Find out how to fix cracked plaster walls. Learn what a keyway is and get tips on repairing the wall with screening wire.
LESLIE: Well, in older homes with plaster , one thing that you’re probably going to see at some point is cracking.
Now, plaster can last a good 200 years and that’s pretty much as long as it’s been around.
TOM: Well, that’s correct. But to get to that ripe old age, it definitely needs some TLC from time to time. Joining us now is a guy with the knowledge to do just that: it’s our friend, Tom Silva , the general contractor from TV’s This Old House .
And Tommy, there’s hardly an old house that doesn’t have cracks in the walls. This is something that’s pretty normal but how do we stop it from getting so bad where those cracks start to develop into chunks of plaster that could rain down on our heads?
TOM SILVA: Well, you’re not going to stop the plaster from cracking. It’s an old house; they get a lot of movement. A windy night, the house is shifting around. You get temperature changes, expansion and contraction. It’s going to crack.
But how do you fix a crack is a different situation. You can net it, go over it, glue it back to the lath.
TOM: Lath, mm-hmm.
TOM SILVA: Because the keyway behind that plaster wall will break from the vibration of the house, from the wind and the movement.
TOM: Now, let’s talk about that. You mentioned the keyway; I think that’s important to understand. Because old homes typically have wood lath, so that’s wood sticks and I think they look like tomato stakes. And they’re attached to the wall and then the plaster, when it’s first put on, it pushes through that lath and then spreads out and sort of locks behind it. So that, in effect, is the key.
TOM SILVA: That makes the key, yeah.
TOM: That’s the keyway. And those keys actually wear over time.
TOM SILVA: Yeah. Because of – the house is shaking. Think of it: on a windy night, that house is moving. Or if your kids are playing and you’re bouncing on the floor, the house is moving; it’s shaking. I mean you can see it if you’re sitting in a room and knew someone who walked across the room or feel it, you can actually feel the – well, think of – wind will do the same thing on your side walls.
TOM SILVA: So you’ve got to basically know now what you’re going to do, how bad is the crack and how do you fix it? You could simply, in some cases, just drag a little – if you had like a little screwdriver or old beer-can openers with the V-notch, you can drag a little bit out and you could put some plaster or patch in the wall and then paint it; it’d be fine.
TOM SILVA: But if it’s really bad, you may have to cut a piece of the old plaster out and insert a piece of wallboard into the plaster laths. In that case, you’re going to need a thinner piece, like 3/8-inch, because you don’t want to be too thick. And then you can feather it out around it with a piece of – with some joint compound. But I always like to take a piece of screening wire and cut it much bigger than my patch and blend it right into the wall and then hide it that way.
TOM: So the screening wire is kind of like that perforated drywall tape that we have today, right? It’s sort of a …
TOM SILVA: That’s right. You get – what I sometimes – what I’ll do is I’ll go to the hardware store and I’ll get a roll of plastic – what do you call – vinyl screening wire.
TOM: Window screen.
TOM SILVA: Window screen.
TOM SILVA: And I can have a big piece, so I can actually do a whole wall.
LESLIE: So are you almost creating a netting in the event of delamination?
TOM SILVA: Yep. Yep. Yeah. And that netting will bridge any gaps and that netting gets stuck onto wet drywall. So the easiest way to do it is – if the plaster is really loose, you put these plaster buttons in and you can fasten it back to the wall or you sometimes …
TOM: So that’s kind of like a washer, almost, that pulls it back in.
TOM SILVA: Yeah, a perforated washer, right.
TOM SILVA: And you screw it down. You screw it in gently because you don’t want to break the big – you don’t want to break out a chunk of plaster so you defeat your purpose.
TOM SILVA: There’s also a product that you can glue the plaster back to the lath by drilling a series of small holes next to the crack and inject it with a caulking gun.
LESLIE: And sort of injecting it in?
TOM SILVA: And that basically is an acrylic adhesive that you have to wait overnight. Use these big plastic rings and you screw the plaster gently back to the lath. And you’ll see the adhesive come out all the little holes that you drilled. You leave it for 24 hours, sand it lightly and then you wire it or tape it, go over it with joint compound.
TOM: Now, let me ask you about the joint compound, because these are originally plaster walls. You can get plaster and mix that up, you can buy standard spackling and joint compound out of the bucket. Does one do a better job than the other when you’re going over old plaster?
TOM SILVA: No. I’ve actually taken – joint compound is amazing stuff; it’ll stick to anything. I know; I’ve got it on my shoes.
But no, it’ll stick to anything; it’s fantastic for that kind of stuff. So you don’t have to worry about doing too much scraping and sanding. It won’t stick to dry – I don’t – I wouldn’t go over wallpaper or anything like that, because the wallpaper will delaminate.
TOM SILVA: But lots of times when I’m trying to patch an old plaster wall and I’m worrying about it sticking, I actually take plaster – dried, powdered plaster – and mix it with my joint compound. Now you’ve got two things that are going to basically dry up differently. The plaster is going to set up and harden.
TOM: A lot quicker, yeah. Mm-hmm. Yeah.
TOM SILVA: Yeah, a lot quicker. So you want to make sure that you use a faster-setting joint compound or you can use the pre-mixed. A faster setting means that you can get a 20-minute, a 45-minute or a 90-minute. It comes in a powder; you mix it with water or you mix it into the water. And then you can mix your plaster into that.
So you – now you’ve got great adhesion, you’ve got a material that will go on easy and your plaster will dry harder than the joint compound. So the problem is is you’ve got to make sure that you can sand it. So you may have to sand it smooth if you don’t trowel it off.
TOM: What a great trick of the trade. Tom Silva from TV’s This Old House, thanks for stopping by and helping us keep those old plaster walls in great shape.
TOM SILVA: My pleasure.
LESLIE: Alright. You can catch the current season of This Old House and Ask This Old House on PBS. For your local listings, some great step-by-step videos on this project and others, visit ThisOldHouse.com.
TOM: And This Old House and Ask This Old House are brought to you by GMC. GMC, we are professional grade.