Don't leave slate on the roof when replacing with shingles

Q. The slate roof on our 90-year-old house needs replacing with shingles. Before putting on the shingles, is it better to remove the slate and then add the new shingles or put the new shingles right over the slate? Velma Liveratti Wilmerding, Pa.

A. Assuming the old state is true-blue material, I'd remove it first. Slate will break if you try to drive nails through it. If an old roof is wood or composition, not slate, veteran roofers generally remove two or more old roofs before reroofing.

If you use wood shingles or shakes when you reroof, you may need to "strip" the old roof unless it already has spaced sheaving. But if the new roofing is a composition type, apply it only to a solid stable substrate, first removing and replacing any bad boards.

Roofers who guarantee their work prefer placing unit shingle roofing on roofs having a minimum pitch of 3 inches in a foot. Since your old roof is slate, undoubtedly its pitch is steeper than the minimum required.

Roofs with pitches of 1 in 12, 2 in 12, or less require rolled roofing or built-up, hot-mopped types.

Most nationally known roof manufacturers have qualified representatives who provide written roofing specifications for clients. When an owner intends to have his roofing competitively bid, he should provide the bidders with common specifications. Otherwise, competitive bidding is almost meaningless.