What's Behind Your Plaster Walls and Ceilings
3 pros!
Live in an older home? You might have to deal with plaster instead of drywall.
Lath and plaster
If your home was built before 1950, you most likely have traditional lath-and-plaster walls.
Wooden lath strips are nailed across the framing and then plaster is applied to them. The plaster that oozes between the lath strips is called the key, which locks the plaster and lath strips together.
Plaster walls and ceilings consist of three coats of plaster:
• the scratch coat. It's the first coat where you key the plaster into the lath and then scratch it to create a mechanical hold for the next coat.
• the brown coat. It's the second coat that's used to even out the scratch coat. The first two coats are both base coat plaster.
• the veneer coat. The final coat is generally a plaster with fine sand to create a smooth finish.
Lath-rock and plaster
If your home was built around 1950, you could have one of several versions of lath-rock and plaster (the precursor to drywall).
This was a plaster board of varying sizes that was nailed onto the framing and coated with a traditional three-coat plaster in the beginning years. You'll know if you have this because your walls will be an inch thick or more. In later years, they switched to a much thinner veneer coat over the plaster board.
Calcimine
If your home was built before the 1940s, then you will also have calcimine (or chalk paint, an easy way to think of it).
Calcimine eliminated the 60-day cure time for fresh plaster mixes of that time. The chalk allows the lime in the plaster to off-gas with no ill effects, whereas oil paint will blister and peel off as the lime gases escape if painted before 60 days.
Why should it matter to you? If the calcimine gets wet from a roofing or plumbing problem, the paint or wallpaper trying to hold on to the calcimine just lets go. It's most likely what you've been staring at that eventually lead you here. If you have a weird bubbly spot usually accompanied with brown staining (tannin leached from the wet wood), you have had water intrusion. Calcimine gets wet, things let go.
This is also the cause of cracks turning into mountain ridges. Plaster is hygroscopic, it seeks out moisture. A crack develops from settling then the plaster draws in any ambient moisture, the calcimine gets wet and the edges of the paint or wallpaper begin to slowly curl away from the crack, thus the mountain ridge.
About this Experts Contributor: Steve Irish has been plastering in the Seattle area for more than 25 years as Lux Interiors. You can follow the company on Google Plus.
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