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size of roof sheathing

mikemjm's picture

I'm getting ready to do a roof and I've noticed a lot of the roofs in the area have "dips" between the rafters or trusses, which I would guess is from using too thin of a plywood or OSB sheathing. My addition will have trusses 24" O.C. with a 8:12 pitch on the main roof and a 5.45:12 pitch on a shed dormer, both with asphalt shingles. I'm in the Mid-Atlantic region so snow load is a consideration, too. The lumberyard spec is for 7/16" OSB. From everyone's experience, is that sufficient or should I up the size to 1/2" or even 9/16"? I'll be using "H" clips between the trusses.

Thanks in advance for you input.

(post #101700, reply #1 of 8)

Most of us use 7/16" OSB for wall sheathing. Every manufacturer of OSB, I've worked with, requires full support (blocking) on all edges of the 7/16" sheathing. We don't consider OSB to be a suitable choice for roof sheathing. H clips do not meet the edge support requirements. The better builders will only use 5/8" CDX plywood for roof sheathing, some use 1/2".

Industry is always offering new products. Many builders use an OSB like product on floors, such as Advantech. This is a product that may look like ordinary OSB but is quite different. It's too heavy to handle on a roof and too expensive. There may be some new OSB roof sheathing products that I haven't used but ordinary wall sheathing would be a poor choice. The builders that use 7/16" OSB for roof sheathing are the ones to avoid.

Beat it to fit / Paint it to match

Beat it to fit / Paint it to match

(post #101700, reply #2 of 8)

I use 5/8" CDX plywood with H-clips. If it were available out here I might use 5/8" Advantech T&G.


7/16" OSB on a roof would flex way too much under my weight. That's the bottom-of-the-barrel material to use for a roof deck.

(post #101700, reply #3 of 8)

5/8" CDX Ply as a minimum for any roof. The clay tile roof we did on the last job used 3/4", as required by the roof tile manufacturer.

(post #101700, reply #4 of 8)

That answers my question. 5/8" CDX plywood, at a minimum, appears to be the way to go.

Thanks.

(post #101700, reply #5 of 8)

Why CDX?  Consider Struct #1, especially if wind or seismic considerations apply.  It can also be less expensive. 

 


 


-- J.S.


 

 

 

-- J.S.

 

(post #101700, reply #6 of 8)

Around here 1/2" is pretty standard with roof trusses 2' O.C.

I've never seen any of the dips you talk about.

The first precept was never to accept a thing as true until I knew it as such without a single doubt. [Rene Descartes on scientific method]

(post #101700, reply #7 of 8)

If you don't want to see the rafters/trusses in a couple of years, don't put them 24" oc. The roof I just framed is 16" oc with 5/8 osb on it. In this case, I believe that slightly over building is worth every penny. I'm certain that knowone will look at that roof and say "who in the heck produced that mess." Good luck.

(post #101700, reply #8 of 8)

mike... we used to use 1/2" when the framing was 16" oc


and 5/8" when it was 24" oc ... in both cases we used  H-clips


now we use only  5/8" Advantech  tongue & groove..


this eliminates all the extra blocking requirements we have in our wind zone  ( 110 mph )


there is also a product made by CanPly which is a  modern T&G in a 1/2" plywood


if i could get that i would use that on any of our 16" OC framed roofs


i might even use it on some of our 19.2" OC frames.. but i would continue to use the 5/8" on the 24" oc roofs


Mike Smith Rhode Island : Design / Build / Repair / Restore

Mike Smith Rhode Island : Design / Build / Repair / Restore

              www.mfsmithbuilder.com