Calculate roof snow load to EN 1991-1-3: shape coefficient by pitch, exposure and thermal factors. Try the free calculator.
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About this EN 1991-1-3 Snow Load on a Roof Calculator
This calculator determines the persistent and transient design snow load on a monopitch roof to EN 1991-1-3 Clause 5.2. It selects the shape coefficient for the roof pitch from Clause 5.3.1 and Table 5.2, applies the exposure and thermal coefficients, and reports the load on the horizontal projection of the roof together with the line load over a tributary width. A chart plots the shape coefficient and the roof load across the full range of pitch, with the entered pitch marked.
- Structural engineer. Set up roof loading by entering the ground snow load, pitch and coefficients, and read the design load straight away.
- Design reviewer. See which pitch band the shape coefficient came from, which is the most common source of error in this calculation.
- Engineer comparing options. Read off the chart how steepening a roof reduces the retained snow before committing to a geometry.
Every coefficient is shown with the clause behind it and units are carried through the calculation. It is an engineering-grade calculator you can audit, adapt and save to a project page in CalcTree.
More info on EN 1991-1-3 Snow Load on a Roof
Inputs
The characteristic ground snow load for the site, the roof pitch, the exposure and thermal coefficients, and the tributary width over which the line load is wanted. The ground snow load must come from the National Annex for the country, zone and altitude.
Shape coefficient
For a monopitch roof the shape coefficient is constant across shallow pitches, reduces linearly through the middle band, and falls to zero for steep roofs where snow no longer stays on the surface. The calculator selects the band from the pitch entered, and the chart makes the transitions visible.
Exposure and thermal coefficients
The exposure coefficient reflects whether wind removes snow from the roof or the site is sheltered, with values given in Table 5.1. The thermal coefficient allows a reduction where high heat loss through the roof melts snow, and is otherwise unity.
Outputs
The snow load acting on the horizontal projection of the roof, and the equivalent line load over the tributary width entered. Both carry their units so they can be taken directly into a frame analysis.
Common Calculation Errors to Avoid
- Reading the load as acting on the slope. The snow load acts on the horizontal projection of the roof area. Applying it along the rafter length overstates the total load.
- Guessing the ground snow load. It is a mapped value that depends on country, zone and altitude, and it must come from the National Annex rather than being interpolated from a neighbouring region.
- Leaving the exposure coefficient at unity by default. A windswept site and a sheltered site sit either side of the normal value, and choosing deliberately can change the load materially.
- Missing the pitch band transition. The shape coefficient changes behaviour at the band boundaries, and a roof close to a boundary is worth checking either side.
- Treating this as the only snow case. This is the undrifted case. Drifted arrangements are separate and frequently govern, particularly against obstructions and at changes of roof level.
- Applying a monopitch coefficient to another roof form. Pitched, multi-span and cylindrical roofs each have their own shape coefficients under Clauses 5.3.2 to 5.3.6.
Engineering templates
Common calculators
Design guides
FAQs
Where does the ground snow load come from?
From the National Annex for the country, using the zone map and the altitude of the site. It is not something this calculation can derive, and it is the single input that most affects the result.
How does the shape coefficient vary with pitch?
It is constant for shallow roofs, reduces linearly through the middle band of pitches, and is zero for steep roofs where snow slides off. The chart on the page shows the whole relationship with the entered pitch marked.
What exposure coefficient should I use?
Table 5.1 gives values for windswept, normal and sheltered topography. Choose from the actual site conditions rather than defaulting to normal, since a sheltered site attracts a higher load.
Does this cover snow drifting?
No. This is the undrifted load case for a monopitch roof. Drifted arrangements, local effects at parapets and obstructions, and snow overhanging an eave are covered by separate clauses and must be checked as well.
Does the load act on the slope or the plan area?
On the horizontal projection. This is a common source of error, because wind and imposed roof loads are not all treated the same way.
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