Longevity of Plywood and OSB Roof Deck?
I recently completed a roof project for a relative. This was roof #3 for the home- tore off the previous two layers. The home is about 40 years old with roof trusses and 1/2 (maybe 7/16) plywood sheathing, no clips. Plenty of roof and eave ventilation.
As expected, the roof deck seemed to be quite brittle. A 220# guy with a bundle of shingles in hand could have stepped through the roof quite easily. One almost did.
Architectural shingles were chosen for the job, and I have heard that the architectural shingles can’t be roofed over again.
After completing this project, I am wondering how long plywood decking lasts. I suspect that this particular roof could not be torn off again (not that I would be doing it).
I have worked on plenty of newer homes with roof trusses and OSB roof deck- some with wide spans and an 8/12 pitch that puts the peak well above the ceiling below. If plywood doesn’t age so well, then how well will a bunch of wood chips and glue fare on a roof over time (long after the builders warranty)?
Replies
danski0224,
Plywood isn't a good roofing material. Your mother was right, just because everybody is jumping off a bridge,.....
To me the ideal method is skip sheathing..I know a lot of skipped sheathed barns that have orignal shingles in great shape after 100 years. I know of no barn with plywood that lasts more than about 30 years or so.. Houses seem the same. Skipped sheathed houses just seem to last longer!
It's the ventilation to the back side of the shingles that does it.. PLUS!!!!!! Solid wood is more durable than wood that is glued together!
But you have to be either very creative or determined to skip sheath a roof nowdays.. What I did was buy rough sawn boards direct from a sawmill. I paid less than 1/2 what the same thing in 1/2 inch plywood cost and had a 1 inch thick roof deck instead of just a half inch of glued up wood fibers.
The cool thing is it's real easy to do a roof on a windy day. No longer do you need to struggle with 32 sq.ft of sail area trying to blow you off the roof. The wind doesn't have too much effect on a narrow board.
Problem with that is that you can't lay any kind of composition shingles on skip sheathing. All manufacturers require that spaces between boards be snug
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Did you use graded, stamped lumber or isn't that a requirement where you live?Also, the humidity level in an actively used livestock barn is WAY above what should be found in the attics of most homes, so the comparison isn't that good.I do agree that old 1x skip sheathing is very stable. My buddy just ripped off three layers, one cedar shingles and two asphalt, from an 80 year old house. The old 1x was in excellent condition. He covered it with OSB as per the code here.+++++++++++++++++++++++++++++++++++++"Doubt is not a pleasant condition, but certainty is absurd." Voltaire
No sheet lumber material is gonna hold up well if it continually gets wet unless it's marine plywood. The reason roof sheathing rots is because the roof covering leaks and people either don't know it or ignore it until it starts to fail. Another cause could be too much moisture in the attic.
Edited 6/4/2006 4:05 pm ET by Matt
There was only one spot of rot- in a valley by the gutter. All else was dry.
I have had similar doubts about the longevity of roofs with sheet sheathing, plywood or osb. I have also seen plenty of roofs with board sheathing. It is not for nothing that most of these roofs get a fresh covering of osb after they have been stripped. The boards that are used to sheath roofs are not the highest of quality and after baking on a roof for twenty five years any knots or weaknesses in the grain are often areas of failure. It is often easier to cover up the whole thing than to selectively repair these spots.
I recently redid my roof twenty three years after I built the house. We used 5/8 cdx over 16"oc framing. This sheathing was virtually perfect after that time. It is my last roof on this house and if the next owner replaces the roof before it leaks, I expect that the sheathing will still be good.
Schelling,
Skip sheathing (or boards) can be as good or poor as the owner wants.. Given a choice of solid one inch thick wood to nail into or a half inch of plywood most would accept the solid wood.. I can buy FAS quality pine boards and still pay much less than I would for 7/16ths OSB. The "trick" is you need to buy direct from a sawmill.
Frenchy, that's a good tip.
