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Question DetailsAsked on 1/10/2018

Water leaks from under my window sill during a heavy rain. Any ideas what could be wrong?

The inside wall is wet and the paint is wrinkled right under the window sill. Any ideas why this happens?

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4 Answers


The outside window sile is more an likely rotted and alowing water to leak to the inside below the window I have seen this befor. Replace the outside window sill and your problems will be over

Answered 10 months ago by codeman62003


As other comment said, rotten frames or brickmold (the outer trim) can cause this. Other common causes, in no particular order - many of them relate to rain or roof runoff hitting the window orthe wall right above it, so if you have a significant roof overhang which prevents that even in heavy rains, that eliminates pretty much all but the surface water around the foundation.

1) leaking caulk or water seal around window (especially) at top, letting rain or roof runoff blowing against the house run in around the window, seeping to the bottom around the window frame, and coming out into the house at the bottom

2) leaking/missing/worn window weatherstripping letting water get past it, then accumulate in the bottom track of window, then either run off the sill or leak through the joints in the window frame at the ends of the sill and come out underneath the sill

3) blocked drain passages (almost exclusive to metal or molded plastic/fiberglass frames, not wood) - so the water accumulating in the bottom track after it runs off the window piles up in the track and leaks inside. THese passageways are commonly rectangular holes in the bottom o fthe track - either in the bottom or on the "outside" face of the track trough, leading through the interior of the molded bottom track/frame and exiting outside at or more commonly right under the outside lip of the track/bottom frame, where it is supposed to drop free to the ground. Can become clogged with dirt and tree debris, or with moss. Can also be caulked over at the exit by people who do not realize those openings are drain exits.

4) vertical sash type window closed wrong way - with the outer window unit being on the bottom rather than at the top

5) vertical sash window, with one unit not properly closed due to an obstruction, or just not latched tight (or missing its latch) so the weatherstripping is not effectively compressed to keep the water out

6) one of the more common causes, for basement windows which sit close to ground level or even below with a window well, is water accumulating around the foundation and building up to the bottom of the window rough opening, and leaking in under the window

7) rarely, a leak at an overlying window or vent, or a roof leak, or a leaking overhead water or hydronic/steam heating pipe, getting into the wall and running down into the wall till it hits the underlying window, running down around the frame, and leaking out at the the bottom sill

You can find a number of previous similar questions with more exhaustive answers, and some with a longer checklist of potential causes and how to track them down, in the Home > Windows link, under Browse Projects, at lower left.

Here are a few links to some o those which appear to maybe apply most directly to your case, including one with a bunch more links to similar questions:

Answered 10 months ago by LCD


We do not have an overhang on our roof. 1926 Tudor revival. when the rain hits that side the water comes in from underneath the sill. They are casement windows that open inward. Thank you both for your advice, but maybe this will give You a little more insight to my issue.

Answered 10 months ago by cynthgom1


OK - leak could still be overhead - in the wall or overhead window even, but most common place is a leak in the flashing/caulking over or alongside the window, which then leaks down around the outside of the window unit and comes you at the bottom when it his the bottom framing of the rough opening - the 2x4 or 2x6 or 2x8 (whatever wall thickness you have) forming the bottom of the rough opening which the window fits into - or the vapor/water barrier if the rough opening correctly had that but it was not designed properly to drain such leakage outside. Or maybe it was originally, but someone caulked over the exit point under the outer sill.

If you crank the window full open (may mean releasing the opening arm so it fully opens), especially if wood, you may well be able to see staining (usually brownish) or wetness (if done right after the rain event) on the sill or frame - INSIDE of the weatherstrip. Staining or wetness of the window unit OUTSIDE the weatherstrip generally means nothing because that area is not designed to by dry. Look all around on the window unit framing and the window itself inside of where it seals when closed for signs of leakage - also look for weatherstrip which is worn, fully compressed and not rebounding when the window is open, maybe torn where it goes into the frame, etc. You can also (at night when dark) shine a BRIGHT flashlight around the seal gaps and see if the light comes through - but your eye has to be right up to the gap, so means someone outside right up to the window.

