Plaster Loose Spots
By Kendall Holmes
"The plaster walls in my 1907 house seem basically sound, but I need to fix a number of cracks. Also, there are a couple of places--about two feet square--where the plaster feels loose. Any suggestions on how to repair these problems?"
Last week I offered some tips on fixing cracks. Now let's fix those loose spots, which are pretty common in houses of your vintage.
Let's start by taking a peek behind your walls. For roughly a century starting in the 1830s and 1840s, the most common system for attaching interior plaster to walls and ceilings consisted of nailing 1/4" by 1 1/4" wood strips (known as lathing) to studs and joists. These strips of wood were held about 3/8" apart from each other, forming slots.
To apply the plaster, a scratch coat consisting of sand, lime, and cattle hair was troweled onto the lath and pressed through the slots with enough force to push some of it onto the back-side of the lathing--forming "keys" that hold the plaster in place. Next, a brown coat was applied to establish flat and plumb surfaces. Finally a smooth finish coat of lime and gypsum was applied.
The soft spots in your walls are caused by broken keys--perhaps because your house has moved, but more likely because someone or something banged hard into the wall at some point in the past.
Why didn't the plaster simply crumble and fall apart when it got banged into? You can thank the horsehair in the scratch coat for that.
To fix these bulges or soft spots, you'll need to buy some plaster washers. These are 1 1/4" steel disks that get screwed into the soft plaster, to pull it back against the lathing.
If you're lucky, you may be able to find plaster washers at a local paint, hardware or drywall supplies store. Or you can buy them online from the Charles Street Supply Company, a Massachusetts hardware store known to old-house lovers around the world because it carried these washers when few others did.
To repair your soft spots, first you need to press the plaster against the lathing, using the palm of your hand. Then reattach the plaster to the wood lathing using drywall screws and your plaster washers. As you attach a few screws, you'll notice that the washers keep the screws from pulling through the plaster. Use plenty of screws and washers--one every six inches or so.
Once you've pulled the plaster back in place, cover the washers--and any plaster cracks--with fiberglass mesh tape. Then follow the steps I detailed last week to cover the tape with a smooth topcoat of plaster-like joint compound.
Patching cracks and soft spots in plaster walls may not be the most fun way to spend a weekend--but the results are rewarding.
Ken Holmes is an award-winning print and web journalist and editor, as well as a former contractor.
