I am a truss manufacturer in an area of the country that often has some pretty severe winters. It concerns me how little some of the local builders seem to know about snow load design. What are some of the things that need to be considered?
Whenever we discuss building codes and design loads with builders and designers we always point out that the loads specified by the building codes represent absolute minimum values and might not be appropriate for every application. We believe it is important for builders to understand the intention of the building codes with respect to loads: it is to ensure that the building is constructed to adequately support the loads to which it might be exposed over its service life.
The current snow load criteria (IRC 2012/2015 R301.2.3, IRC 2012/2015 figure R301.2(5), IBC 2012/2015 1608) begins with a base value (i.e. ground snow load), to which several modification factors are applied. These factors account for variables such as site exposure, building importance, roof slope and the thermal properties of the roof. Application of these factors to the ground snow load generally reduces the magnitude of the snow load for which the roof is to be designed. In addition to the various modification factors, a complete snow load analysis should also consider the potential for unbalanced loading, snow drifting, snow accumulation in valleys, ice damming and additional loading due to rain-on-snow situations. These conditions, when applicable, increase the magnitude of the snow load on at least a portion of the roof. The ground snow load and each of these factors are generally site specific and vary from project to project. It is important to understand this and to use the proper values.
A complete and thorough snow load analysis can be complicated and is beyond the capabilities of the average homeowner or contractor. To simplify matters, some code jurisdictions require the ground snow load be used as the roof design load without further modification. Drifting provisions may also be applicable, but we have rarely seen these provisions enforced for residential projects in which a professional building designer is not involved. An in-depth analysis is not necessarily required, but the simpler and broader the assumptions, the more conservative the loads should be. The code enforcement officials are certainly available to help in these matters.
For additional information on snow load, see How Much Snow Can a Roof Handle in SBC Magazine as well as SBCA’s Load Guide. It is an Excel spreadsheet that can be downloaded. The tab titled “C7-Snow” includes discussions on most of the issues relating to snow loads and trusses.