New sheathing on a roof

Stan in Oly, WA · Sep 15, 2020

What would happen if I were to put new OSB sheathing directly over the single layer of 3-tab roofing that is on a rental house I own? It probably would make it technically illegal to do a roof-over when that roof needed to be replaced because I don't think the city the house is in allows more than two roofs. There's little likelihood that I would do that even if it wasn't a bad idea in some other way, but I've wondered for a long time about what would be the drawbacks to doing that.

Thanks, Stan

Sean Feeney · Sep 15, 2020

Why are you doing it ? to save a strip.

Mule Meat · Sep 15, 2020

No, no and no again.

1. You attach new sheating over shingles. The sheating will not be "anchored" good. A good wind storm can rip it off easily.

2. Weight. You do it. You best get an engineer in to do the math on the max load your trusses can handle.

Myself. I would rather do the job right the first time than have a roof come down on my head when 2' of wet heavy snow piles on top of it

dpendzic · Sep 15, 2020

you would need longer nails to anchor the new OSB--but i would be worried about weight and trapping moisture between the old shingles and the new OSB

Stan in Oly, WA · Sep 15, 2020

Sean Feeney; I'm not going to do it. Okay?

Stan in Oly, WA · Sep 15, 2020

Mule Meat; Preferring to do a job right rather than having a catastrophe doesn't actually put you in a narrow category. Only lunatics would rather have a catastrophe than do a job right. It's the choice between doing a job right and the POSSIBILITY of a catastrophe that separates the good, the bad, and the ugly. Thanks for the information, though. That's all I was looking for.

Stan

Stan in Oly, WA · Sep 15, 2020

dpendzic; At the risk of making it sound like this is something I'm going to do, I have responses to your good observations. First, longer nails wouldn't seem to be too much of a problem. Second, if the local building code allowed two roofs on a house, wouldn't roofing, OSB, roofing be the same as OSB, roofing, roofing as far as weight is concerned? That would be the case sometime in the future if somebody decided to put another roof over the OSB and three-tab of the previously properly installed roof, as the code allowed.

Stan

MarkB_MI · Sep 16, 2020

As I recall, a square of shingles weighs around 240 pounds. That's a lot of unnecessary weight to leave on the roof, which is one reason I will never put a second layer over old shingles. (The main reason I hate two layers is I've had to do several two-layer tear-offs, and I refuse to leave that job to the next guy, who just might be me.)

Blackhole49 · Sep 16, 2020

You don't say why you would do this. The only reason I can think of is because the original OSB is rotten. At that point, all you are doing is doubling the weight on a rotten roof. If the original is that bad, better take it off and see what else is damaged.

Stephen Newell · Sep 16, 2020

I wouldn't do it. Shingles are incredibly heavy. You would be adding unnecessary weight to the roof framing. What would be cheaper and better if you can do it is to put a metal roof on. That would be best done over shingles. The existing roof helps dampening the noise from a metal roof.

kcm.MN · Sep 16, 2020

(quoted from post at 16:13:27 09/15/20)Second, if the local building code allowed two roofs on a house, wouldn't roofing, OSB, roofing be the same as OSB, roofing, roofing as far as weight is concerned?

[i:7b3971f604]As far as weight[/i:7b3971f604], yes -- pretty close. The difference is that the OSB/sheathing is the "foundation" of the roof. If you think of it in that sense, then the better your foundation, the better your roof. You want as little air as possible being able to get a grab and ripping the roof off your building.

To add to this thought, let's throw in the used course of shingles (or whatever). The fasteners may not be new and they may, in fact, be the devil to get some of them out, but remember that many of the older fasteners are on their downward approach in life. They will become weaker as time goes on, which would compromise the newer roof on top.

And lastly, I personally would never do such a thing because that removes ALL possibility of being able to examine the roof, all the way down to the roof framing. Remember those older fasteners? The wood at those points often seems to deteriorate more quickly than the rest of the framing.

