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Roof Sheathing: Overlay the old?

Jigs-n-fixtures's picture

One of the buildings I take care of was built as "Low Cost" housing in the seventies.  The roof is sheathed with either 3/8 or 7/16-inch plywood. 

It is now warped and wavy. 

I have it under contract to resheath and reroof.  The contractor has asked if it is possible to leave the old sheathing in place, and install the new 5/8-inch sheathing over the top of it.  He is arguing that the old stuff, is so flimsy that the new stuff will flatten it out, and it lets him keep the house fairly dired in during the process. 

What do you folks think?

roof sheathing (post #206928, reply #1 of 6)

7/16" plywood was usually the sheathing the code required, but anyone with a little common sense should know that it is too flimsy to be a proper material.  I have sheathed roofs with 5/8" material and realized I should use 3/4" plywood for proper rigidity and longevity. As for the renovation, the existing sheathing should be removed entirely, as it is a liability and will hinder the correct installation of the new plywood.  Any issues with underlying factors can now be properly addressed.  As  for the type of sheathing, it should be plywood. Waferboard has a built in life span and also has other disadvantages, such as it is much heavier than ply, it has a slippery and unsafe surface on one side, which is sometimes placed face up, a mistake if it causes you to slide off the roof.  It is very tough on saw blades and drills and also produces an annoying , fine sawdust when cut.  Add to that the fact that it is more difficult to nail into and you should conclude the only thing it has going for it is the cheaper price. Does the saying penny wise, pound foolish mean anything? Bottom line, the roof is the first line of defense against the weather, so it only makes sense to build it right, especially with the correct materials.

I see that you have never (post #206928, reply #5 of 6)

I see that you have never used Huber's Advantec.



Oh Well,

We did the best we could...

I don't like the idea of (post #206928, reply #2 of 6)

I don't like the idea of leaving the old stuff. If nothing else it leaves a place to trap moisture.


I don't agree with the other poster who was trashing OSB and suggesting 3/4" plywood. Using 1/2" OSB is fine - No need to go way overboard.

I'd have to be there..... (post #206928, reply #3 of 6)

but my gut feelings are  that 7/16 osb installed with thoughtful lapping over the old would be a good fix.That would include stripping the old roof bare among the details...........


Yeah, so long as the old (post #206928, reply #6 of 6)

Yeah, so long as the old sheathing's not rotten or delaminated I can't see that removing it is better than leaving it in place.  The new stuff should be lapped over the old joints, just like with flooring.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Generally, that would be my (post #206928, reply #4 of 6)

Generally, that would be my first choice. It has many advantages to the owner and the installer, as long as there is no rot in the old sheathing



Oh Well,

We did the best we could...