Although sags along roof ridgelines can be discovered at any time, in northern climates they are often attributed to the effects of a past snow load. To make an educated analysis of the actual cause of this phenomenon, one must have a basic understanding of the structure and the loads to which it has been subjected.
Roof sags generally appear as a downward bow of the ridgeline that progresses in a gentle curve uniformly from its highest points at the roof’s eave ends to its lowest at the approximate center of the ridgeline. The amount of sag between the ridgeline’s highest and lowest points can vary from just an inch or so to over a foot. The characteristic shape of the sag is due to the fact the elements on each end of the roof are stronger than those between due to their attachment to an adjacent structure, or as the result of their cladding with sheathing or similar materials.
Residential roof structures are either hand-built on site using dimensional lumber and nails (stick-built) or assembled using prefabricated, engineered trusses. Although it is not impossible for an engineered truss-constructed roof to sag, most sag problems occur within stick-built structures.
The most common cause of roof sags in a stick-built structure is improper or poor construction practices. The cross-sectional shape of a typical residential roof is a triangle. The sloped portions of a roof’s triangle are called rafters, while the horizontal elements are joists. The properties of the triangle make it very strong; however, if any one of its sides or connections fails, its strength is lost. Improperly cut and fastened joints at the ridge beam or rafter ends will reduce the structure’s ability to support loads. The most easily found evidence of such poor construction practices can usually be found where the rafters attach to the ridge beam. If uniform gaps are found between the bottom of the end of the rafter and the ridge beam, this is an indication that an excess load created the movement. However, if the gaps are irregular in size, with some tight while others large, then it is probable that this is the result of workmanship issues rather than overloading. Also, new and sudden movement such as that caused by overloading will show evidence of shiny nail shafts, and cobwebs will not be found within the gaps.
In some structures, the joists will not run parallel with the rafters, and therefore, the roof is inherently weaker. In these structures, alternative horizontal members, called collar ties, must be installed to provide the necessary strength. If these critical elements are not in place in such a construction, it will be prone to sag under its own weight.
Sags can also occur as the result of improperly sized rafters. Building codes specify minimum rafter sizes dependent on their length, wood type, and design load. Using undersized rafters can cause them to sag individually and results in an overall sag along the roof’s ridge line. Poor attic ventilation can exacerbate this problem.
Another factor to consider is the weight of the roof covering. Multiple layers of asphalt shingles increase the roof’s dead load and thus make it far more susceptible to sagging when a snow load is then applied.
Building codes regulate the design snow load that must be considered during the planning and construction of structures. These design loads vary from state to state and range from as low as 0 pounds per square foot (psf) in a few southern states to as much as 300 psf in some Alaskan communities. Typically, northern states have snow load requirements that vary between 30 and 90 psf.
Snow and ice varies in its density, and therefore, so does its weight per given volume. Neither snow nor ice will ever exceed the density of water: 62.4 pound per cubic foot (pcf). Typically, snow will weigh much less, and according to the University of North Dakota, it will weigh between 15 and 20 pcf depending on whether it has drifted and compacted.
Given this data, it can be estimated that for every 1 inch of snow, a roof will be subjected to a load of between 1.25 and 1.67 pounds . Therefore, roofs in most northern climates, if properly designed and constructed, must be able to accommodate between about 18 and 72 inches of snow, depending on the prevailing code requirement.
When estimating snow loads, drifting must be considered. During snow storms with strong winds, drifts can form on the leeward side of a roof’s ridge. These drifts can often double the snow load on those affected areas, but even larger multiples are possible given winds and roof design.
Although snow loads are generally not the root cause of roof sags, they contribute to the problem, just like all other loads that the roof must bear. However, a properly designed and constructed roof structure should not sag unless it can be shown to have experienced loads well exceeding the community’s code requirements.