Snow Sliding and Snow Guards

Snow sliding is the release of accumulated snow and ice from a sloped roof, often suddenly and in large masses. Smooth, steep surfaces such as metal panels, glazed tile and slate shed snow readily. This helps limit the load on the roof but creates hazards below: falling snow and ice can injure people, damage vehicles, plants, gutters and lower roofs, and block exits.

When and why snow slides

Snow tends to slide when its bond with the roof weakens, typically during thaw, after rain, or when sun or building heat warms the roof surface. A layer of meltwater at the roof surface acts as a lubricant. Roofs over heated spaces, dark coverings in sunshine, and steep pitches all increase the likelihood of a slide.

Snow guards

Snow guards are devices fixed to the roof to hold snow in place or break it into smaller pieces so it melts gradually or falls in smaller amounts. Variants have been used for more than 300 years in snowy regions. Types include:

• Individual guards (pads or brackets) installed in staggered rows across the roof.
• Rails or bars (snow fences) that run horizontally across the roof, sometimes in several rows.
• Clamp-on systems for standing seam metal roofs that attach to seams without penetrating the panels.

Modern guards are made from metal, polycarbonate or both and can be coloured to match the roof.

Design considerations

The number and layout of guards depends on roof pitch, length of the slope, surface material, and expected snow load. Guards are not meant to retain all snow indefinitely; a fully held snowpack adds load and can promote leaks as it melts. Engineers check that the roof structure and fastening can carry the retained snow, and manufacturers publish layout guidance based on testing.

Where guards are used

Guards are common over entrances, walkways, parking, decks, lower roofs, and gutters, and wherever people or property below could be struck by falling snow and ice.

Related issues

Sliding snow can tear off gutters, vent stacks, and rooftop equipment. Placing penetrations near the ridge, using diverters above chimneys and vents, and fitting heavy-duty gutter brackets help reduce this damage.