Decking Thickness


Insurance claim is getting processed on my roof. I have 3/8" that is 16" on centers that is in good shape. The roofer says that it is inadequate per new codes and I don’t necessarily disagree. It is straight, but having been up there, it doesn’t fell particularly stiff.

Sidenote…what is the 2015 code say about requirements for thickness?

Can the roofer just install the new plywood over the top to get up the required aggregate thickness in this case (i.e. add another layer of 3/8 plywood) or does the old plywood have to come off and be replaced with the code approved thickness for today’s code? Or can he just add another layer of the required thickness plywood or does the who roof deck have to come off?

Roof construction is engineered trusses.



3/8 plywood is normal where I live, but if it doesn’t meet code where you live than it doesn’t meet code.
If it’s in good shape and the installer is careful they can install on top, all joints have to break on trusses.


Thanks. Can the installer just add the additional layer or 3/8" to get to code or does he have to install a full 5/8 new sheet?

Still needs to use clips and landing on the trusses should be pretty easy.



I don’t know, you’d have to call your city to find out.


I have a call into them now. Thanks.

Happy Easter everybody!!!


If going to the expensive of labor to install new decking and are going over top of the the existing install 5/8, the material cost difference is Minimal and will end up with a far superior job. Nobody today is installing 3/8 for anything even if it was allowed.


APA still stamps 3/8" for 16" and 24" decking. I wouldn’t do it. If it still performs, leave it alone. If you want to install new sheathing, then install new sheathing, but tear off the 3/8. If you go over it, you will have backed out fasteners that push the 3/8 down, but do not penetrate it. I’ve seen a few in my 30 yrs and they all have that problem. Do it right.



I thought there was something in the 2015 IRC that specified decking thickness but I have to confess…finding a straight answer is damn near impossible.

That is a good idea about the fasteners pushing down the decking. I guess you could screw the sections together, but at that point its probably more work than just removing the old plywood.


3/8 plywood normally peels right off, alot easier just taking it off than fully removing all of the shingles, felt, and nails.


I would question live load vers dead load snow load ???