Snow Load Calculator

About Snow Load Calculator

Snow Load Calculator computes the balanced roof snow load using ASCE/SEI 7 Chapter 7 — the standard American structural engineers actually use. Enter the ground snow load, pick the exposure, thermal and importance factors, set the roof pitch and surface, and read the design load in psf and kN/m2. It works offline and nothing you enter ever leaves your phone.

This is an educational estimate, not a substitute for a licensed structural engineer.

HOW IT WORKS
Roof snow load is a chain, not a single multiplication:

The ground snow load pg is the starting point — an input you look up, never something an app can guess. Take it from the ASCE 7 map, the ASCE Hazard Tool for your address, or your local building department. Much of the mountain West is a case-study area where only the local authority can supply pg.
Flat roof load follows ASCE 7 Eq. 7.3-1: pf = 0.7 x Ce x Ct x Is x pg. The 0.7 accounts for wind scour and melting. Ce is the exposure factor (Table 7.3-1) from terrain and wind. Ct is the thermal factor (Table 7.3-2), from 0.85 for a heated greenhouse to 1.3 for a freezer. Is is the importance factor (Table 1.5-2), 0.8 for a farm shed and 1.2 for a hospital.
Sloped roof load is ps = Cs x pf. The slope factor Cs (Fig. 7.4-1) holds at 1.0 until the roof is steep enough to shed, then falls to zero at 70 degrees. A slippery unobstructed roof (standing-seam metal, slate) starts shedding at 5 degrees on a warm roof; shingles, gravel or snow guards hold to 30 degrees. Cold roofs hold longer.
Two provisions catch the flat end. Below 15 degrees a minimum load case pm applies (Section 7.3.4). Where pg is 20 psf or less on a nearly flat roof, Section 7.10 adds a 5 psf rain-on-snow surcharge.
Separately, the app converts a measured snow depth into a load using the snow density — fresh, damp, settled, wind-packed, very wet or ice. Eighteen inches of settled snow weighs about 23 psf; very wet snow at the same depth weighs about 70 psf.

Worked example: with pg = 35 psf and Ce = Ct = Is = 1.0, pf = 0.7 x 35 = 24.50 psf. A 6:12 shingle roof is 26.6 degrees, below the 30 degree threshold, so Cs = 1.00 and ps = 24.50 psf (1.17 kN/m2). On a 16 ft by 40 ft two-slope roof — 1,280 sq ft — that carries 31,360 lb, or 15.68 tons.

WHAT YOU CAN DO

Enter pg and see it in psf, kN/m2 and kg/m2
Choose terrain and exposure and watch Ce change
Pick the thermal condition and risk category for Ct and Is
Set the roof pitch and choose a slippery or obstructed surface for Cs
See the minimum low-slope load and rain-on-snow surcharge applied automatically
Convert a measured depth plus condition into the load on the roof now, as a percentage of the design load
Open Details for a full breakdown, the assumptions applied, Table 7.3-1 and the drift formulas

AN ESTIMATE, NOT AN ENGINEERING CERTIFICATION
This app is for education only and is NOT a substitute for a licensed structural engineer. It tells you what the code says the snow may weigh; it tells you nothing about what your roof can carry. It computes the BALANCED load case only. Drift, sliding, unbalanced, partial and ponding loads are not calculated — and drift is what actually collapses roofs, piling at two or three times the balanced load. The ground snow load is jurisdiction-specific: your local department's value governs, and in many mountain regions a case study is required by law. Clearing snow off a roof is itself hazardous. If you believe a roof is overloaded, leave the building and contact your local building department or a licensed structural engineer.

Sources: ASCE/SEI 7 Chapter 7 "Snow Loads" — Eq. 7.3-1, Tables 7.3-1, 7.3-2, 1.5-2, Fig. 7.4-1, Sections 7.3.4, 7.6-7.8, 7.10, Eq. 7.7-1.

PRIVACY
The app collects no personal data and works fully offline. Ads are served by Google AdMob, which may use an advertising identifier as described in Google's policies.

Download Snow Load Calculator to see how a code snow load is built — then take it to an engineer.