Why does the plaster crack beneath the window sill?

Question

Sorry if this is an obvious question.

Can anyone tell me why the dry lined plaster in our house so frequently cracks beneath the edge of the windows?

At first I thought it may be relating to the heat changes or something over time; however, we've recently had a room stripped down, new plasterboard put in and it's been re-skimmed, and three weeks later, it's cracked just beneath the edge of the window.

The cracks are barely visible, and don't seem to get any worse over time - they just crack and stay cracked.

Answer 1

I've seen this happen often at the corners of windows and doors. My first question would be - is there a seam in the plasterboard where the crack is occurring? If not, is the crack just through the skim coat or is the board underneath actually cracked?

If you just plaster over the crack, it will certainly crack again. I would try either taping over the crack with mesh tape and plastering over the tape or if it's just a hairline crack, caulk it and paint it. The caulk should be flexible enough not to crack again.

Answer 2

What's the soil like where you live?

I grew up in the Cdn prairie. The soil there was sandy clay. I spoke to a geologist who said that houses built in Edmonton actually "floated" - that is, they'd shift seasonally, sort of swaying back and forth on the soil.

He was very amused at the lengths to which people went to patch cracks in walls, etc, only to have them re-appear the next summer. Everyone assumed the houses were shifting, and the #$@))@#$ foundation contractor didn't do a good enough job, etc. but in reality it was the nature of the soil.

If you're on ground like that, then the cracks will always appear at the corners of windows, doors, etc. because those are natural flex points if the house moves a bit.

Answer 3

It's no big deal -- just stress cracks in the plaster that form your house settles, or when the humidity and heat cause the wood to expand and contract.

If it bothers you cosmetically, you can just skim the cracks with plaster. Avoid the pre-mixed plaster that they sell at the big-box stores, as its difficult to spread it really thin.

Answer 4

Wood casement around your windows swell and contract with weather changes and humidity in the house. This causes the plaster in the lower right and lower left corners of your windows to crack and shoot the crack downward. Sometimes it's a hairline and sometimes a small chunk. Only 2 ways to stop it is either replace entire wooden window casing(terrible idea) or remove the plaster over the corners, chisel the wood casing deeper into the wall, fill the area with compound, then mesh and skim over the entire area. Paint and smile.

Answer 5

I've seen this in areas where the foundations are built on a shrink-swell clay or above a shale formation. Doorways and window openings are the weak spots in the walls, so that's where you see the cracks. Get with someone in your local USDA or Conservation District office, they can tell you if soil type could be the culprit.