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Sooner or later, every remodeler will be asked to reroof over an existing asphalt shingle roof. But before adding a third, or even a second, layer of 300-poundper- square shingles, you have to make sure the roof structure can handle the load. The chart presented here (facing page) is designed to help you do just that. The top section of the chart applies to situations where you are adding a second layer of shingles; the bottom section is for three layers (in those cases where code allows — and the customer insists). You may be surprised when you use the chart, because many older buildings were never built to comply with building code guidelines in the first