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When installing a roof cap on a roof where there are three layers of shingles, do i trim away just the first layer of shingles or should i trim away all three layers to slide the cap underneath.

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2 Answers

First off, three layers of shingles indicates that you need to redo the roof completely soon. You're at your max, and frankly I think even two layers is a sign of lazy roofers.

That said, vent caps are most effective when installed so that the water runs onto them from the upper shingles and then back ONTO the lower shingles, as such:

enter image description here

So, you should definitely put the cap under ALL layers of shingles above, and over ALL layers of shingles below.

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I am not sure the OP mentioned a re-roof, only the roof jack (which with 3 reroofs already, likely shows as much or more wear than shingle layer one). That said, I would be inclined to let the urgency of a total reroof (of which code will not permit a 4th layer) be the determiner: if reroof is imminent, go down a layer or two, if long term, carve down to (but not in to) the felt and built back up to flush--which could be tricky to accomplish, maybe more so with laminateds? Alternatively, wouldn't screwing a pristine jack onto the existing be feasible if the reroof is very soon? –  Lex Oct 26 '12 at 0:04
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Given that multiple layers of shingles should be overlapping each other, I think it will be difficult to get the vent under all the shingles on the top half of the cap without cutting shingles and risking water intrusion. I'd suggest going under just the top layer of shingles above the cap. And of course have it exposed on the bottom half, like Greebo's photo shows.

When you get the roof replaced (they should remove everything down to the plywood and start fresh next time), have the vent properly installed under the shingles above and over the singles below.

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You have to remove a few of the shingles that will go above the top of the vent, which is a bit tricky but not all that hard, giving yourself more space than the vent needs to get it in place. Then attach the vent, then reattach the shingles, sans a few where the vent now goes. –  The Evil Greebo Aug 23 '12 at 18:07
    
I will probably try just removing the top layer when installing the roof cap. And see how that works out. If that fails, i will try removing the other two layers. Thanks for the tips! –  al lee Aug 25 '12 at 0:52
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