Q&A – Changing from Cement Tile to Composition Shingle and I Have Questions

This California homeowner is getting ready for a reroof and has some concerns about the OSB and skip sheathing and turned to the experts for answers. 

Rod, a homeowner in California, is concerned about a potential air gap between the OSB and plywood near the rafter tails as they prepare for a reroof and asked: 

Doing a reroof. Changing from 45-year-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-inch plywood. It is nailed onto the top of the rafter and extends about 2.5-3 feet 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-inch plywood because it's not 1/2 in thick. There will be an air gap. I point this out to the roofers and they 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-inch 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-foot 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! 

Our resident roof expert, 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. 
 
Hope this helps! 

Also from Go Roof Tune Up, Marty Stout, 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. 

Another roofing expert, John Kenney from Cotney Consulting Group, had this to say: 

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. 

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