I’m building a porch and have framed the shed roof with 2×6’s spaced at 24 inches (span is 7’6″). As per code in my area (Ottawa, Canada), I plan on using 3/8″ plywood for sheathing. The roof covering will be asphalt shingles.
I just read somewhere (actually in Taunton Press’ Roofing with Asphalt Shingles book) that one should consider using sheathing thicker than code to avoid sagging between rafters.
Any framers/roofers with opinions on whether 3/8″ plywood will sag in this application? The pitch on my roof is 3.5″ in 12″. And naturally being in Canada, the roof will see lots of snow in the winter!
Thanks in advance.
Replies
Ours is half inch EGP in LA. I think 3/8" is too thin.
Boris
"Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1934
I am in upstate NY ski country, and they won't let us sheathe roofs with anything thinner than 5/8".
ez,
My house is 1/2" with trusses on 24" centers and it still has sagged some in the 25 years since the house was built. My roof is 4 x 12 pitch. For a small porch like yours, I would almost step up to 5/8" (if available) or thicker.
Just my DIY opinion,
Bill Koustenis
Advanced Automotive Machine
Waldorf Md
3/8 plywood
24'' O.C rafters
3-1/2 on 12 pitch
Lots of snow
Definately will sag!
Unless you put blocking inbetween the rafters.
>> "blocking"
Yes, the code does call for blocking or H clips (otherwise the book I'm looking at says 1/2" for "unsupported" panels). I plan on using H clips - should have said that in my original post. Still thing 3/8" is too thin?
3/8" is a joke. How much are you going to save over using 5/8"?
1/2" will sag over time too on 24" centers.
Joe H
OK. 5/8" it is. I feel much better about that.
Thanks for all the replies guys. First time on this forum - what a great resource!
Take it back and exchange for thicker material. Double layer of 3/8 does not solve the problem.
Whenever you are asked if you can do a job, tell'em "Certainly, I can!" Then get busy and find out how to do it. T. Roosevelt
>> Double layer of 3/8 does not solve the problem.
If the two layers are glued and stapled together it does.
Of course that's the slow, annoying, and expensive way to solve the problem.
Edited 7/6/2004 10:56 pm ET by Uncle Dunc
Yes, but getting proper contact between the layers, using the correct glue, and holding it until the glue sets, is much more difficult than putting up one sheet of thicker material.
Whenever you are asked if you can do a job, tell'em "Certainly, I can!" Then get busy and find out how to do it. T. Roosevelt
True, and if you don't get it stuck together right, you've made a lovely home for moisture and microbes.
Since the rafters are 24" oc, pencil out the cost of in-betweening them to 12" oc vs. thicker ply.
If they're exposed on this porch, appearance is another consideration for going 12" oc. Visually that makes them more of a mass than individual details.
-- J.S.
I thought about "in-betweening" to get 12" on centre. given that 2x6 may be a bit light for a 3.5 in 12 slople spanning 8', maybe that is a good solution.
Thanks for the responses guys! I just double-checked my book and it does say 3/8" plywood (or 7/16" OSB). Of course, I might be reading it wrong since I'm not a professional builder ;)
I agree that 3/8" seems too thin. Since I have the stuff bought already can I just put two layers on the roof. Or should I do the right thing, return it to the store (I think Home Dept will take it back) and buy some 5/8" sheathing? The nice thing about doubling up would be that it would be easier for me to pull up the sheets given that I'll be roofing the porch myself.
Emmanuel
No, you didn't read it wrong; I checked the code too--Table 9.23.15.6.A says 9.5mm PLYWOOD and OSB O-2 GRADE, with edges SUPPORTED (12.5mm unsupported). My experience says this is not sufficient.
I pulled my crew off a job being run by another GC because he wanted us to sheath a roof almost exactly the same as the one you describe with HALF inch OSB--but over a heated living space. I warned the client their GC was cheaping out on them (I had a separate, direct relationship with that client as GC/Design/Build on another building on the same lot and was the HO's designated framing and roofing sub for the main house) but by the time the client got up to the site, another crew had been brought in and the stuff was on so the HO elected to leave it.
It sagged over an inch and a half between trusses after one winter. It will have to be replaced in another two or three years....
I'm in Mont-Tremblant; we have higher snow loads than you do in Ottawa, but the difference isn't a lot and your snow is heavier since it's wetter.
For 24 OC with a 3.5 in 12 roof, you need MINIMUM half inch PLYWOOD; 5/8 is better. If you go for OSB, you should get O-2 grade and it HAS to be 5/8.
Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
i think it is laughable that the codes there even allow less than 1/2". For the 224" spacing, I would want 5/8"
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24oc?
my code allows 1/2 ...
I don't.
5/8th here ... especially if I have to walk on it ......
huge difference between 1/2 and 5/8th.
Jeff
Buck Construction, llc Pittsburgh,PA
Artistry in Carpentry
I`m in NY here...last I checked 1/2" was code....haven`t followed it in years.....I don`t use anything less than 5/8" minimum....3/4" on my own house and the more discerning customers.....24" centers? I might consider 1"....then again, I`ve never built anything spaced that far apart.
EDITED TO ADD: 2 x 6 rafters? I`d probably kick them up to 2 x 8 as well....aint much of a pitch there.
J. D. Reynolds
Home Improvements
"DO IT RIGHT, DO IT ONCE"
Edited 7/7/2004 12:22 am ET by JAYBIRD
Some builders use 3/8" down here.I stepped thru the sheathing twice in one day before I learned to walk on the rafters only.it always amazed me how cheap a buider would try to be on a 400,000$ home.
ANDYSZ2
I MAY DISAGREE WITH WHAT YOUR SAYING BUT I WILL DEFEND TO THE DEATH YOUR RIGHT TO SAY IT.
Remodeler/Punchout
The Ontario bldg code; 24" oc, edges supported = minimum of 3/8" Plywood..... use a minimum of 1/2" plywood if edges unsupported.(support= H clips) For the great amount of snow we get here in central Ont, Canada, I stongly suggest using the thicker material, even a 5/8" osb is less costly than plywood.. Good luck Hube
in snow country 5/8 for sure - and one other thing - try to find 5 ply (country plywood) vs 4 ply (city/Home Depot plywood) 5/8. 5 ply is stiffer, less likely to warp, more stable w/temp changes, etc.
e2canoe, Is this an exposed rafter ceiling,where you will see the under-side of the roof sheathing? If so, use 3/4" ply for your roof sheathing, that way 1 1/4" roofing nails will not penetrate the underside of the sheathing and be exposed on the porch side, They're not that nice to look at!!
Geoff
play it safer and use 7/8ths. nails
Spheramid Enterprises Architectural Woodworks
Repairs, Remodeling, Restorations.
No. Not exposed. Ceiling joists are installed.
I think I'll just add some more rafters to make it 12" on centre. Given 2x6 and 3-1/2 on 12 pitch, I think that would be stronger. Keeping 24" on centre and putting all that dead weight of 3/4" (someone suggested 7/8" and even 1"!) plywood on top of the 40 PSF snow loads in this area is making me nervous.
Besides, I'm not young anymore and don't want to be pulling those heavy sheets up there! ;)
E
it matters like this.... the thicker the sheathing, the earlier us carpenters get retired.