Accurately Calculate Flat Roof Load

If you are considering building a home or other edifice with a flat roof and live in an area where there is snow during the winter, you must calculate the capacity of the roof for snowfall and other roof loading variables. Unlike sloped roofs that allow snow and other buildups to fall to the ground, flat roofs tend to collect snow and other debris. In cold climates, an entire season’s worth of snow may accumulate on a flat roof without any melting or evaporation. For this reason, it is crucial that your flat roof be able to support the weight of the snow, not only for the structural integrity of the building but also for the safety of people in and around the building.

Step 1 - Determine the Ground Snow Load

Determine the amount of snow likely to fall on the ground. This data is dependent upon snowfall information collected over the decades. Rather than try to estimate on your own, consult with your city hall and records department for the ground snow load in your particular area. This information is also available in the ASCE 7-05 Code, available online.

Inspector's Notes:

The ASCE 7 Standard Minimum Design Loads for Buildings and Other Structures is the document the International Building Code (IBC) relies on for its structural and nonstructural requirements. ASCE 7-05, was the standard referenced in the 2006 and 2009 IBC’s but has now been substituted by ASCE 7-10 in the 2012 IBC. Per the code updates, there are some changes in the nonstructural requirements for commercial buildings that may affect your project.(IBC, ASCE 7-10, 2012) IBC code is subject to change so it falls upon you to keep up to date on it.

Step 2 - Determine the Exposure and Thermal Factors

The load of the snow on a roof tends to vary from the load of snow on the ground. Because wind levels tend to be somewhat higher on roofs, there may be some snow lost due to wind. Additionally, heat from various sources may contribute to some melting of snow that has accumulated on top of the roof.

Consult the ASCE 7-10 manual for the exposure factor for your home and environment. Ranging from “fully exposed” to “sheltered,” this factor helps to determine how much snow will be lost during the winter months due to exposure.

The thermal factor is dependent upon the heat produced by your roof. This value is also available in the ASCE 7-10 guide, depending upon whether your roof is cold or warm. Warm roofs lose heat in such a way that it melts snow, while cold roofs lose heat that doesn’t contribute to melting.

Step 3 - Determine the Importance Factor

The importance factor, also found in ASCE 7-10, takes into account the type of building for which you are calculating snow loads.

Step 4 - Determine the Flat Roof Snow Load

For roofs that are flat and with 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 ground snow loads greater than 20 pounds per square foot, the roof load is equal to 70% of the product of the exposure, thermal and importance factors, the ground snow load, and 20 (pounds).

If you have any concerns as you are determining your flat roof snow load, consult with a structural engineer or roofing specialist.