5 Key Factors to Consider When Using a Snow Load Calculator

roof structures

Snow loads can be dangerous for roof structures and should always be considered when designing. There are procedures and guidelines outlined in ASCE 7-10 to help guide structural engineers in calculating these loads and applying them appropriately. A roof snow load calculator can easily determine how much weight your structure can support. Enter your location and national annex; the software will do the rest!

Roof Pitch

The roof pitch is an important factor in determining the snow load that your building can handle. It’s also a factor in deciding what roofing materials are appropriate for your roof and how well it drains water.

Roof pitch is also referred to as slope. Technically, the slope is a measurement of rise over run and is expressed as a fraction. To calculate your roof’s slope, you need to measure the run length of the roof and the height of its peak in feet. Then enter the ratio of these two values in one of our calculator’s fields (e.g., Roof Pitch). 

Roof Length

While roof pitch plays a significant role in snow load calculations, roof length is also significant. A shorter roof will retain less snow and carry a lower snow load than a longer roof. It is important to measure the length of a roof from the ridge (or eaves) to the building walls if possible. You can do this using a ladder or by measuring along the under-purlin strut, which is located between the rafters. It is also essential to know how long the rafter spans are and to use this information when calculating your roof snow load. Using the span/length ratios will produce a much more accurate snow load calculation than relying on span/height ratios. This will reduce the chances of overestimating your snow load and causing premature failure of the roof structure or even the entire building.


The ground snow load (psf) is the weight of the snow on a surface. It can have disastrous effects on a structure if not considered during design. A formula, thickness x density, determines it. The mapped design ground snow loads provided by this tool are based on a regression curve fit data from stations located at each latitude and longitude. Because the modeled data is averaged over 4 km (2-1/2 mile) grid cells, terrain features such as hills or valleys may not appear in the minimum design snow loads. Therefore, it is recommended that the designer check with local building officials for site-specific minimum ground snow load values. The design engineer can also conduct a field measurement to determine the minimum design snow load in those cases where the mapped maximum ground snow load is insufficient.

Wind Speed

The wind speed affects the force of snow blowing against a structure. It also influences the snow density. If the wind is strong enough, ice will form in the snow and increase the weight of the load. A structural engineer must consider these load factors when designing a building or other structures. The procedures and guidelines set forth by ASCE give them direction on calculating and applying these loads to the structure. To help with this, SteelMaster launched the automatic data retrieval feature in 2010 that allows you to enter your zip code and find the ground snow load design (pg in ASCE7) for your site. 

Snow Density

Snow density is a variable that can significantly impact the snow load calculation. Density is determined by how much water is in the snow. Wet snow is much more dense than dry, wind-packed snow. This is why collecting and weighing your snow samples before using them in calculations is important. This will help you understand the importance of the density factor in the equation. Design ground snow loads are calculated from the snow depth and density.