roof loads, section properties, bending and deflection limits.
Purlin Geometry
Roof Loads
Area loads are converted to line loads using the entered purlin spacing. Wind uplift is screened separately from gravity loading.
Purlin Section Properties
Enter the actual section modulus and moment of inertia for the purlin section being screened. The bending stress factor is user-selected and should reflect the applicable design approach.
Ready to screen the purlin span.
Purlin Span Result
Preliminary Governing Span
—
Entered Span
—
screened
Bending Span
—
limit
Deflection Span
—
limit
Bending Utilization
—
screening
Deflection
—
at entered span
Load and Response Summary
Gravity Loading
Roof dead load—
Roof live load—
Additional load—
Purlin self-weight—
Total gravity line load—
Uplift Loading
Wind uplift line load—
Dead-load stabilizing line load—
Net uplift line load—
Gravity moment—
Uplift moment—
Detailed Purlin Screening
Check
Demand
Limit
Utilization
Status
Steel Roof Purlin Span Preview
Distributed roof loading
Simply supported preliminary span
The diagram represents a simplified simply supported purlin. Real roof purlins may have continuous spans, bridging, lap connections, restraints and other conditions.
Gravity Line Load
The roof area loads are converted into a line load on the purlin using the purlin spacing.
w = q × s + wp
Where q is the roof area load, s is purlin spacing and wp is purlin self-weight.
Simply Supported Bending Check
For a uniformly distributed load on a simply supported beam, the preliminary maximum bending moment is estimated as:
Mmax = wL² / 8
The corresponding bending stress is estimated from the section modulus:
fb = M / Z
The calculator compares this stress with the user-selected bending stress limit.
Deflection Check
For a simply supported beam under a uniformly distributed load, the preliminary maximum deflection is:
δ = 5wL⁴ / 384EI
The allowable deflection is calculated from the selected span ratio.
δallow = L / Limit Ratio
Preliminary Maximum Span
The calculator estimates separate span limits from bending and deflection and uses the smaller result as the preliminary governing span.
Lbending = √(8Mallow / w)
Ldeflection =
[384EI / (5w × Limit Ratio)]1/3
The governing preliminary span is the smaller of these two screening values.
Wind Uplift Screening
Wind uplift is converted into an upward line load using purlin spacing. Purlin self-weight and entered roof dead load are treated as stabilizing downward load in this separate screening.
wuplift =
qwinds − qdeads − wp
If the result is positive, the simplified model has a net upward line load.
Important Limitations
This is a preliminary steel roof purlin span screening tool.
It assumes a simply supported beam with a uniform load for the basic calculations.
It does not perform a complete cold-formed steel or hot-rolled steel member design.
It does not calculate lateral-torsional buckling.
It does not calculate local, distortional or global buckling.
It does not design bridging or sag rods.
It does not account for continuous-span behavior.
It does not account for purlin laps or sleeve connections.
It does not calculate web crippling or bearing resistance.
It does not calculate fastener capacity.
It does not calculate roof-sheet restraint.
It does not determine the correct section size.
It does not automatically select code-specific load combinations.
It does not account for snow drift, concentrated loads or complex roof geometry.
Actual wind pressure and load direction must come from the applicable project design criteria.
Engineering warning: This calculator is for preliminary span screening only. Final purlin design requires the actual section, restraint conditions, load combinations, buckling checks, connections and applicable structural design standard.
Frequently Asked Questions
What does the Steel Roof Purlin Span Calculator calculate?
It screens a purlin span using roof loads, purlin spacing, section modulus, moment of inertia, bending limits and deflection limits.
Can it estimate a maximum span?
Yes. It calculates preliminary bending-controlled and deflection-controlled span limits and uses the smaller value.
What section properties are required?
The calculator uses section modulus Z and moment of inertia I for the purlin section being screened.
Does it include purlin self-weight?
Yes. An optional purlin self-weight in kg/m is converted to a line load and included in the gravity and uplift screening.
Does it check wind uplift?
Yes. It converts the entered wind uplift pressure into a line load and subtracts the entered stabilizing dead load and purlin self-weight.
Can this be used for final purlin design?
No. It is a preliminary screening tool. Final design requires complete member, restraint, connection and code checks.