New roof or another layer of shingles?


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Old 06-20-12, 09:31 AM
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New roof or another layer of shingles?

I have a high peaked roof in Brooklyn NY (hot humid in summer, possible snow and ice build up in winter). There are already a few layers of shingles. 3 roofers claim a new layer of shingle could be put on with 2-1/2 in nails. One roofer claims a new roof is necessary.

1) Do you think a new roof is necessary, or should I go with a new layer of shingles.

2) They each claim a different thickness of plywood ranging from 3/8 to 3/4 should be used. Does it really matter as long as they meet the shingle makers specification?

3) The original roof was cedar shakes. Would a plywood roof require a new ventilation system like attic vents or even a blower? The eaves over the soffit are not very big and the roof is not flat, it has a single high peak (with one dormer), so maybe ice dams will not be a problem.

4) There is no roof insulation. The wiring is old armored cable, no problems but old. Would an electrician be able to replace the wiring from the roof? Could insulation be added to the plaster lath below? Or are these bad ideas?
 
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Old 06-20-12, 09:35 AM
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Wait a minute - you're talking about adding shingles and you're also talking about adding plywood. I'm confused, which is it?

Two layers of shingles is the most I'll ever go so you're apparently beyond what I'd do in the first place.
 
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Old 06-20-12, 09:47 AM
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1). yes I would say that a new roof is necessary.

2). the bottom line is that you should go with the shingle mfg specs. 3/8 ply or 7/16 osb is usually the minimum, when you tearoff and are overlaying a roof that has slatted 1x sheathing, as many of the older wood shingled houses have. Wood shingles are "sometimes" counted as wood sheathing in some areas, so they may or may not allow you to apply up to 2 layers on top. Not that it's a good idea, mind you!

3). can't answer that when you did not mention what existing ventilation you currently have. Do you have soffit vents? Ridge vents? Gable vents? Attic fans are almost always a bad idea. Ice dams are a problem on houses most where the distance between the top plate and the roof is narrow. This is because heat loss from the house in those areas will melt the snow on the roof, causing ice.

4). if you mean attic insulation, that could be added anytime, but I'm sure the electrician would prefer to do his work first. If you have attic access inside, the electrician would go in from there. but if it is easier for him to make a hole through the roof, and you could coordinate that with your roofers, that would be an option. Changing wiring usually means making a lot more holes in the walls than you probably imagine.
 
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Old 06-20-12, 05:08 PM
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is the sheathing spaced or tight?
 
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Old 06-20-12, 05:42 PM
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There's normally a gap in the sheathing around here - there are clips which hold the spaces for you.
 
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Old 06-22-12, 07:40 AM
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"existing ventilation you currently have." Currently there are vents below the soffits, the Soffit extents about a foot and a half beyond the exterior wall. There are strips of wood that run along the house which are not air tight.

My concern is that the original roofing system consistes of Cedar shakes, an air gap, and plaster lath interior wall, which I think would let a lot of air and moisture escape. I think, but don't know, replacing with plywood would block the air. So I am wondering if additional vents would not be a good idea. While I trust the tradesmen to do their job, I wonder if they are contemplating the situation decades into the future. I also wonder if a blower would be good during the summer.

Like all politicians I am tempted to kick the can down the road by adding a cheap layer of asphalt shingle. Maybe 20 years from now they will have a super new roofing material I wished I had bought.

"you're talking about adding shingles and you're also talking about adding plywood. "

All work would be done by the roofer not me. The options are removing the 2 previous roofs and replacing it with asphalt shingles over plywood. Or to just add a third layer of shingles. The third layer of shingles would need longer nails (possibly a bad sign).


"is the sheathing spaced or tight?" I don't know. The original roof was furring strips covered with cedar shakes. No sheathing. Over that is a layer of crumbling asphalt shingle. If you are referring to the roofers suggestion to add new plywood, I don't know, My guess is I would have to do what the roofer wanted or risk voiding his guarantee.
 
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Old 06-22-12, 10:03 AM
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The air ventilation should be under the plywood and then exit either thru a ridge vent or gable vent.
It's always been my understanding that more than 2 layers of shingles voided the warranty and most codes...... but I'm a painter, not a roofer.
 
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Old 06-22-12, 11:46 AM
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Twenty years down the road, you will have to replace your shingles anyway.

An old cedar shingle roof would not have any sheathing below it. Just the 1x furring strips.

There are a lot of different types of roofing materials out there now. If you want to be cheap about it, then 3 tab asphalt shingles are going to rule. Either way, strip down to the furring strips and put plywood on. I don't know what the appropriate thickness would be. If the furring strips are in good shape still, they will provide a lot more support to the plywood than just having the rafters below would.

What is right under the roof? Is it living space or attic space?
 
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Old 07-16-12, 08:47 AM
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The high pitched roof is over a living space. The shape of the roof in the room below follows the shape of the roof, although at the top of the room the roof has a 3ft flat portion where lighting ect is mounted, so there is a triangular portion at the very top of the roofline with a small substantial airspace. I have noticed some houses in the area have a small vent on the front (not the roof) of the house.

The roof is probably 100 years old by now so I am thinking replace it just to correct any problems that might be there.

The issue for me is the venting as that is speculative and even the roof's experience might not be good enough.

In my mind right now there is nothing but cedar shakes and then asphalt shingles. The new system with be plywood roofing paper and shingles. That changes the ventilation situation, possibly not for the better. So I wonder if venting is needed?
 
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Old 07-16-12, 09:06 AM
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Changing your roof (putting on plywood) will not change your existing ventilation at all. (you probably don't have any, and if you do, it's either a gable end vent or a can-style roof vent.) When the roof is torn off, sheathed and shingled, roofer will usually add venting as needed, but you would want to specify that on the contract. (i.e. exactly what type of ventilation they will add and how much / how many).

If your house is old, you may not have very good insulation at all up there, so adding venting where there previously was none "could" change the dynamics of the room, for better or worse. For instance, adding venting to a space like that might help with heat in the summer but it could also cause frost or condensation problems in the winter if there is not adequate insulation in the space.

It's probably a situation where you would need to rely on the experience and judgement of your contractor... and he may not be able to make an informed decision until the day he tears off the roof and can actually look into the attic space. It's hard for us to say when we can't see it for ourselves.
 
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Old 07-16-12, 07:07 PM
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Thanks for the advice. GF
 
 

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