Plaster Walls Cracking


Old 09-20-00, 12:38 PM
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All of the exterior walls on the second floor of my house have gently curved plaster joining the walls to the ceiling. The plaster is in excellent condition in the three bedrooms and one of the bathrooms. However, in the second bathroom the cracking in the plaster is so extensive that it resembles a spider web. The previous owner said that they had tried to fix this with a new coat of plaster but the cracks came back. Any suggestions?
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Old 09-20-00, 08:54 PM
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I would believe the previous owner. Plaster is brittle, and doesn't allow as much movement as drywall before it cracks.

The source of the movement is likely to be frost heave in the winter. What happens is that as the ground freezes in the winter, it initially expands down to -4 degrees Celsius, then shrinks below -4 deg. C. As the ground is frozen to your house's foundation, it lifts the foundation differentially, depending on what areas are warmer and which are colder. Also, if you have teleposts supporting a beam under your main floor, then these won't move because the ground under the cement under them probably won't be frozen. So, the middle of your house stays put while the exterior walls go up and down, and this usually results in cracks in the weakest spots on interior walls, which is the top corners of interior doorways.

There is actually a method to minimize frost heave whereby you bury 2 inch thick slabs of polystyrene insulation around the perimeter of your house. The idea is that you're actually containing the geothermal heat under the insulation to prevent the ground from freezing. I know they've done this is some US states with good success, but I don't know how far north this method would be effective.

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