In short, it's not ever a good idea to leave older roofing on a newer roofing job.

dpendzic · Sep 16, 2020

the old 3 tab shingles are probably 2.4 PSF--another layer would add that again, plus the weight of the OSB second layer...granted this is all probably less than your snow load but timber is great for taking a short term overload--the dead load is constant and can contribute to deflection
I still don't like the trapped air space that can contribute to condensation. If you can get all the permeability ratings of the materials you can determine where the condensation can occur knowing the inside and outside ambient temperatures
generally in cold weather climates you want the less permeable material on the inside and more permeable materials toward the outside so moisture can migrate thru without condensing

ss55 · Sep 17, 2020

As the old sheathing continues to fail, the new roof will fail early too. A rule of thumb is that an overlaid second layer of shingles only lasts 2/3 as long as a complete tear off and re-shingle. I wouldn't expect the overlay of sheathing roof to last half as long as a complete tear-off, re-sheath and re-shingle.

Overlaid shingles have not been to code in many areas for decades now. Even if it is to code now in your area, it may not be in the future when you need to sell. A buyer's inspector will easily spot an overlay. What kind of red flags and doubts about the rest of the building will the discovery of overlaid sheathing set off?

1948CaseVAI · Sep 17, 2020

ss55 has a great point. Even if your roof does not collapse or rot off prematurely you would not be able to sell the building in my area. Code here allows only one layer of shigles. We have no such thing as a second layer here and the only way to sell a non-compliant building is to pay cash because lenders won't touch it.

Stan in Oly, WA · Sep 17, 2020

ss55; That's good advice. I never planned to actually put OSB over the existing roofing, but I've got to get a layer of OSB or plywood on top of the old sheathing which is not entirely sound. I'm always curious about why things are done a certain way and not a different way. Posing the question resulted in a lot of good information. I'm dreading a complete roofing job at a location where a rooftop delivery of materials is not possible. I've reroofed many houses but only on the last one I did myself did I get a rooftop delivery. It was so much easier than hauling materials onto the roof myself that I vowed I would never do a roof any other way. But here it is four or five years later and there's no other way to do it (except to get someone else to do it, I guess.

Thanks, Stan

TomA in CA · Sep 17, 2020

I like the idea of a metal roof over everything.

ztrmowers · Sep 18, 2020

why would you want to do that

Billy NY · Sep 18, 2020

Stan, based on experience from a long career in construction, I can understand value engineering a job, possibly eliminating some work within any project, this is not one of them. No fault in exploring options, just that I don't see this being feasible for the same reasons mentioned below. I have never seen a roof detail on a project that would even remotely come close to overlaying sheathing and shingles onto existing shingles. I think it's very risky, better to strip it down and do it right, + when you do that, you can inspect the current underlayment, repair etc.

Stan in Oly, WA · Sep 18, 2020

Hey Billy; Thanks for the good, well grounded advice...as usual. What's your opinion of Stephen Newell's suggestion about putting a metal roof on over everything? I've put on metal roofs before, and found it pretty easy. There are some other considerations that might preclude the use of metal roofing, but that's a different issue. What do you think about it from an engineering/practicality viewpoint?

Stan

Billy NY · Sep 19, 2020

Worthy of researching to see what the manufactures say about installing their products like this, attaching furring strips of appropriate size for the metal roofing fasteners to fasten to. We did a friends house like that and recently his brother did a garage at the family farm like that. The house was done in 2003, and I do not know of any problems to date. This type of installation is seemingly more common, I'd still want to research a bit, but I'd have to think it's a viable solution vs the other idea you posted.

This was an interesting question as sheathing over existing shingles, then installing shingles on the new sheathing, is not something I've ever heard contemplated or done before. All of us have probably seen some sketchy roofing work done somewhere along the line but I cannot recall ever seeing that done.

New sheathing on a roof

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