⌂ RoofingCalculatorHQ

Snow Load Calculator

Calculate roof snow load to BS EN 1991-1-3 UK National Annex. Returns characteristic snow load on the ground (sk), shape coefficient (μ1), exposure (Ce) and thermal (Ct) factors, and design roof snow load (s).

Snow Load Calculator (BS EN 1991-1-3 NA)

Calculate design snow load on a pitched roof from ground snow, exposure, thermal coefficient and pitch — to BS EN 1991-1-3 UK National Annex.

Default: Birmingham — sk = 0.50 kN/m²

Roof snow load (s)
0.4 kN/m²
Characteristic ground load: 0.5 kN/m²
EN 1991-1-3 · Formula: s = μ1 × Ce × Ct × sk
mu1
0.8
Ce
1
Ct
1
Formula: BS EN 1991-1-3 NA / Approved Doc A

What this calculator does

This tool computes the design snow load on a pitched roof to BS EN 1991-1-3 UK National Annex and Approved Document A. It returns the characteristic snow load on the ground (sk) for your site, the shape coefficient (μ1) based on roof pitch, the exposure factor (Ce), the thermal coefficient (Ct), and the design roof snow load (s) in kN/m² ready for structural-engineer use.

Enter the characteristic ground snow load sk from BS EN 1991-1-3 NA figure NA.1 and the §NA.2.8 altitude correction. Pick the roof pitch in degrees, the topographic exposure, and the thermal class. The calculator returns s in kN/m² directly usable in steel, timber, and tile-batten design.

How the snow-load math works

BS EN 1991-1-3 §5.2 applies a single equation to convert ground snow to roof snow:

s = μ1 × Ce × Ct × sk

Where:

  • sk is the characteristic snow load on the ground at the site, from figure NA.1 plus the §NA.2.8 altitude correction.
  • μ1 is the shape coefficient — 0.8 for slopes up to 30°, then linear to 0 at 60°.
  • Ce is the exposure coefficient — 0.8 windswept, 1.0 normal, 1.2 sheltered (NA Tbl NA.4).
  • Ct is the thermal coefficient — 1.0 for standard buildings, lower values only permitted for high-temperature roofs with documented thermal calculations.

The result s is the uniformly distributed snow load on the projected horizontal area of the roof.

Reference test cases

Locationsk (NA.1)PitchCeCtμ1s
Birmingham, 100 m AOD0.50 kN/m²20°1.01.00.80.40 kN/m²
Edinburgh, 80 m AOD0.60 kN/m²30°1.01.00.80.48 kN/m²
Glasgow, 50 m AOD0.65 kN/m²22.5°1.01.00.80.52 kN/m²
Manchester, 50 m AOD0.50 kN/m²35°1.01.00.670.34 kN/m²
Aberdeen, 35 m AOD0.65 kN/m²45°1.01.00.400.26 kN/m²
Aviemore, 220 m AOD0.95 kN/m²30°1.01.00.80.76 kN/m²

Each row reproduces what the calculator returns for the inputs in the leftmost columns.

Characteristic ground snow load (sk) — where to find it

BS EN 1991-1-3 NA figure NA.1 maps the UK into five zones at sea level. Figure values:

  • Zone 1 — 0.30 kN/m². South coast from Cornwall to Kent.
  • Zone 2 — 0.40 kN/m². South Midlands and Welsh borders.
  • Zone 3 — 0.50 kN/m². Central England and northern Wales.
  • Zone 4 — 0.60 kN/m². North England, southern Scotland, Northern Ireland.
  • Zone 5 — 0.70 kN/m². Central and northern Scotland.

For sites above 100 m AOD, apply the altitude correction:

sk(A) = sk + 0.1 × A/100 + 0.05 (kN/m²)

Where A is the site altitude in metres above ordnance datum. The correction applies separately to each zone — a 250 m site in zone 3 gives sk = 0.50 + 0.25 + 0.05 = 0.80 kN/m².

The calculator accepts the corrected sk directly; pull the value from the NA before you enter it. The Met Office and most chartered engineering practices keep tabulated zone-and-altitude reference sheets.

Shape coefficient (μ1) — pitch dependence

BS EN 1991-1-3 §5.3.2 sets μ1 for monopitch and duopitch roofs:

  • 0° to 30° — μ1 = 0.8.
  • 30° to 60° — linear from 0.8 to 0, equation μ1 = 0.8 × (60 − α)/30.
  • 60° and above — μ1 = 0.

