Thursday, June 20, 2013

Plaster Repair

We started the long process of repairing the plaster and lath in the stairwell and upstairs hallway. The old plaster has come loose from the lath in many places as the house has settled, shifted, warmed, and cooled over the last 150+ years. Sometime this movement leaves a crack as evidence other times it takes a tap or gentle press to find the troubled areas.

Over years of dealing with these old walls we've developed a little technique that seems to work for us. We work our way around the wall with our hand gently tapping and pushing on the wall listening and feeling for signs of movement. If we find a spot, we mark it with painter's tape. Once we have an idea of all the areas needing attention we'll have a good idea of the quantity of supplies needed for the repair job.

In the picture below there was a spot underneath the light switch that moved easily with a tap. This is a bad location to have a loose wall since it's in the line of sight and could easily be bumped, causing a crack in any new paint.

Here is the process we use:

First, drill a hole with a Phillips head bit. Apply firm pressure at first to break through any top coat then ease up a bit to grind out the rest of the sandy plaster. It's important to ease up on this first hole since you might be between the lath and don't want to suddenly punch through any further than you need to.

If you hit the lath you usually end up with a fairly round hole. If you missed the lath behind the wall your drill may go into the wall a bit further than expected; try another hole about 3/4" up/down or to the left/right of that location depending on which way the lath runs. In the wall I'm working on, the lath slats run horizontally. Once you hit the lath you can estimate where your other holes should go keeping in mind that lath is typically about two inches wide with about a 3/8 inch spacing between courses. Sometimes it's a little guesswork finding the lath, other times there are sections exposed that give you an idea of the location and spacing.

Keep pressing around the area to see how far the loose plaster extends. Drill/grind into the loose area trying to keep the holes on the lath. Try to avoid getting too close to the edges of the wall. The holes on the left of the photo below are about as close as you want to get, the very top hole near the light switch was actually too close to an edge and ended up cracking off a piece of the plaster around the switch hole, oops.

After you've made all the holes you feel you need it's very important to vacuum them out, a shop vac works great. Move back and forth over the holes to get all the loose debris out of the way. Give a gentle press on the wall while vacuuming to free up any loose material. If the wall is very loose or crumbling use caution, sometimes the vacuum may suck chunks of the wall off. If a piece does come off easily while you are vacuuming, don't worry, that piece probably wasn't going to hold well and probably needed to come off anyhow.

The next step involves squirting some goo behind the plaster so it spreads onto the lath. We've had good results with the stuff pictured below. "PowerGrab" is very inexpensive and a bit watery compared to other construction adhesives. I don't think I would use it for anything heavy duty but the fact that it's a bit thin is actually a benifit for this application. It needs to be thin enough to slide into the nooks and crannies between the plaster and lath yet cure with a reasonable hold.

Some attention should be given to how the tip of the tube of goo is cut when opening it. You want a cut straight across making the cross section small enough to fit in the hole yet large enough to seal itself on the edges of the hole without pressing itself closed against the lath. Sometimes the edges of the tube ever so slightly crush the edges of the hole forming a nice seal.

Once everything fits inject some adhesive into the holes. Depending on the condition of the wall you may get a few different results.

- You confidently squeeze adhesive into the hole and it erupts right back out. Stop, adjust the nozzle, it's not sealing properly on the hole.

- The wall may bulge or heave slightly. You can fill it a little more but don't push your luck, you may be breaking the wall even more.

- You keep squeezing and don't feel any back pressure. You are probably in between the lath and pumping adhesive into the abyss, stop.

- You feel firm back pressure and might see some goo spreading to other holes or out the side of an adjacent crack. This is what you want to happen.  Once you see this, stop pumping the gun but keep the pressure on for a moment before releasing and backing out of the hole. The adhesive should slowly creep a bit further.

Make your way around the loose spot injecting adhesive through the the holes where you hit the lath. Wipe off any excess that gets on the wall.

After injecting the adhesive you'll need to anchor the plaster down to the lath. Anchoring it will tightly sandwich the adhesive between the two materials. There are lots of things you could use to anchor; I've used metal sheets, plastic scraps, sticks/shims, milk bottle caps. Your anchor should be fairly rigid to spread out the force yet have enough flex allowing you to fine tune how tight it's screwed to the wall without crushing it.

Here I've used some milk jug caps for a small trouble area.

In the pic below I'm using a sturdy paint stirring stick. I have a pre-drilled hole in the stick, I put the screw through the stick and drive it into one of the plaster holes that's central to the area I'm trying to secure.

I shimmed up the ends to spread the force out a bit more.

You may notice some of the adhesive squiring out of the hole as you tighten down the anchor, this is good, especially if it's oozing out of adjacent holes. Don't be overly concerned that you might be gluing your anchors to the wall and they may rip off more wall when you try to remove them; because the adhesive isn't heavy duty this rarely happens. I can usually remove the anchors after a few hours but I frequently leave them overnight with no issues. Just take it slow when peeling them off and use a putty knife if you have to, although I never have.

After you have the holes injected and the anchors in place stick something into the exposed holes to clean them out a bit; I use the cap from the tube of PowerGrab. This is done to keep the holes from filling to the brim with adhesive. The hole should be mostly empty so it can accept a fair amount of patching compound. The purpose was to fill in the void between the plaster and lath, not fill up the hole.

Cracks in the plaster get a little extra attention. I gouge it out slightly using a phillips drill bit on a drill. With the drill tilted slightly I run the bit along the crack to grind it out and open it up.

Holes are made on both loose sides of a crack, again, try to land them on the lath. Try to span the crack while placing your anchors.

Once the adhesive has setup remove the anchors. Fill gaps, craters, and gouged out cracks with patching compound and feather out with a skim coat. The filling and feathering on rough sandy plaster is a bit of an art form, a topic for another blog post.


  1. Really your blog is very interesting.... it contains great and unique information. I enjoyed to visiting your blog. Its just amazing.... Thanks very much

  2. I am glad you have shown the images from the working place itself, its gonna be more helpful for all. And I must say an well explained post on plaster repair it is.