Pros and cons: Covering instead of stripping that old roof

Home Improvement
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In a nor’easter, your home’s first line of defense is its roof. When it’s time to replace the asphalt shingles, can you save money and install new ones right over the old?

John Lojek, commissioner of inspectional services in Newton, said Massachusetts Building Code doesn’t directly address the question of a “go-over.’’ It says there can be no more than two layers of shingles on a roof, Lojek said.

“Obviously, if the shingles are badly curled and brittle, you don’t want to install a new layer right over them,’’ Lojek said. “And the shingles have to be installed according to manufacturer’s instructions.’’

(Help Desk: Lessons we learned while shopping for a new roof.)

Sean Green, president of Storm Works Roofing & Restoration in Belmont, said nailing new shingles over old ones will void most manufacturers’ warranties.

“I always recommend stripping old roofs before installing a new one,’’ Green said. “You want to inspect the decking and re-nail or replace it where you need to. It’s also the only way you can install the paper and ice and water shield.’’ The paper underlayment and ice and water shield are crucial to preventing ice dam leaks.

Green said go-overs add a lot of weight to the roof. The additional shingles also absorb more heat, which wears the new shingles faster than if they were installed directly on the wood sheathing.

He said go-overs were more common when three-tab shingles were used. Today’s architectural shingles wouldn’t allow a go-over to lay flat, leaving them susceptible to being blown off in a storm, he said.

Many homeowners used to opt for a go-over to save the cost of stripping and disposing of the old shingles — around $5 to $7 per square foot today, said Steve Johnson, a partner at Roof Hub in Boston. A typical roof in this area is roughly 1,500 square feet, so the savings would be significant.

“It is never advisable to shingle over existing shingles,’’ Johnson said. “We do not offer a go-over as a service because it can’t be warrantied.’’

Jim Morrison can be reached at [email protected]. Subscribe to the Globe’s free real estate newsletter — our weekly digest on buying, selling, and design — at Follow us on Facebook, Instagram, and Twitter @globehomes.