The 30°-to-60° linear reduction reflects empirical UK measurements: at 35° an asphalt-shingle or natural-slate roof retains about 67 percent of the maximum balanced snow before sliding; at 45° about 40 percent; at 55° about 13 percent.

If the roof has snow boards, snow fences, or other obstructions that prevent sliding (common on grade I listed building slate to comply with Historic England guidance), hold μ1 at 0.8 regardless of pitch. The calculator applies the standard sliding curve; override manually if obstructions are present.

Exposure coefficient (Ce) — site topography

NA Tbl NA.4 gives:

  • Windswept (Ce = 0.8). Flat unobstructed terrain — coastal sites, ridges, exposed hilltops. Wind reliably blows snow off the roof before it can accumulate to maximum.
  • Normal (Ce = 1.0). Suburban and urban sites with neighbouring buildings or trees providing partial shielding but not full enclosure.
  • Sheltered (Ce = 1.2). Dense conifer plantation, urban courtyards surrounded by tall buildings, secondary roofs lower than nearby tall structures.

Most UK residential and commercial design uses Ce = 1.0. The windswept reduction is conservative to claim and requires documented site assessment by the engineer.

Thermal coefficient (Ct) — usually 1.0

The NA permits Ct < 1.0 only for high-temperature roofs with documented heat-transfer calculations. For all standard buildings, Ct = 1.0. The calculator offers cold-ventilated (Ct = 1.0 in BS practice — no penalty) and unheated (Ct = 1.0) options for consistency with the ASCE method but applies Ct = 1.0 for the actual UK calculation.

When the design needs more than this calculator

BS EN 1991-1-3 also requires consideration of:

  1. Drift loads (§5.3.3 – §5.3.6). Drift surcharges at roof steps, valleys, abutments, parapets, and behind rooftop equipment. Drift analysis is mandatory for any L-shape, T-shape, or stepped roof. The drift surcharge formula uses ls = 2 × b1 (drift length) and μ1 = 0.8 + 0.6 × b1/h (drift coefficient).
  2. Local effects on overhangs (§6.3). Cornice loads at the eave, applied as a horizontal line load.
  3. Snow loads on snow guards (§6.4). The guard must be designed for the down-slope component of the snow above it, with safety factor 2.0.
  4. Exceptional snow loads (Annex B). Mid-Scotland sites should consider exceptional events with return periods longer than 50 years.

For any of these, refer to a chartered structural engineer (CEng MIStructE).

Approved Document A and Building Regulations

Approved Document A §1B(d) requires structural design to satisfy BS EN 1991-1-3. For a single-family house designed to standard methods (timber roof to BS 5268 or BS EN 1995-1-1, masonry walls to BS 5628 or BS EN 1996-1-1), the snow-load calculation is part of the structural design statement filed with the Building Regulations submission. LABC and approved-inspector schemes will check the calculation against figure NA.1 and the relevant altitude correction.

For Permitted Development extensions and conversions, the structural calculation must still be done — Permitted Development relieves planning permission, not Building Regulations.

Comparing to NHBC and BBA

NHBC Standards chapter 7.2 references BS EN 1991-1-3 directly for new-build housing covered by NHBC warranty. NHBC inspectors will check that the design statement uses figure NA.1 and the altitude correction.

BBA Agrément certificates for engineered roof systems (e.g. metal cladding, structural insulated panels) typically state the maximum sk the system has been tested to. For sites where the design sk exceeds the BBA limit, the system must be over-engineered or replaced with one tested to a higher load.

TRADA timber-frame guidance treats sk values up to 1.0 kN/m² as standard for UK timber-frame design; sites above need bespoke analysis.

