How many layers of shingles?
How many layers of shingles can be safely supported on a wood framed residential roof?
Most building codes limit the number of layers of shingles to two. As a structural engineer, I design for two, but I prefer one layer in practice.
Each layer of shingles weighs 2 to 3 pounds per square foot (psf). So, two layers of shingles equates to 4 to 6 psf of load on the roof. This is the equivalent of 3 to 6 inches of snow depth.
However, since the weight of shingles is sustained (continuous) compared to the short duration (temporary) weight of snow, the shingles actually have a more adverse effect on the roof framing. When designing for short duration snow loads, the allowable stresses in the lumber may be increased by 15%. When designing for a sustained load, the allowable stresses are reduced by 20%. That is a difference of approximately 43% in the stress and sagging effects of short duration versus sustained.
Please note that this is a quick and dirty comparison for educational purposes, but it demonstrates the increased effect of a sustained load versus a short duration load. And, as one can see from this demonstration an additional one or two or MORE layers of shingles can really have an adverse effect on the ability of the roof to safely support snow and ice.
Stick with the code requirements on this – NO MORE THAN TWO LAYERS OF SHINGLES ON A ROOF.
By the way, how many layers in our photo? (See below)
Four!!!! But we have seen more!