Image of plaster repair in progress

How To Repair Lath & Plaster Walls

After numerous decades of faithful service, the old lath and plaster walls and ceiling in your home have started to fail.  Sometimes this exhibits itself as a section or sheet of the wall / ceiling loosening from the underlying lath.  Sometimes a section of the plaster has fallen off completely.  The question of how to repair plaster walls tends to arise, nervously, around these times.

So, what gives with this gravity challenged plaster & how do you deal with that?  That’s the focus of this tip.  Standard little nicks, picture hanging holes, small holes etc. can all be mended in a similar fashion to your now-standard drywall walls.  That’s not what we’re discussing here – we’re talking about when big chunks of plaster have either become detached from the lath, or are about to fall off altogether.

The Trouble Signs:

You can tell when sections of your wall or ceiling are about to fall off when you see areas that are protruding or bubbling out from where they were, sometimes with cracks or ridges along the outlines.  When you gently push on those areas, you find ‘soft spots,’ where sheets of plaster seem to be floating independent of the rest of the solid wall below.  Why does it feel that way?  Because they are!

This is a situation that is best dealt with sooner than later – it’s not going to improve by itself, and it’s quicker and easier to deal with at an earlier stage.  If those bits of plaster haven’t actually fallen off the wall  yet, it will be easier to fix now than in a little while, when they have actually fallen off.

A Quick Study in the Anatomy of the Plaster & Lath Wall:

A very brief summary of the construction of plaster walls & ceilings helps give some background to the situation.  Before the advent of large convenient sheets of drywall, walls were hand plastered.  Stud wall frames were built, in a similar manner to today’s framed walls.  Today, the wall cavity may then be outfitted with electrical cables, plumbing lines, HVAC ductwork, network & home automation wiring, insulation, vapour barrier, etc.  Back in the days of plaster, you’d get your electrical (knob & tube?) & plumbing in there, but usually nothing else at this stage.  Maybe some ducts, but that’s really all beside the point!

As a base for the plaster, lath was then nailed horizontally across the wall studs.  Lath is simply strips of wood, rough cut, generally around an inch to an inch and a half wide, by around a quarter inch thick.  These were nailed to the studs with gaps of around a quarter inch between them.

Image depicting plaster keys in lath
Plaster ‘keys’ in lath

A thick first coat, or ‘scratch coat’ of plaster was then applied to the lath, squeezing through it so that some of the plaster oozed through the gaps between the pieces of lath and drooped down behind the back of the lath a bit.  When it hardened, this plaster between and behind the lath created ‘keys’ that held the plaster in place.

Then later they might apply a second base (‘brown’) coat, and they’d certainly apply a thin top coat of very hard plaster to finish the wall.

So far so good, right?

Well, sure … but … Over the decades, the walls get bumped in to.  Kids jumped on the bed & the floors, and they also banged lots of stuff in to the walls.  The adults did a little bit of that as well.  The house settled a bit in to it’s foundation.  There were a few minor tremors, both natural and man made when they replaced the road and the sidewalks.  Etcetera and so on and so forth.

Image depicting broken plaster keys in lath
Broken plaster ‘keys’ in lath

The keys holding the plaster on to the walls slowly started cracking, and the sheet of plaster once held so nicely to the lath by the plaster keys started to become a little more free-standing, then it started to crack and became a little more free and a little less standing, falling away from the broken keys.  Now you’re staring at it saying “I’m pretty sure this doesn’t look exactly right any more.”

So that’s the background, but the real question is how to remedy the situation!

Easy.  Levitation generation.

Or, if you lack that capacity …

Fixing your Walls

Where the plaster has already started coming off in slabs, then you carefully remove any bits that are still hanging on but are beyond saving.  Sometimes there are pieces that are loose, but not loose enough that they can’t slide back nice and tight against the lath again – bigger pieces of those are likely worth saving, but smaller ones can go – it’s a bit of a judgement call based on how much is gone, how much is capable of being saved, how big of a piece you’re considering trying to save etc. – a bit more guidance follows below … but for now, the main trick:

You take standard coarse threaded drywall screws (usually 1 5/8″) inserted in to special plaster repair discs, and carefully secure the plaster that’s still capable of being saved by screwing through the plaster in to the lath, thereby squeezing the plaster snugly once again against the lath with the disc.

If none of the plaster has come off yet, and you can push the loose section back tight against the lath, then you gently do that and then secure it wherever loose with the screws and discs.

