Should I put new plywood ontop of old, or rip and replace?

superfly · 07-09-2009, 02:47 PM

I'm having my tar and gravel roof replaced, and as part of the contract I was having my old 3/8 plywood removed and replaced with 5/8. Turns out the old plywood is still in good shape ie no rot at all. The roofer would like to just put the new plywood on top of the old one. I understand it's easier for him--is there any downside to this for me? As a possible negative, the roof will be heavier, but will a 3/8+5/8 (i.e. I inch roof) be any better than a 5/8?

**Ed the Roofer** · 07-09-2009, 03:01 PM

I think that by leaving the old sheathing on, there is too high of a risk for the new decking to get improperly nailed into the rafters correctly, even with snapping chalk lines.

The old rafters may not run true and could be twisted slightly and the framing nails could be on the mark of the chalk line, yet still miss hitting center.

Also, without the roof already being torn off, how can you possibly know that there is no decking delamination or structural instability, which would telegraph through and sump the new decking as it conforms to the existing decking shape.

3/8", in my opinion rarely survives more than 20-30 years without significant deterioration and defects.

Ed

superfly · 07-09-2009, 06:32 PM

Thanks for the quick reply Ed. They're ripping off the old stuff as we speak. After ripping some of the stuff off, it didn't look all that great, so it was definitely solid advice!

**Ed the Roofer** · 07-09-2009, 11:37 PM

Quote:

Originally Posted by superfly

Thanks for the quick reply Ed. They're ripping off the old stuff as we speak. After ripping some of the stuff off, it didn't look all that great, so it was definitely solid advice!

Once in a while, I guess right.

By the way, you are welcome and good luck on the remainder of the project.

Ed