Calculating flat roof load

If you want to buy a home with a flat roof, or you want to build your own, and if the house is situated in a zone with heavy snows, then you should calculate the capacity of the roof for snowfall and other roof loading. On a flat roof it’s very difficult for the snow to slip off. It’s crucial that the roof should be able to support the weight of the snow.

Ground snow load

Determine the amount of snow likely to fall on the ground. Collect this data from the internet. This information can also be found at your local city hall.

Exposure and thermal factors

The load may be different from the roof and the ground, because the winds are higher on the roof, and some snow may be lost. Also the heat loss from the house may melt some snow.

You will need to consult the ASCE 7-05 manual for the exposure factor for your home and environment. This will tell you how much snow will be lost during the winter months due to exposure.

Importance factor

This factor can also be found in the ASCE 7-05 guide, and it takes into account the type of building you live in.

Flat roof snow load

For ground snow loads lower than 20 pounds per square foot, the roof snow load is equal to 70% of the product of the exposure, thermal and importance factors and the ground snow load. For load greater than 20 pounds, the roof load is equal to 70% of the product of exposure, thermal and importance factor, the ground load and the 20 pounds.