Free Online Tool
Roof Load Calculator
Calculate total roof structural load in psf — dead load, live load, and snow load with ASCE 7 slope factors. Includes rafter spacing guidance. Free, no install.
Roof Parameters
Shingles + decking + framing: typically 10–20 psf. Add solar panels: +3–5 psf.
Total Roof Load
47 psf
2×6 Rafter Spacing Guidance
12" or structural engineer required
Based on 24-ft span at 47 psf total load. Verify with local codes.
Pitch → Slope Factor (Cs)
Related Calculators
The Wet Snow Problem: Why Your Roof May Be More Dangerous Than You Think
Engineers design roofs for a maximum ground snow load, but the type of snow matters enormously. Fresh light powder weighs about 3–5 lbs per cubic foot. Wet, dense snow can hit 20–30 lbs per cubic foot.
A 12-inch accumulation of wet snow on a 1,000-sqft roof area can add 15,000–30,000 pounds — 15 tons of unexpected load. If your roof was designed for the minimum 20 psf, this can be 150–300% of its design capacity.
Ice dams compound the problem by trapping additional meltwater on the roof, which can add another 5–10 psf over the affected area. Signs of overloading: cracking sounds, sagging ridge beam, doors suddenly binding.
Solar Panels: The Load Addition Nobody Plans For
A standard residential solar installation (10 kW, ~30 panels) adds roughly 3–5 psf to the areas it covers. That sounds small, but you need to verify your roof can handle it — especially in snow regions where panels prevent snow from sliding off.
Roof load with solar in heavy snow region:
At 71 psf, a structural engineer review is mandatory before installation. Most reputable solar installers will request a structural letter — be suspicious of any installer who skips this step.
Frequently Asked Questions
How much weight can a roof hold per square foot?
Most residential roofs are designed for 40–50 psf total load (dead + live + snow). Never exceed your roof's design load — wet, heavy snow weighs up to 20 lbs/sqft and can quickly overload roofs in moderate snow regions.
What is the dead load of a typical roof?
Typical residential roof dead loads: asphalt shingles = 2–3 psf, sheathing = 2.3 psf, framing = 2–3 psf, insulation + ceiling = 3–5 psf. Total: typically 10–15 psf for standard construction. Solar panels add 3–5 psf.
How does roof pitch affect snow load?
Steeper roofs use a slope factor (Cs) that reduces snow load. Per ASCE 7: pitches up to 6/12 use Cs = 1.0. From 6/12 to 17/12 (~55°), Cs decreases linearly from 1.0 to 0. A 12/12 roof has Cs ≈ 0.57.
When do I need a structural engineer for roof load?
You need a structural engineer when total load exceeds 40–50 psf, you're in a heavy snow region (50+ psf), adding solar panels, cutting rafters for vaulted ceilings, or spanning over 24 feet.
What is the difference between dead load, live load, and snow load on a roof?
Dead load is permanent weight of materials. Live load is temporary maintenance load (IBC requires 20 psf minimum). Snow load is geographic design load based on pitch and region. Total = dead + live + snow. Wind uplift and seismic loads require additional engineering analysis.
Disclaimer
This calculator provides estimates based on ASCE 7 and IBC guidelines. It is not a substitute for professional structural engineering. Always consult a licensed structural engineer before making structural modifications or adding significant loads to your roof.