Water stains on window sills can be a source of anxiety for home buyers, and it's the home inspector's job to help determine if the stains are the sign of a major problem or not.

There are three common causes of water stains on windows:

1. Leaving the windows open.

The windows get left open, and water pours in through the window during a rain storm. This type of staining will often leave fairly uniform water staining across the window sill.

2. Condensation.

If it's a wood window, you'll see stains at the corners of the window sashes and the window sills if the stains are caused by condensation. You'll also find the worst stains on the windows that are most likely to be damp, such as in bathrooms or bedrooms just outside the bathrooms. The windows at the north side of the house will be worse than windows on the south side.

3. Leaking windows.

If water is actually leaking in to the house around the opening for the window at the siding, this will typically show up as staining at the corners of the window sills. The photo below left shows what a window sill may look like with minor water leakage in to the wall; the photo below right shows a window sill with major damage from water leakage. A home inspector could use a moisture meter to help determine if the stains are currently damp.

The first two causes of stains are fairly straightforward and easy to prevent; remember to close the windows before it rains, and lower the humidity in your home. Here are a few tips to lower the humidity in your home:

  • Turn off your whole house humidifier
  • If you have one, use your kitchen exhaust fan while cooking. This includes using your oven, if it's gas - they add a considerable amount of moisture to the air.
  • Turn on your bathroom exhaust fans during showers and leave them on for a half hour after every shower. If you don't have a bathroom exhaust fan, get one. While the building code allows an openable window as a substitute for a fan, I don't.
  • If you have a crawl space, make sure that a proper vapor barrier is installed on the crawl space floor.
  • Install an HRV or a continuous exhaust fan. Either one of these will dramatically lower humidity levels in a home.

The third cause of staining at windows, leaking water from the exterior, is the one that home buyers should be concerned about. A window can leak from just the slightest defect in flashing at the top, and unfortunately, it's not easy for home inspectors to know if a window is going to leak just by looking at the siding.

If the flashing above a window is installed properly, all of the water coming down the siding will be diverted around the sides of the window. The windows that will be exposed to the most water are the windows that aren't protected from rainwater by soffits and gutters - such as the window shown at left.

The photo below shows the proper path for the water to take; I know this is kind of a 'no-duh' issue, but actually thinking through this stuff helps me to know which windows I really need to pay particular attention to during home inspections.

Here's a close-up view of the window flashing, showing the path that water is supposed to take... but this window has a nasty detail in the flashing that will be prone to leakage. Do you see it?

Here's another close-up view, pointing out the exact issue with the flashing.

As you can see in the photo above, if the caulking at the J-molding around the window fails, the window is going to leak. Big time. As a matter of fact, it has failed at this particular window, and that's what is causing the major water staining at the bottom of the sill, which is what was shown above at the beginning of this post. Could you tell just by looking at the exterior of the window? I couldn't.

The repair for this condition is to have the flashing redone, so the window isn't relying on the caulking to keep water out. This project will probably only take about an hour or two to complete, but it would have taken the original installer an extra two minutes to get it right. My first thought was that the installer was either lazy or a bonehead, but at the time this window was installed, which was about twelve years ago, this was just the way it was done.

If I saw an installation like this on a relatively new home, I'd call it a boneheaded installation.

Reuben Saltzman, Structure Tech Home Inspections - Email- Home Inspector Minneapolis