If you do not see any staining there - or wetness when the water is showing up during or right after a rain like do above check immediately after the rain stops or when it is blowing away from the window but the window unit would still be wet from the leakage - but if no wetness or staining then is probably getting in around the window trim/flashing, or from above higher up the wall. or as other comment said, if the trim or outer frame is pretty rotten, can penetrate straight into the wood and through the voids and cracks caused by the rot - though in that case you would have visible cracking/peeling of the paint and generally of the wood as well.

One test you can do, though this means more wall wetting, but if you (if you can access the outside) play a slow flow of water from a hose (no spray) over the outer sill for a minute or soper location (or till water appears inside), then over the weatherstrip area between outer sill and the bottom of the closed casement window, then on the bottom part of the window glass, then over the sides of the casement weatherstrip one at a time, then the top weatherstrip gap over the casement window, then around the sides and top of the window one by one - in each case noting whether any water is coming through before you move up to the nect highest possible leakage area, you should be able to see where it is coming from. This of course assumes water is not coming from a damaged siding spot or window or roof leak overhead, running down through the wall to the window opening and just happening to come out there because that is an opening letting it get into the house.

Don't forget, if there is a bottom track of any sort (not likely in a casement window) check if water getting into it off the bottom of the window drains to the outside.

Check also for gap around the casement unit - if the window mounting is coming loose, it could be tilted in the opening and not sealing nicely against the weatherstripping. You can rub some flour, chalk dust, or such on the weatherstrip all around, in a moderately heavy coating, then close the window and reopen it - the dust should have been transferred to the frame of the operating window unit all around - if no contact at one area that means water can freely blow through there.

Another couple of options to check on that possibility, assuming you have done the checks above and not found any leak through the window unit itself. You can remove the interior trim above the window and check above the window unit (between it and the rough opening in the wall) or wetness, wet insulation, etc indicating the water may be comding from above the window (though a leak at the top of the outer trim can also sometimes wet this area).

For a definitive answer putting a hand-sie hoelin the drywall above the rough opening (realizing there will be a 2x at the top so you need to be say 3-4inches above the top of the window trim so you can put your hand in and feel around for wetness (right after a rain causing leakage).

You can also non-destructively check for water there by using a cell phoneor tablet camera returned to the near-infrared (newer Apples have this recalibration option built-in, many other phones need an App to recalibrte the camera sensitivity) - commonly but not always will show wet insulation and wood in a wall. or rent a thermal infrared camera at Home Depot, tool rental place, or some auto parts stores for about $30-70 per half-day/day. Lets you see heat differences in the wall, where the wet zone will have poorer insulation so will show up as a warmer zone (in winter) or colder zone (if A/C is cooling the house) from outside because more heat loss through the damp part of the wall. From inside the wet zone will look cooler (if inside is warmer than outside), or hotter (if air is hotter outside than inside).

If you cannot find the source, then you would need to have a professional Window contractor look at it, though it is hard to get many to come out to fix just one window. Sometimes it means taking the window out of the wall at a time when the wall would still be wet, to see where the water is coming from. If going to that trouble, commonly both because you have invested (for normal size windows) at least half of the cost of a new window, and because till it is out you do not know i the window unit itself is realistically fizable or not, one would normally just plan on installing a new window, checking in the process (once the rough opening and any water barrier in there is out) if there are signs of leakage from above in the wall. Taking it out and replacing the water barrier and window and flashing and caulking would eliminate any cause except leakage from above - which should be readily evident in staining and maybe dampness of the ddrywall and wood at the top of the rough opening when the window is removed.

Course, if the problem IS coming from above the window, that mens you replaced a window needlessly - for $500-1000 commonly for a normal sized bedroom window.

One other thing - exposed windows like that are a bad idea - putting an awning or shed roof overhang over it might solve the problem (assuming the leak is at the window and not just a caulking failure) unless you are in an area where the rain pretty much drives in sideways routinely.

Answered 10 months ago by LCD

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