COP:roof-drainage; snow

5.4 Snow 

The installation of metal roof cladding in snow areas does not require any increase in the capacity of gutters, but it does require the installation of snow guards, so the gutter will remain free to drain the melt-water. As the gutter needs to withstand the dead load when it is full of water, and because the weight of snow will be less than that of water, no additional strength is required in the gutter if snowboards are provided.

Gutters without snow guards are vulnerable to leakage because they can be blocked by snow, but with snow guards, the size of the openings should allow melted snow to escape.

In snow areas, all internal gutters must have snow guards. See 3.8 Snow Loads.

The UDL imposed by the additional weight of snow will vary because one m³ of fresh snow weighs approx 100 kgs or a load of 1 kN. Fresh snow will be partially melted by rain and will be a combination of ice and snow. See 3.8 Snow Loads.

Because one m³ of ice weighs approximately 900 kg (or a load of 9 kPa) and one m³ of fresh snow weighs approx 100 kg (or a load of 1 kPa), it is reasonable to assume that approximately 100 mm of snow on roof cladding will impose a load of 0.5 kPa. (50% snow/ice)

When temperatures are prolonged at sub-zero, melt-water can refreeze and build-up as an ice dam, particularly if the roof is insulated. In such conditions, to prevent the ingress of water, an impermeable membrane should be installed and supported between the last two purlins to discharge into the gutter.

Purlin spacings should be reduced at the eave to allow for the added snow load, which is likely to be greatest at this position.

In snow areas, the snow load must be added to the downward wind load, and purlin spacings must be reduced at both end and intermediate spans by the amount as shown on the load span graphs. See 3.14.5 Wind Load Span Graphs

Special designs are required for spoutings and their supports in snow areas. See 5.5.2 Gutter Support Systems.

Leakage due to snow can occur at penetrations that are not designed in accordance with this Code of Practice.

Penetrations such as chimneys or vents from a heated building should be placed at a ridge or in the peripheral area around the roof because diurnal temperature fluctuations can provide a freeze/thaw cycle that will severely test any sealed hole in the middle of the roof cladding.

All penetration flashings larger than 600 mm in width in snow areas must use a cricket or diverter design. See 8.3.2 Diverter Or Cricket Designs

Snow loads are subject to variation due to drift and melting, and roof areas expected to trap significant quantities of snow present complex loading patterns. The design loads should take account of drifting of snow due to wind, but wind loads need not be combined with the snow load.

Roofs with internal gutters, valleys or high parapets, saw tooth or barrel vault roofs, can all retain accumulated snow through drifting due to wind and sliding snow. Where snow can be expected to accumulate, or where the shedding of snow is prevented, special loading should be applied as snow slide and drift can impose a load of up to two or three times greater than normal on the roof cladding and the structure.

Projections such as gutters, flashings and chimneys present obstructions that accumulate snow, and should also have special designs to resist loads from sliding snow.

When an additional building is to be built alongside an existing one, or where it is likely that snow can slide onto a lower roof, a revision of the original roof design loadings must be made.


5.4.1 Hail 

Like snow, hail can cause blockage and subsequent leakage. For this reason, sumps and rainwater heads should have leaf guards that are the same size as the sump, provide a large free overflow area and should not be tapered, because of the wedging action of hail.

Snow gutter boards should be provided for all internal gutters, as a baffle, to allow hail melt water to drain safely without overflowing the gutter.

In all areas prone to hail—spouting, gutters, and their supports—should have additional fixings. See 5.5.2 Gutter Support Systems.