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The site's ground snow load, from your local building code's ground snow load map.
From the map for the site, not for the nearest city — the two differ substantially in mountainous country, which is exactly why large parts of the western United States are marked as case study areas where the map declines to give a value at all. In those regions the figure comes from a site-specific study or the local building department, and a number interpolated off the map's edge is not a design value.
Accounts for wind exposure and terrain, which affects how much snow accumulates on the roof versus blows off.
It runs from about 0.7 for a genuinely windswept roof in open country to about 1.2 for one sheltered on all sides, and it depends on the roof rather than on the region: a roof surrounded by taller buildings or dense conifers keeps its snow because there is no wind to move it. The wind can only reduce the load where it can actually reach the roof, so the sheltered case is the one to assume unless the exposure is demonstrable.
Accounts for heat loss through the roof that can melt accumulated snow.
A normally heated building gets 1.0; structures kept just above freezing, unheated structures and freezer buildings take progressively higher values, because the snow on them does not melt away. The counter-intuitive part is that better insulation makes this WORSE — a well-insulated roof loses less heat, so it keeps more snow, and the same detail that reduces the heating bill increases the load the structure has to carry.
Based on the building's risk category, per ASCE 7.
It scales the load with the consequence of failure: a structure representing low hazard to human life takes less, ordinary buildings 1.0, and buildings that must remain operational after a storm — hospitals, fire stations, emergency shelters — take more. It is a property of what the building is FOR rather than of how it is built, so it is decided at the start of a project and applies to the snow, wind and seismic cases alike.
Flat-roof design snow load
21.93 psf
They open the calculator with your figures already in it
Ground-to-Roof Snow Load Calculator: 21.93 psf — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- ASCE 7 flat-roof snow load: pf = 0.7 × Ce × Ct × Is × pg, where Ce is exposure factor, Ct is thermal factor, Is is importance factor, and pg is ground snow load
Inputs used
- Ground Snow Load pg
- 31.33 psf
- Exposure Factor Ce
- 1
- Thermal Factor Ct
- 1
- Importance Factor Is
- 1
What this calculation does not cover
- The page returns pf from the equation alone, and ASCE 7 also sets a minimum roof snow load pm that governs instead on low-slope roofs: where the ground snow load is 20 psf (about 0.96 kPa) or less, pm is Is times pg, and above that it is 20 psf times Is. No floor is applied here, so the page's default pg of 1.5 kPa with an exposure factor of 0.9 returns 0.945 kPa, which is 19.7 psf and already below that 20 psf minimum.
- This is the balanced load spread evenly over the whole roof. Drift against a parapet, a roof step or a rooftop unit is a separate ASCE 7 calculation that puts a triangular surcharge over a limited width and can reach several times pf, so a rafter or beam sized on this figure can still be badly under-designed where the snow actually piles up. Sliding snow off an upper roof, and the rain-on-snow surcharge of about 5 psf (0.24 kPa) that applies to low-slope roofs where the ground snow load is 20 psf or less, are added on top of pf rather than contained in it.
- pf is a pressure on the horizontal projection of the roof, not on the sloped surface. Applying it to the true roof area counts the pitch twice: at 6:12 the sloped surface is 11.8 per cent larger than the plan area, so the snow total comes out that much high. Dead loads run the other way and do act on the sloped area, which is why the two are never taken over the same area.
- Ce, Ct and Is are multiplied exactly as typed and are never checked against the tables they come from. Entering Ce as 0.7 rather than 1.0 removes 30 per cent of the load on the strength of a wind exposure that has to hold for the life of the building, and conifers grow, a taller neighbour goes up, a screen wall gets added. The field also accepts Ce up to 1.3, beyond the 1.2 top of the ASCE 7 exposure table that its own help text describes, without objecting.
- The form used here, with a separate importance factor, is the pre-2022 one. ASCE 7-22 rebuilt the snow chapter around reliability-targeted ground snow loads mapped for each risk category and dropped Is from the flat-roof equation, so a pg read off those newer maps and then multiplied by Is of 1.2 applies the risk category twice and overstates pf by 20 per cent. Which edition your jurisdiction has adopted decides which of the two pg values belongs in the box.
Add the equipment this sizes
This result is a specification — 21.93 psf — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.1.1
Regulatory standards & verification citations1
- ASCE 7 flat-roof snow load: pf = 0.7 × Ce × Ct × Is × pg, where Ce is exposure factor, Ct is thermal factor, Is is importance factor, and pg is ground snow load
Which documents these citations point at
Standards referenced: ASCE 7 (American Society of Civil Engineers, United States).
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