Cracking under window sill

15 Jul 2013
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United Kingdom
Hi, after some advice please. Been decorating living room and under bay window between where wall meets the window sill underneath it keeps cracking. There was some gaps so filled with foam and used easy fix was going chaulk over but it just keeps cracking. I think the wooden sill is expanding as it's a south facing window. What's the best to use to stop cracking?


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Realistically, they all seem to do that, especially in cheap modern places.

Often, there are brackets fixed to the underside of the sill which are fixed to the wall behind the plaster.

But they still seem to crack, and convention seems to be that folk just caulk the gap, and then forget it, hoping no one will ever look under the sill until the next decorating project.

In truth, most wouldn't even both caulking it, if they believed the average person was not going to spot it.

Of course, your sill does not protrude very much, which makes the crack more obvious.

In the real world, folk would just use filler on any damaged plaster, and caulk the gap.

But you could try fitting brackets to the underside and chip the plaster away - but I don't think you want to go to those extremes.
Brown CT1 sealant will be less likely to crack. It is almost as flexible as silicone. Generally known as MS polymers.

I would recommend that you create a 3mm channel under the sill and then back fill with the CT1. Be advised that it very sticky though. I use white spirit on my finger to smooth it. Also be aware that you cannot paint oil based varnish directly over it. You would need to use a water based paint/varnish first.

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