Is this a substitute for a structural engineer or my local snow-load map?

No. This tool is for preliminary planning and estimating only. For anything load-bearing, get your official ground snow load from your local code's map and have the final roof design confirmed by a licensed structural engineer before you build.

What's the difference between ground snow load and roof snow load?

Ground snow load (pg) is the baseline figure for a site from an official code map. Roof snow load (pf or ps) is what actually accumulates on the roof after adjusting for exposure, thermal condition, building importance/risk category, and roof slope -- it is usually lower than the ground figure once slope reduction applies.

Does this calculator use the UK National Annex snow-load map?

Not automatically -- you look up your site's characteristic ground snow load from the UK National Annex map yourself and enter it. This calculator's code-based mode is not yet populated with the UK's own factor tables, so always confirm your final figure against BS EN 1991-1-3 or a structural engineer before you build.

Snow Load Calculator — Eurocode 1 UK National Annex (UK)

Snow Load Calculator: Ground-to-Roof Snow Load Estimate

This calculator gives a preliminary estimate of the load a roof carries from snow -- either from a simple snow depth and density, or from a code-based ground-to-roof conversion using exposure, thermal and importance factors plus a roof-slope reduction. It's built for early-stage planning and rough pass/fail checks against a stated roof capacity, not as a substitute for your local building code's official ground-snow-load maps or a licensed structural engineer's sign-off before you build.

How to Use the Snow Load Calculator

Choose a computation mode: simple depth x density, or code-based ground-to-roof conversion.
In code-based mode, select the design standard for your region (ASCE 7, NBCC, Eurocode 1, or AS/NZS 1170.3).
Enter the roof length, width and pitch (slope).
In simple mode, enter the snow depth and pick a snow-type/density preset (fresh, settled, wind-packed, ice, etc.).
In code-based mode, enter your code-tabulated ground snow load, then set exposure, thermal condition and risk/importance category.
Optionally enter the roof's stated load capacity to get a pass/fail utilization check.

The Math Behind the Calculation

Simple path: snow load (pressure) = depth x density x gravity (9.80665 m/s2). Worked example: 0.3 m of settled snow (250 kg/m3) on a 10 m x 8 m roof pitched at 20 degrees gives a snow load of about 736 Pa (0.74 kN/m2), and a total snow weight on the roof -- adjusted for the sloped surface area -- of about 6,386 kg.

Code-based path: flat-roof snow load pf = 0.7 x exposure factor (Ce) x thermal factor (Ct) x importance factor (Is) x ground snow load (pg), then reduced further by a slope factor for pitched roofs. The ground snow load pg always comes from your own local code's official map or table -- it is never invented by this calculator. This exact factor set is only implemented for ASCE 7 (US) today; NBCC, Eurocode 1 and AS/NZS 1170.3 are selectable but use the same generic calculation as a stand-in, since each code's own factor tables are not yet populated, so always confirm a code-based result against your official national standard.

UK Design Standard

UK roof snow loads are governed by Eurocode 1 (BS EN 1991-1-3) with the UK National Annex and its snow-load map -- selectable in this calculator's code-based mode, though only ASCE 7 (US) currently computes with a fully populated factor table here, so cross-check any code-based result against BS EN 1991-1-3 and the National Annex map directly.

Pro Tips

A steeper roof pitch reduces retained snow load, since snow sheds more easily -- the slope factor in this calculator lowers the sloped-roof result below the flat-roof value as pitch increases.

A sheltered roof (surrounded by trees or taller buildings) traps more snow than a fully exposed, wind-swept one -- the exposure factor in code-based mode accounts for this.

A structural engineer's assessment of a roof's snow-load capacity in the UK typically costs GBP 250-950 for a single-property review, depending on roof complexity and access.