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.
Can I put new T111 on top of the old T111? Will it hurt to leave the old up there?
Hi Annette – My answer is a cautious yes! First, for those reading this who are unaware, T-111 is an inexpensive plywood siding that features a vertical shiplap pattern. Being a wood siding product, it needs a lot of maintenance! Weak points are usually at the overlap with sheets above it as the “Z” flashings between the boards may leak if not installed well, and cause the siding to delaminate and/or rot.
When adding new siding, its OK to leave the old siding in place and in fact may even be necessary as sometimes it is necessary to reinforce the framed walls below. Just remember that openings around windows and doors need to be extended outward to make up for the additional thickness.
Lastly, while T-111 siding is one of the least expensive siding products, it needs painting or staining every few years and is generally not nearly as durable as many others. If you are considering new siding, I’d suggest reviewing all options before proceding with a 2nd layer of T-111 siding.