Layers of Roof Shingle: How Many are Too Many?

LESLIE: Ellen in Arkansas is on the line with a roofing question. How can we help you?

ELLEN: I’ve got a roof job coming on this in a couple of weeks and they’re doing my roof and they’re doing up my siding.

TOM: OK.

ELLEN: Now, I had a big oak tree in my backyard. I had it cut down. But in the meantime, since it was there so many years, it left mold – the green stuff – growing on my shingles, right?

TOM: Yep. Mm-hmm.

ELLEN: The guy that’s going to do my roof said that – “Oh, that’s OK. Well, you can get up to three layers of shingles before you have to take the old shingles off.” And I said, “Well, mold strikes a negative vibe in me because my husband and I have allergies. Would that affect us?” They said they’re going to put the new roof over the old roof and the mold. I told them, “No, that’s not going to happen.” He said, “Well, what we’ll do, we’ll pour – spray it with bleach – the mold – and we’ll rake it and then we’ll just cover it with the new roofing.” And my question is: since it is so much money, is that wise?

TOM: Well, here’s the thing. Yeah, you can put three layers of shingles on but it’s a really bad idea to do that, aside from the fact that you’ve got some sort of growth on this roof. Whenever you put a second layer of shingles on, the first two layers, in your case, tend to retain a lot of heat and especially in an environment like Arkansas where you have really hot summers.

ELLEN: Oh, yeah.

TOM: As that heat is retained, it accelerates the deterioration of the top layer. So in my experience, if you had a roof that typically would last 20 years, you put a shingle – a layer or two of shingles underneath, you’re going to have it last more like 13 to 15 years. I’ve seen it cut a quarter to a third of the life off by doing that.

So it’s always smart to remove old layers. They’re probably trying to avoid it because it’s expensive to remove old layers. They’ve got to pull it off and they’ve got to get rid of it but it absolutely is the best way to do a roof replacement. And if you’re planning on being in that house for most of the life of the roof, it’s well worth it.

ELLEN: Yeah.

TOM: Now, in terms of the moss or the mold or the mildew or whatever is on there, there are many, many things that can grow on a roof, depending on the environmental conditions.

ELLEN: Yeah.

TOM: And it’s not always mold, although people tend to call it that. There’s a product called Spray & Forget that we have a lot of experience with.

ELLEN: Yeah. Mm-hmm.

TOM: And you spray it on the roofing surface. And it tends to smother all of those biological growths and stops them from coming back. It will kill what’s there, it will naturally just die off and go away and then there’s a bit of a residual effect to it that stays on there. And if you apply that about every two years, you’re not going to ever see any mold or moss or mildew or algae or lichen or anything else grow on that roof shingle.

It’s SprayAndForget.com. OK?

ELLEN: OK. I appreciate it.