Snow Drift On Roof

Slipperiness of the roof.

Snow drift on roof.

Ground snow load s s. Snow drift loading adjacent to roof obstructions. Asce 7 specifies drifts for simple geometries based on the size of the upwind snow source area fetch and ground snow drift is a 2 hfor more complex geometries a reasonable approach is to match the cross sectional area of the odd shaped drift to that for a roof step with the same fetch and ground load 50 faq 4 odd drift geometries. Less snow will accumulate on a slippery roof it will slide off instead.

Asce standard 7 requires designers to assume that the high winds that cause snow to drift could come from any direction. Horizontal extent is smallerof 6hdor 6h s 30. Windward snow drifts occur when wind blows snow from a lower elevation roof towards the wall of an adjacent higher roof. For small gaps we have an inconsistency.

Density snow density γ. If your roof is located in windswept areas or even north of the treeline the wind will cause snow drifting making less of it accumulate on your roof. This is true of typical smaller roofs but as the roof s plan dimensions become large the roof increasingly emulates the ground. Drift height is smallerof hdfor upper roof snow source and h s 6 space below aerodynamic shade line.

Rain load s r. Importance factor i s. Snow drift loading adjacent to roof obstructions. There are three elements needed for roof snow drift formation.

An area of aerodynamic shade geometric irregularity on the roof where the drift can form and grow a source of driftable snow upwind of the geometric irregularity and wind speed sufficient to cause transport of driftable snow across the aerodynamic shade region. Leeward snow drifts occur when wind blows snow off of a higher elevation roof down onto an adjacent lower roof. Leeward drift results from snow blown off a high roof onto a lower roof. The peak snow load on the lower roof is about nine times the load on the upper roof.

This accounts for the wind s tendency to remove snow from the roof by drifting while snow is generally not able to drift up onto the building from the ground.