Free Roof Load calculator with clear step-by-step results.
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.
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
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.
Codes treat them as non-simultaneous: nobody is carrying out maintenance during the design snow event. Combining them would be unrealistically conservative.
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.