Renovation & Design

Leaking bay window hard to diagnose

Extensive repairs may be needed if area shows signs of rot or mould

QUESTION -- We have a bay window in our kitchen made up of three separate Pella triple-pane windows. The two side windows open and have not given us a problem. The large centre window juts out from the floor on the bottom and on the top reaches the edge of the eaves. This window leaks a lot in a heavy rainstorm. You can see the eavestrough overflowing above the centre window. We have caulked extensively and cleaned the eavestrough regularly. The rain comes in where the vertical window frame hits the horizontal sill. I'm not sure of the technical names if any.

We want to fix this but we aren't sure what the problem is. Could it be a roofing problem? Someone suggested we add another downspout as the window is fairly close to an existing downspout. The water seems to like to come inside rather than rush to the spout.

-- Penny Bryans, e-mail

Answer -- As I look out my window at the pouring spring rain that we are currently experiencing, it appears to be an ideal time to deal with your chronic problem. It is unusual for a leak of this nature to continue for that long without extensive rot being visible in the wooden components of the window. If the window is still in serviceable condition, there is much less concern with the leakage than there would be if significant damage is seen. However, if the window frame, wall, or support structure below the window is soft to the touch in any area, replacement of the window or entire bay area may be required.

The insidious thing about moisture intrusion, particularly at exterior walls and windows, is that the point of entry into the living space may be quite different from the initial area of leakage. You are correct that the leak could be a roofing problem, but it is just as likely that it is a problem with the window seals, eavestroughs or due to a missing or loose flashing. I will offer some possibilities but it is impossible to diagnose the true nature of the problem without a physical inspection, ideally on a wet day like the day I'm writing this.

Many bay windows in older homes, especially smaller ones in kitchens, have been installed long after the home was initially constructed. To accommodate these types of retrofit units a support structure has to be built underneath as well as a small roof, if the unit extends beyond the existing eaves. In your case, it sounds like the eave was of sufficient depth to cover the top of the window, but just barely. If the window framing has been installed directly up to the underside of the older soffit, there may be no flashing installed over top. With no room for a flashing in this area, there is always a possibility of wind-driven rain or water leaking down from the eaves above, sneaking into any small gap or hole between the window frame and the soffit. This is particularly a concern if the eavestroughs overflow due to blocked downspouts or excessively heavy rains. What can happen in that case is that the overflowing water may travel along the front and underside of the trough, by capillary action, and leak into any gap at the junction of the window and the soffits. Alternatively, if this overflow is extensive enough, the water may run down the outside of the window until it finds its way in through a worn weatherstrip or small opening in the exterior of the window.

From your description of the point of entry, it appears that the leak is occurring near the top of the window and running down the vertical frame or mullions, until it works its way inside at the sill. Unfortunately, this is only a guess, at best, and the leak could also be occurring much higher up on the roof and running in through the window. This can occur as the window frame may be the easiest point of entry in the building envelope in this area. The leak could also be at the side of the window frame and only occur when the wind is strong enough and from the right direction to blow the water through small gaps between the frames of the different window units in this bay area.

As previously stated, the main concern with the leakage in your kitchen bay window is potential damage to the wooden components of the window and surrounding area. If these give way or feel rotten when probed with a screwdriver or scratch awl, then the potential for a major repair is high. If significant leakage has occurred for the two decades you have noticed this issue and rotten wood is discovered in or near the window, the entire unit may have to be rebuilt. Implications from years of leakage include the presence of extensive mould and rot in this area. The only way to determine this for certain is to enlist the services of a CAHPI inspector or qualified general contractor to inspect the area, ideally on a rainy day.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the president of the Canadian Association of Home & Property Inspectors -- Manitoba ( Questions can be e-mailed or sent to: Ask The Inspector, P. O. Box 69021, #110-2025 Corydon Ave., Winnipeg, MB. R3P 2G9. Ari can be reached at (204) 291-5358 or check out his website at


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