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Sagging Plaster

Mike Lamb

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  • 3 months later...

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.

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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).

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