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...

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