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Q.I intend to reroof over the original asphalt shingles on a 30-year-old house. I would like to install heavy 40-year architectural-weight asphalt shingles, without stripping the existing shingles. I am a little worried that the new shingles may add too much weight to the roof. The 4:12 roof has 2x8 rafters, 16 inches on-center, that are 16 feet long from the ridge to the eaves. Will a second layer of shingles pose any problems?

A.Christopher DeBlois, a structural engineer with Palmer Engineering Co. in Tucker, Ga., responds: The simple answer is that if the roof is fine now, it will almost certainly be fine when you add the new roofing.

Most residential roofs are sized or designed for basic loads of 10 pounds per square foot (psf) for dead load (self weight), and 20 psf for live load ( primarily snow, ice, and rain). The old roofing — standard asphalt shingles — probably weighs about 2.5 psf. The 2x8s themselves weigh about 2.3 psf depending on the species of wood, and 3/4-inch plywood or 1-by roof decking weighs about 2.5 psf. Add a pinch for 30# felt, and you get an existing load of about 8 psf. This assumes that you don’t have insulation or a finished ceiling on the rafters, but I am guessing that a 30-year-old house probably has a roof over an unfinished attic.

The fancier and heavier architectural shingles will weigh in at about 4.0 psf. Adding all this up, I get a total load of 32 psf (20 live load plus 8 existing dead load plus 4 new dead load). That’s less than a 7% increase over the standard design capacity of 30 psf, so you’ll probably be fine.

A more detailed analysis of your question depends on the answers to at least five questions:

  • What is the species and grade of the rafters?
  • Is the framing in good condition?
  • Did you measure the 16-foot rafter span following the slope, or did you measure horizontally?
  • Is there a finished ceiling under any or all of the roof?
  • What is the ground snow load where you live?

In order to give a more precise answer, I’ll make some assumptions. I’ll assume the rafters are #2 Southern Yellow Pine (SYP) or of equivalent quality, and I’ll assume the material is still in good condition. Since you said "16 feet long" instead of "16 foot span," I’ll assume you measured the 16-foot rafter length up the slope. At a 4:12 pitch, this gives a design span of 15 feet 2 inches. To give myself a little wiggle room, I will add another 2.5 psf to the existing dead load for a drywall ceiling with insulation.

Now I have enough data to actually calculate the capacity of the roof. For #2 SYP rafters at 16 inches on-center and spanning 15 feet 2 inches, I calculate an allowable load of 45.2 psf. Note that I have taken advantage of two upgrades permitted by the wood design code: a 15% strength increase available for multiple member use (typical for rafters and floor joists), and a second 15% load duration increase that applies for snow loads that are only present part of the year. Subtracting 10.5 psf for the existing framing and roofing, and 4.0 psf more for the new roof, and rounding down, I am left with an allowable roof live load of 30 psf.

A roof load this high can only be caused by snow. In this case, I conclude that you can add the new roofing if you live in a locale with ground snow loads of 30 psf or less. For the curious, the dividing line would fall approximately as follows: south of Pennsylvania, south of Michigan and Wisconsin, south of the middle of Iowa, south of South Dakota, and west of the Dakotas at an elevation below 5,000 feet above sea level.

If the framing details for the house you’re reroofing match my assumptions, then you should be able to add the new layer of shingles. If you’re in doubt, either consult a local engineer or tear off the original shingles.