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Question DetailsAsked on 10/19/2014

How do I repair interior drywall window sill which rotted due to moisture?

The interior window sill is made of drywall from what I can tell. I have a picture but not sure how to post it. I ended up peeling half the paint and drywall in the sill. I need to repair this crumbling mess.

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3 Answers


To post a picture, response back using the Answer This Question link right below your question. The picture can be posted using the leftmost yellow icon right above the Answer box.

3 ways to fix -

1) if you dug down to firm drywall, you can patch with drywall compound (available in pint and larger containers at building supply and home improvement and some hardware stores) - see how on the web, lots of videos on drywall patching on Youtube and This Old House and elsewhere. Will take several coats to get it filled and smooth if you dug well in. Then after dried several days prime with drywall primer and paint 2 coats minimum in that environment.

2) cut it out entirely and replace with new drywall, joint compound for the corners with fiber joint tape, prepare and sand joints, and prime and paint.

3) get a handyman to do the fix and painting

Answered 5 years ago by LCD


Source: Here is a picture of my window sills, can I cut a small piece of drywall and patch or should I replace the whole ledge?

Answered 5 years ago by ladyrepairit50


Looks like you have some mold up along the sidewall drywall also - the dark spots in the flaked surface by the window trim.

If you have a contractor do it - I would have him remove all the drywall on both sides and the bottom of the window recess and replace it, then prime and repaint using waterproof paint - oil-based and containing mildewcide additive. And of course get rid of the source of the water - either caulking around windows and trim outside or replace weatherstripping on window at a minimum if coming from outside, or replace entire unit if falling apart or laced with rot and leaking like a sieve.

If doing yourself and you are not experienced doing drywall corners (and to avoid having to buy a sheet of drywall and transporting and cutting it), I would dig out the rotted and soft /mushy material to firm drywall, then use premixed drywall patching compound (might take a quart container or two for your area, I would guess - pint will not be enough) like Dap Wallboard Joint Compound - you can find lots of how-to videos on Youtube and This Old House and elsewhere if you google - drywall patch . Likely to take a couple of passes to get filled in, because it sags as it dries. IF you open up holes through the drywall into voids in the wall cutting away the bd material, you can stick duct tape up underneath (sticky side upon the underside of the drywall - use longish peices) to seal the hole and keep the compound from falling through - put a piece on each side of the hole partly filling the hole from two sides but leaving a gap so you can reach up under to stick the tape strips to the underside of the drywall, then to fill the remaining part of the hole your fingers were reaching through, cut a patch of tape that fills the bottom of the hole and plu in sticky-side down against the other strips you just put there. IF the hole situation is not conducive to that or is small, you can wad up a plastic bag or few and jam it in the hole so it is tight against the bottom of the hole, then use that to gently put a layer of compound over to partly fill the hole, let dry, then finish off filling with second layer.

I would use at a minimum waterproof paint (including up to the bottom of the window trim) - you could go one step further if this is window condensation/melting ice and put in a liner of vinyl flat trim material as a waterproof liner - though then it would just run down the wall or pool in front of the window.

If the water is from condensation (which I doubt since at sides also), then you need to control the humidity at that window, or at least wipe the moisture up daily or twice daily before it weeps down onto the drywall - or replace with a more energy efficient window to eliminate the problem. Leaving drapes/curtains open at least part of the day to evaporate it will help too - preferably after wiping down with paper towels to remove the bulk of the moisture.

If from condensation, you probably ( if not replacing entire unit) need to clean up mold at the base of the window glass and use some paint to seal that wood before it rots - Kilz is probably the best paint for this.


Answered 5 years ago by LCD

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