Roof Load
Free Roof Load calculator with clear step-by-step results.
About Roof Load
Converts covering and structure mass into a dead load, adjusted by the slope factor because a pitched roof has more surface than its plan area. Snow and maintenance loads are not additive - the greater of the two governs - and the result is expressed per square metre of plan, which is how rafters and trusses are checked.
Formulas
Dead load on plan
kN/m2 = (covering + structure) kg/m2 x 9.81 / 1000 / cos(pitch)
Design load
Design = dead load + greater of snow load and imposed load
Worked example
With the default values below, Roof Load returns Design load on plan of 1.506. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Roof plan areaminimum 0.5 m2
- 90 m2
- Roof covering massminimum 1 kg/m2
- 55 kg/m2
- Structure and insulation massminimum 0 kg/m2
- 25 kg/m2
- Design snow loadaccepts 0–10 kN/m2
- 0.6 kN/m2
- Imposed maintenance loadaccepts 0–5 kN/m2
- 0.6 kN/m2
- Roof pitchaccepts 0–70 degrees
- 30 degrees
Results
- Design load on plan
- 1.506 kN/m2
- Dead load
- 0.906 kN/m2
- Total load on the structure
- 135.56 kN
- Equivalent total mass
- 13.82 t
Step by step
- 1Values used
- 2Dead load on plan
- 3Design load
- 4Design load on plan= 1.506 kN/m2
- 5Dead load= 0.906 kN/m2
- 6Total load on the structure= 135.56 kN
- 7Equivalent total mass= 13.82 t
Assumptions and limits
- The result is derived only from the 6 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or compounding frequency into account.
- Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
- Structural roof loading must be assessed to the applicable design code with correct load factors, combinations and site-specific snow and wind data. This is an indicative check only, not a design.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Structural roof loading must be assessed to the applicable design code with correct load factors, combinations and site-specific snow and wind data. This is an indicative check only, not a design.
Frequently Asked Questions
Why take the greater of snow and imposed load rather than both?
Codes treat them as non-simultaneous: nobody is carrying out maintenance during the design snow event. Combining them would be unrealistically conservative.
What do common coverings weigh?
Roughly 20-25 kg/m2 for metal sheet, 45-55 for concrete interlocking tile, 35-45 for natural slate and 10-15 for bitumen shingle. Check the manufacturer's figure.