Plaster and Drywall Cracks
Cracks are common in plaster and drywall because they are brittle and rigid but are supported by materials that are not equally as rigid. Plaster and drywall are attached to flexible wood structures on foundations that rest upon compressible soil. Wood expands in humid weather and contracts in dry weather. Plaster and drywall are too rigid to move with the wood and therefore do crack.
New cracks indicate recent changes. Increased traffic, nearby construction, children wrestling and maybe even the spin cycle on the washer will cause them. Sometimes it’s simply because the floor joists have sagged.
Most drywall cracks can be eliminated by just applying joint compound, but plaster cracks should be widened to 3/8 inch down to the lathe, cleaned out and moistened with Elmer’s glue diluted 50% with water. If sections of plaster are disconnected from the lathe, they can be secured with “plaster buttons” prior to plastering the crack. Press fresh patching plaster, not joint compound, into the crack so that it is forced into the spaces between the lathes.
When plaster ceilings are full of cracks, apply drywall directly over the plaster and don’t bother patching. It’s just not worth the effort. Make sure the drywall is screwed to the floor joist or strapping and not just to the lathe.
Buildings built between 1930 and 1950 may have metal lathes under the plaster. You’ll notice when you try to hang a picture! You can distinguish between metal or wood lathe by hitting the wall. Metal lathe walls tend to be much stiffer.
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