Frequently asked questions

What is the typical roof snow load in the UK?
Design snow loads in the UK are modest by continental European standards. BS EN 1991-1-3 NA figure NA.1 gives sk values from about 0.30 kN/m² in coastal Cornwall and Pembrokeshire to over 1.20 kN/m² in the Cairngorms. For Birmingham at sea-level (sk = 0.50 kN/m²) on a 20° pitched roof with normal exposure, the design roof snow load (s) works out to about 0.40 kN/m². Glasgow at 50 m elevation (sk ≈ 0.65) on the same roof gives s ≈ 0.52 kN/m². Site altitude matters more than UK latitude — the NA gives an explicit altitude correction equation in §NA.2.8.
How do I read BS EN 1991-1-3 figure NA.1?
Figure NA.1 maps the UK into five characteristic ground snow load zones at sea level: zone 1 (0.30 kN/m², south coast), zone 2 (0.40 kN/m², south Midlands), zone 3 (0.50 kN/m², central England), zone 4 (0.60 kN/m², north England and southern Scotland), and zone 5 (0.70 kN/m², central and northern Scotland). The NA then adds an altitude correction sk(A) = sk + 0.1A/100 + 0.05 (kN/m²) for sites above 100 m AOD, where A is the site altitude in metres. A site at 400 m AOD in zone 3 carries an sk of 0.50 + 0.40 + 0.05 = 0.95 kN/m². Highland sites above 600 m can reach sk values of 1.5 kN/m² or more.
What does the shape coefficient μ1 do?
Per BS EN 1991-1-3 §5.3.2, μ1 reduces the characteristic ground snow load when the roof slope is steep enough that snow slides before reaching maximum accumulation. For a duopitch or monopitch roof, μ1 = 0.8 for slopes up to 30°, then linear from 0.8 at 30° to 0 at 60°. Above 60°, the design assumes no snow accumulates. The breakpoint is steeper than the ASCE warm-roof curve because the UK NA accounts more aggressively for snow sliding off slate and tile. The calculator applies the standard μ1 curve; for snow-fenced or obstructed roofs, hold μ1 at 0.8 regardless of pitch.
Do I need to worry about drift loads on a typical UK house?
Yes for any house with a roof step, valley, parapet, or rear extension lower than the main roof. BS EN 1991-1-3 §5.3.3 to §5.3.6 require drift analysis at all of these geometries. The drift surcharge can easily quadruple the local snow load on the affected band, and it is the single most common cause of partial roof failures in UK heavy-snow events (notably 2010, 2018, and 2024). For a simple gable house with no extensions or roof equipment, the calculator output is sufficient. For any L-shape, T-shape, or stepped roof, refer the design to a chartered engineer with the §5.3.6 drift equations.
Which exposure category should I pick?
BS EN 1991-1-3 NA Tbl NA.4 sets Ce based on topography. Windswept (Ce = 0.8) applies to flat unobstructed terrain such as coastal sites and exposed hilltops where wind reliably scours snow off roofs. Normal (Ce = 1.0) covers most suburban and urban sites where neighbouring buildings provide partial shielding but not full enclosure. Sheltered (Ce = 1.2) applies to dense conifer plantations, urban courtyards surrounded by taller buildings, and roofs lower than upwind features within 10 building heights. Most UK residential design uses Ce = 1.0; the windswept reduction is reserved for sites where you can document the site exposure. The default is to use Ce = 1.0 unless site exposure justifies otherwise.
Does the calculator work for listed buildings and conservation areas?
The calculator handles the structural design load, which is determined by climate and site geometry — these are independent of listed status. Where listed buildings differ is in detailing: snow guards, snow boards, and snow-fences are normally required by Historic England guidance for grade I and II* slate or pantile roofs near pavements and pedestrian access, regardless of slope. The structural load itself is identical to a non-listed building of the same geometry. For listed building consent applications involving roof works, the structural calculation should be in the engineer's design statement and the Historic England Adviser will check it against BS EN 1991-1-3 NA. The calculator output is the right starting point for that calculation.
When is rain-on-snow important in the UK?
Rain-on-snow surcharges are not currently a separate clause in BS EN 1991-1-3 NA, unlike ASCE 7-22 §7.10. UK practice instead relies on a higher characteristic value at low altitudes. The NA-derived sk already reflects the wettest snow events. However, low-pitch industrial and warehouse roofs in mid-Wales and the Pennines should be checked for ponding-induced increase in load via the rain water clause in BS EN 1991-1-1. For pitched residential roofs above 7°, ponding is not a factor.
How does the UK snow load compare to other European countries?
UK snow loads are the lowest in Europe outside Mediterranean coastal Spain and Portugal. Typical UK sk runs 0.30–0.60 kN/m² at sea level. Compare to Germany sk = 0.65–2.55 kN/m² (Schneelastzonen 1–3), France 0.45–1.40 kN/m² (regions A1 to E), Italy 1.00–2.50 kN/m² (zones I–III), and the Netherlands a flat 0.70 kN/m² nationally. Even Highland Scotland is roughly equal to lowland Germany. The mild oceanic climate keeps UK snow events brief — a UK 50-year return-period snow load is typically half what mainland Europe sees at the same latitude.

Related calculators