I wouldn't care less if the shingle manufacturer wouldn't warantee my roof. If I layed the boards tight, they'd end up with a 1/2" gap. I'm sure dimensional shingles will bridge a half inch gap.
blue
blueeyedevil,
you do have to use 30# felt though when skip sheathing.. I differ in that I lay one pass at a time rather than all of the felt. Just my once of protection.. You see I attempt to dig my toes into the gap between boards. That can tear the felt so I only lay one pass at a time.. It may be a bit slower, but for scaredy old men like myself its a comfort. Since I'm working on the part I've laid my toes can't dig in and tear what I've already laid..
If you are doing the planning for your roof, order a little more boards than you would plywood that way you'll have enough wood to feel good cutting out any knots etc. and wind up with a little better roof. I found the local sawmill has no market for Tamarack , so I was able to specify Tamarck and since that has moderate decay resistance I wound up with a roof deck much more decay resistant than most roofs! You're right lay the boards tight and natural shrinkage will give you nice ventilation..
If you use those plastic insulation troughs and overlap the top one of the lower ones even if rain should sneak up past the shingles, past the builders felt it will fall on the plastic trough and be carried out past the building envelope..
Frenchy, I don't believe that felt is necessary on roofs. I lived in a house for 25 years that didn't have felt.
I've roofed a lot of roofs for myself and I didn't use felt.
If the water gets through the shingles, the felt sure ain't gonna help.
blue
I have heard felt keeps shingles from sticking to the sheathing. The shingles would be more likely to crack from expansion and contraction if you have skip sheathing without felt.
Just something I heard.
bump
I knew you had a little hack in you, Blue.I've torn off huyndreds of roofs and I can tell you for certain that that line of felt underlayment is all that made the difference between a simple re-roof and a major rot repair on a lot of them. When the shingles are worn out and the water starts to get in a bit here and theere, when you tear the shingles off you get to see the water signs, and you know 65that if that water had all been allowed to soak into the sheathing, you would be replacing it.And that is what I've had to do on a lot of roofs that had no tarpaper. It's there for a darn good reason. Stick to what you know in saving studs and owls. you are dead wrong on noit using underlayment
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Thank you, couldn't have said it better.
I thot you were just trying to start something;)
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ME!, never.....:~)
Here in tile roof country, SOP is felt between the wood and the tile. It's necessary, but not always sufficient. The cement tiles are porous, water soaks thru them to the felt. They mostly shed water, but some can splash upward between them.
Over time, it seems that the tiles must somehow move around a little. Tear them off carefully, and there are wear patterns where they were in contact with the felt. That wears thru and lets a little bit of water into the wood, makes it nice and tender, just the way the termites like it. The old solid sawn stuff may look fine from the inside, but it's eaten away from the top.
-- J.S.
I must confess I too skipped the felt,
instead I used I&W shield everywhere.
Hip roofs 4:12 and 5:12 pitch.
LOL
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Piffin, it's my house and I'll shingle it anyway I want too.
When I started in the trades, I watched hundreds and hundreds of roofs being laid without paper. So, I did what I saw. The houses are still standing. I'm okay with that.
So, where did the water end up that got under the shingles? It didn't find it's way through the paper with thirty thousand holes in it?
Anyways, if someone doesn't re-roof before the shingles start letting water in, they deserve a little rot.
blue
So, You've borne witness to hundreds of hack jobs that will cost their owners more to maintain through their life.it's a shame that so many think they have to chose the cheap and easy way to get by.on your house it doesn't bother me a lick.But when you go public recomending that it be done wrong, you've screwed the pooch. Peole read here expecting good advice, not how to shortcut their way to rotten houses.And yes, most of that water will be diverted off the sheathing and out, unless they let it go for years. And If the do that, you are right, they deserve a little rot, but your recommendation give a lot of rot to those who don't deserve it.
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" Plenty of roof and eave ventilation."
How much is "plenty"? That's pretty non-specific and subjective.
I'd suspect condensation problems from inadequate ventilation if most of the plywood was bad.
The only other thing that comes to mind is a batch of bad plywood. That's always a possibility...
There were 6 roof vents and at least 12 12x6 soffit vents- all unblocked. Two of the roof vents have been replaced with solar exhaust fans.