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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.

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Where do the cracks originate, and what direction do they run? –  stack Aug 5 '10 at 15:40
    
someone sitting on the window sills? I assume they are wood underneath with plaster on top and 5/6 inch wide marble slab on top? –  staticx Aug 5 '10 at 15:46
    
Just below the sill and run downwards –  pm_2 Aug 5 '10 at 15:51
    
There's definitely no-one sitting on them :-) The window sill itself it wood. –  pm_2 Aug 5 '10 at 15:55
    
House shifting? –  staticx Aug 5 '10 at 16:12

4 Answers 4

up vote 3 down vote accepted

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.

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+1 for caulk, which fixed a persistent crack over a window. With matte paint over it, you can barely even tell it's there. –  Niall C. Aug 5 '10 at 20:10
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I wonder if scoring back some of the drywall from around the window and back filling with caulk would be enough of a "decoupling" between the window and wall to prevent the cracking. –  kkeilman Aug 5 '10 at 23:20

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.

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Yes - we do have clay like soil. The wall beneath the plaster didn't have any major cracks in it though. –  pm_2 Aug 5 '10 at 17:01
    
In some areas, if there's a long drought, people have to file for permission to water their house (to keep the clay hydrated so it doesn't shrink irregularly, and crack their fundation). –  Joe Aug 6 '10 at 11:51

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.

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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.

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