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OSB and Skip Sheathing

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March 13, 2023 at 4:41 p.m.

alextolle

Rod from San Jose asked:

Doing a reroof. Changing from 45 yr old cement tile roof to a composition shingle roof for solar installation. Currently, there is no OSB/plywood under tiles. Tiles hang on skip sheathing. In attic looking up, you can see bits of daylight. Seems like all the roofers want to take off the cement tiles and put the OSB decking on top of the skip sheathing. I see no problem with that except for the area above the rafter tails. What is on top of those rafter tails is 1/4 in plywood. It is nailed onto the top of the rafter and extends about 2.5-3 ft up the rafters. This is installed all around the roof on the outer edge. So the new OSB will be nailed on to 1/2 inch skip sheathing boards. But when you get down to the last few feet of rafter by the rafter tails, that OSB will not be resting on that 1/4 in plywood (because its not 1/2 in thick. There will be an air gap. I point this out to the roofers and say no problem. They say the OSB will just bend down to meet the existing plywood and the nails will hold it down. I think that can turn into a problem. That OSB will have a bend and be under tension applying a force always wanting to pull the nails out and over time it will raise up. Should they either replace all that existing plywood with a similar thickness of the skip sheathing or add a thin plywood on top to raise the surface up to the 1/2 in height? None of the roofers see this as an issue. Should I make them change it or are they correct? Also this bend would be occurring over about a 2 or 3 ft length of OSB and that's a lot of bend in a short distance. Also. I would think you could see this bend / slope change when the shingles are on. Help!

March 13, 2023 at 4:41 p.m.

alextolle

Here is what John Stout from Go Roof Tune Up said:

We understand that the original roof is Concrete Tile laid over Skip Sheathing. You want to install proper sheathing over skip sheathing. There is a concern of the lift at the transition of the overhang which now has sheathing. Solutions could be to (a) Remove existing sheathing at the overhang (b) Install a cant of some kind to eliminate the transition.

The ¼ inch transition between the overhang and the main roof should be unnoticeable. If it is noticeable, install ¼ inch sheathing over the entire overhang area for a smooth transition.

As a test, lay out some sheathing and lay your Dimensional Shingles out. This should cover the ¼ inch difference to make it unnoticeable.

We sincerely hope that this info will help you make a decision that is satisfactory.

March 13, 2023 at 4:42 p.m.

alextolle

Marty Stout from Go Roof Tune Up said:

From the information provided the best solution is, “add a thin plywood on top to raise the surface up to the 1/2 in height” is the correct approach.

March 15, 2023 at 7:25 p.m.

alextolle

Here is what John Kenney from Cotney Consulting Group said:

Rod,

I recommend either adding a ¼" layer to even it up or removing the ¼" existing and adding one layer of ½" to meet the skip sheathing height.