My garage roof has shingles that are worn and weathered. Under what conditions would it be advisable to lay a second layer of shingles over the existing layer? I would assume there are cost savings doing this. There is currently only one layer of shingles on the roof and no leaks in the building.
Most people will choose to roof over the first layer of shingles because of costs. If the area around the building is accessible and only one story high, the costs to remove and replace the older shingles is about twice the cost of a roof-over.
Among the things that affect roofing costs: how many stories the building is, the steepness of the pitch or slope of the roof, accessibility to the building and the style or type of shingles.
Typically a garage is a 4/12 pitch, that is, the rafter will have a vertical rise of 4 inches every 12 inches in horizontal length of the wall below it. The steeper the slope of the roof, the better when adding a second layer of shingles. I would not recommend adding a second layer unless the roof is a 6/12 or steeper angle.
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Next, check the size of the rafters and the spacing between the rafters. A two-by-four-inch rafter set every 24 inches along the bearing wall may not adequately support the added weight of a second layer of shingles.
Check the collar ties, the two-by-four or two-by-six boards that span from one bearing wall to the other. There should be one collar tie for each set of rafters. Collar ties prevent the walls from spreading under the weight of the roof. Add collar ties if needed. If the rafters are prebuilt trusses, they should be checked by a qualified home inspector, roofer or carpenter before adding the extra weight.
Inspect the roof’s decking. It would be wise to remove the older shingles and replace damaged decking.
You also have to consider the weight of the shingles you choose. Shingles vary in weight per bundle. Shingles are typically sold per square (100-square feet) and are marketed by the number of years they are expected to perform under normal weather conditions. A shingle that is expected to last 20 years will not be as heavy as a 40- or 50-year shingle. The weight per square can be supplied by the supplier or manufacturer.
Next, consider the longevity of the product. A second layer of shingles will not last as long as a single layer.
Finally, what is the condition of the original layer of shingles? Shingles that are cupped, cracked or are lifting can ghost through the second layer of shingles and are not only unsightly but will cause advanced wear to the new layer.