Adding New Siding Over Old T1-11 – Do I Need to Remove the Plywood?
LESLIE: Dennis in California is on the line with The Money Pit. What can we do for you today?
DENNIS: Yes. I have a house that was built in 1979 and it has T1-11 siding on it and I’m wanting to change the siding on it. I want like a cement-board lap siding but my question is: is it practical or feasible to go ahead and put my new siding on top of what’s there? Or will I be sandwiching in some problems?
TOM: Well, for those that are unfamiliar, T111 is essentially plywood siding and it serves two purposes: it’s the siding and it’s the sheathing. So you do not have to remove that. Now, the downside is that you’re going to have pack out, so to speak, around the windows. The trim will – the windows will be a bit deeper than perhaps you’ve seen in the past but that siding can stay just like that.
You can put Tyvek over the the existing T111 siding and then add your HardiePlank over that. Just follow the manufacturer’s installation instructions. But there’s no reason for you to pull that plywood off because frankly, if you did, I’m afraid that you would have to replace it with just regular plywood and there’s really no point to that. The T1-11 serves a structural purpose, as well as keeping the water out of your house.
DENNIS: Oh. That makes sense. I didn’t think that it actually takes care of the shear, doesn’t it?
TOM: It does. That’s right. It protects it against the shear and the racking forces.
DENNIS: Makes sense. OK. Great. Now I have a direction to go. My concern was is that if I put the solid – if I sandwich something in, was I sandwiching in some moisture or anything like that? And I didn’t want to create problems down the road.
TOM: Yeah. Well, let’s hope not. If you use good siding on top of Tyvek, I don’t think that’s going to be an issue.
DENNIS: OK. Great. Well, thank you very much.