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Ask the Carpenter: Can you add a new roof over the existing one?

Ask the Expert Home Improvement

Q. I have a 22-year-old asphalt-shingle roof. The home is the same age. I have GAF 20-year-rated three tab shingles. I want to replace the roof and was told by a roofer that I could go over the existing layer with a second. He recommended 30-year architecture-style shingles. If I add another layer, will it last, or should I strip and replace the first roof?

D.J., Wakefield

A. Stripping a roof is a miserable process. Roofers love roofing new houses because they don’t have to deal with the “tearoff.” That said, I always advise on stripping to the roof sheathing. A “roof-over’’ adds weight to your existing rafter structure, may void your shingle warranty, and prevents you from installing an ice and water shield.

The best practice is a complete strip and new roof, which would allow you to do the following:

■ Inspect the wood sheathing

■ Install an ice and water shield

■ Inspect and replace the flashings and drip edge.

 

Q. A few years ago I had a new roof put on my house. There has been a lot of moss growing up there this year. I bought some products from The Home Depot that killed it, but it’s still sitting up there. The moss went from green to brown, so I know it is dead. How do I get it off my roof?

BILL CAMIRE

A. It probably needs to be brushed, scrubbed, or washed off. If you need to wash it off, hire someone equipped with fall-arrest harnesses who will do it from the top down with low water pressure.

 

Q. We have a fairly new Bosch dishwasher. It does a great job except for our knives. Several always come out with stains that look like rust spots. We then have to handwash them again. Why is this happening just to our knives?

MAURA

A. This was happening to my knives, too. A lot of cutlery is stainless steel. Knives are often constructed out of a hardened steel so they will maintain their sharp edge longer. The harder steel is more susceptible to rust spots. Also the cleaning tablets that many dishwashers use contain salts, and some folks even use water softeners in their homes — all of which can cause rust.

The best approach is to wash your good knives by hand or remove them as soon as the wash is complete. Don’t let them sit in that damp environment too long.

Rob Robillard is a general contractor, carpenter, editor of AConcordCarpenter.com, and principal of a carpentry and renovation business. Send your questions to homerepair@globe.com or tweet them to @robertrobillard. Subscribe to our free real estate newsletter at pages.email.bostonglobe.com/AddressSignUp.