Double decking for strength

I am in the Houston Texas area. My house was built in 1978. A roofer told me that code for rafter spacing in my area (Deer Park) is 18 oc. Some of mine are much more, like possibly 28 in a couple of places. The roof was replaced less than 10 years ago (I think), has 3/8" decking, is wavy and leaking. The roofer recommended that we remove the shingles, replace any rotting 3/8" decking (with 3/8), and then add a layer of 1/2" decking over that. He said the rafters would support it ok, and it would work. Is this a sound recommendation?

I see nothing wrong with that suggestion. I would probably handle your situation in the same solution. 3/8 is not strong enough for a roof deck no matter of rafter spacing imo.

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Thank you. What about the spacing of the rafters? Should I be concerned about this?

No, I honestly wouldn’t be too worried about it. In my area I am code for new buildings is 24" but is fine to leave as is as long as you are not altering the structure.

Sounds like a good plan, might want to use 5/8 plywood instead of 1/2 for extra 2$ a sheet would add more strength for minimal cost

I wanted to throw this out there. I wouldn’t recommend putting decking over sagging 3/8 or 1/2" decking. Especially on those wide centers. I’ve run across 2 roofs in my 30yrs of roofing that I had to remove the 2yr old roof that someone did this on. What happens is when you put the shingles on, you run the risk of the fastener not penetrating both sheets. It ends up pushing the bottom sheet down, cresting a gap between the sheets. Both homes I worked on had hundreds of fish mouths from the fasteners backing out, because they were under pressure from the bottom decking. I would recommend removing the decking and installing new. It always amazes me that 3/8 plywood carries a decking stamp at 24" OC. Just my experience. I would not do it.

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