If, in screwing one of of the discs in, there’s no bite and the disc doesn’t snug up nicely, odds are that you’ve hit the gap between pieces of lath.  Remove the screw and try again, up or down around 3/4″

If the plaster hasn’t yet fallen off your wall but has progressed to a point where it can’t be pushed back tight against the lath, then you secure around the affected area in the areas where you can get it nice and tight, and then break out the part that won’t cooperate.

When the plaster has progressed to a point where it has become separated from the wall, bits of plaster breaking off from above can fall between the sheet of plaster and the lath.  This prevents the ‘sheet’ of plaster from being able to be pressed back against and re-secured to the lath, and any such section has to go.  You want to get some screws & discs in the good areas to snug it up and prevent plaster being able to fall in there too, before breaking off the rest of the loose bits.  Otherwise you sometimes just make a bigger & bigger & bigger hole.

So What Exactly Are these Plaster Repair Discs and Where Can I Get Some?

I get mine from Lee Valley Tools here:  Plaster Repair Washers from Lee Valley

They’re available elsewhere as well, but for some reason they’re not as widely available as you’d think.  Anyway, it’s always great to have another excuse to have to go shopping at Lee Valley again.  (No affiliation btw).

After Stabilizing the Good Sections of the Wall

Once you have all the remaining plaster nicely secured to the lath with the screws & discs in the affected area – all around the perimeter of the gaping holes where they exist – and enough to have eliminated any soft loose spots where the plaster remains – you can clean away any remaining bits of loose plaster and trim any protrusions above surface level from cracks etc. with a utility knife.  You’ll then want to dust out and vacuum all around the edges and get things nice & clean.

Image of plaster repair in progressSo, you should by now have what’s left of the damaged area attached nice & securely to the lath.  You also have a bunch of those discs clashing with your general decor.  Also maybe a gaping hole or two of exposed lath.

If you’re left with bigger-than-small holes of exposed lath, you’ll want to cut a piece of drywall to fit the hole, then screw that to the lath (& studs where possible) using just the drywall screws (no discs necessary).  Half inch drywall is usually good, but use thicker if that will line up better with the finished plaster surface.

It’s a good idea at this stage to thoroughly check for other damaged areas in the room  – these things often travel in packs.  You’ll be doing some plaster repair & repainting, so best to get it all now.  You can check for other ‘soft spots’ by gently pressing against the plaster with your fingers – you’ll get the feel of a solid wall vs. a soft & loose spot pretty quickly, if you haven’t already.  Usually if you find a few soft spots at this stage, that you haven’t really noticed before, they’re early enough along that a few screws & discs will cure them without creating too many more huge gaping holes.


Image of plaster repairedNow it’s time to have some fun with some plaster.  You’ll want to get your hands on some CGC Durabond joint compound & fibreglass tape, to go with your drywall tools … hmmm … I don’t think this tip is going to extend to the whole “how to finish drywall” thing … but suffice it to say that Durabond is a better choice than many other drywall compounds for your first couple of coats here.  It’s more … durable … and more compatible with the old plaster.  You could go with a plaster of paris, but you’re sacrificing a lot of working time with that.  Durabond works very well in this situation.  What you need to be aware of with Durabond is that it’s TOO durable to sand easily, so just use it to fill the deep areas to be repaired and to bring things generally very close to surface level over the first two or three coats, but make sure you don’t leave any of it above surface level to dry.  Otherwise you’ll have to sand it off, which is no fun.  The last thin coats can then be a standard compound – CGC Dust Control is a favourite as it can contribute to keeping the sanding dust mess to a minimum.  All of these things are available at your local home centre or hardware store.  No affiliation with CGC or the local home centre or hardware store either.  Other brands & retail locations will probably be just fine.

Take your time & enough coats as is necessary to smooth your wall to perfection, prime & paint & you’re done!

Wear a Mask!

Among other general efforts to do it all safely, always make sure to wear a good mask while dealing with the old plaster (and the drywall compound dust).  Crumbling old plaster is gross, dirty, dingy & dusty and sometimes has nasty ingredients in it.  Take precautions or have it tested if you’re not sure.

Protect Your Floor!

Make sure that any crumbling plaster chunks, bits and dust are contained in a tarp or something, and do not land directly on your floor.  Carefully clean any that gets on to your floor with a vacuum before it gets ground in.  Old crumbly plaster bits are highly abrasive to nice hardwood floors & will completely trash them a lot more quickly than you’d like.

Need a Hand?

Good luck with your plaster repair project – if you need a Toronto handyman to lend a hand, you know where to find me!

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