Sagging Plaster

[Mike Lamb]

I have seen this dozens of times but I don't have a good explanation for it. The plaster undulates about every 2 feet in a straight line. There are no cracks and the plaster is not loose. This photo is of a wall but I usually see it on ceilings.

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[Jim Katen]

I have seen this dozens of times but I don't have a good explanation for it. The plaster undulates about every 2 feet in a straight line. There are no cracks and the plaster is not loose. This photo is of a wall but I usually see it on ceilings.

That's sometimes how rock lath behaves.

- Jim Katen, Oregon

[Tom Raymond]

If it isn't rock lath, you will find that the wood lath was installed with several pieces lined up. With that being under a window there is probably also some water infiltration contributing.

Tom

[kurt]

It's one or the other of the above 2 explanations.

It looks like an old bungalow in the city, so my guess is it's wood lath lined up. I see rock lath sag more in late 30's-50's houses.

[john_opwin]

I am not very much familiar with it. I think you should view oldhouseweb(dot)com and plaster-wall-ceiling-solutions(dot)com. I hope it will help you.

[mgbinspect]

Tom is closer to the mark.

This is so typical in Richmond's old row houses and inner city homes, with wood lath plaster finishes, from the late 1800s and early 1900s.

My theory is that this is the result of the three coats of plaster finally coming apart from being dry as dust and dealing with the slow but constant movement of an old brick masonry foundation and footing. The pattern, as already mentioned, telegraphs the square sheets of wood lath.

When I had to make repairs to these old plaster walls, as a disaster restoration contractor, it became apparent that typically the scratch coat was breaking apart right at the surface of the wood lath. The plaster is usually reinforced with horse hair or some other flexible strand material so it remains pretty strong in spite of this slow progressive failure. As I would remove the plaster, the keys of the scratch coat would fall through and back behind the lath as the horse hair let loose of them. The bond between the scratch coat and brown coat tends to remain impressively in tact. The skim coat may be slightly loose as well. Rapping on it with a key and listening for the same sound that reveals loose ceramic tile is usually telling.

Usually, if moisture intrusion is involved, the surface will also have bubbly efflorescence on the surface along with stains and cracks.

In essence, this is a pretty typical condition for a home with old wood lath plaster.

[Chad Fabry]

This is so typical in Richmond's old row houses and inner city homes, with wood lath plaster finishes, from the late 1800s and early 1900s

That's because sawn lath has no tooth. Riven lath with all its irregularities provided a much better anchor.

[mgbinspect]

I'm no familiar with the term, Chad. What is rIven lath? School me...

[Les]

Well, I dis-agree with Chad on this one.

I am quite sure there are regional differences, but my knowledge and experience lead to saw lath having more tooth on the flats, even tho it is uniform. It will produce a more uniform key. Riven lath will produce a stronger key (opinion) because it is irregular as a result of being split.

In my geographic area, sawn lath usually is pine or spruce, sawn with a large diameter circular saw, usually very dry, except when you need to span an arch or column.

I have never personally seen chestnut or other hardwood laths or even riven lath (in Michigan).

[Jim Baird]

mgb,

I think riven is sort of like split instead of sawn. I have seen it in really old buildings. Has a very textured surface.

Rock lath swelling is likely due to moisture getting in the wall.

[mgbinspect]

But, long rectangular patterns of maybe 18" +/- by six to eight feet are usually the tip off that it's rock lath while patterns closer to square tend to be wood lath. That's my experience anyway...

[Tom Raymond]

Unless there was a lot of scrap, then you see much shorter pieces, usually all in the same place.

Tom

[mgbinspect]

I see. That makes sense.

[Rob Amaral]

I've seen most of the 'laths' used in New England here in the Boston area (oldest house I was given the opportunity to check-out was built in 1636). That looks like my house (1958/rocklath) and I see it all the time. Mine even came with a weak finish-coat (Hooray!)