A bay window that projects from the house usually has its own roof, a small sloped section that sits over the glass and ties into the wall above and beside it. In St. Louis, where a wet February can be followed by a hard freeze and then a warm April rain, that little roof takes a surprising amount of weather. When a stain appears on the ceiling inside the bay, or on the wall beside the window seat, many people go straight to the shingles over the window. This page explains why that is usually the wrong first move, and what to check instead.
Leak and repair
Water rarely comes through the shingles on a bay roof. The shingles are lapped and sloped, and the water that hits them runs off the edge. The gap that lets water in is almost always where the bay roof meets the house, at the top of the window and along the two sides. Water that gets past that joint runs down behind the trim and along the header, which is the horizontal board or beam over the window opening. From there it shows up on the ceiling below the bay, sometimes several feet from the spot where it entered.
There are four places to look first. The first is the head flashing, the metal piece that should sit above the top of the bay roof's wall joint and tuck under the siding or house wrap. The second is the counter flashing or step flashing on the sides, where the bay roof's edge meets the wall. The third is the kickout at the lower end of the bay roof, a small angled piece that turns water away from the siding below. The fourth is the nail line through the trim boards, where nails pushed up by repeated freezing and thawing can open a path for water.
Caulk is the tempting fix. Caulk can hold back water for a season, and it can also hide the real opening. A bead of sealant over a missing piece of flashing does not make the flashing exist. The repair should be a piece of metal that sheds water and is tucked where water runs, not a line of sealant along the top of the trim.
Why the ceiling stain misleads
Water follows framing. It runs down the header, along the top plate, and across the top of a window opening before it drops. A stain on the ceiling inside the bay may sit a foot or more from the wall joint where the water entered. A stain on the wall beside the window seat may come from water that ran along the outside of the sheathing and came through at a lower point. This is why people who patch the spot directly above a stain often see the leak return. The stain marks where the water landed, not where it started.
Check whether the stain grows after rain, whether it stays in one place or moves, and whether the window frame itself shows condensation or a dark ring. A frame that stays wet after the rain stops points toward water behind the trim, not toward condensation. A dry frame with a stain that appears only during wind-driven rain points to the top of the bay roof, where wind pushes water up under the flashing.
What to check from inside
Start in the room with the bay, and watch during rain that comes from the direction the window faces. If the bay faces south or west, the wind-driven rain often hits it from the same side as the window. Put a towel or a shallow pan under the stain and note the time. Take a photo of the stain each time it changes, with a tape measure in the frame so the distance from the window trim is clear.
Open the cabinet or access panel under the window seat if the bay has one. Look at the underside of the seat roof and the top of the subfloor. A wet or dark stripe along the top of the wall framing means water has been running along the header rather than falling straight down. Look also at the drywall in the corners where the bay meets the main wall. Those corners are where the joint gives way first, and where water collects.
Do not pull the trim off on your own before the rain check is done. Removing the trim can change how water runs, and it can make it harder for a roofer to read the path. A few clear photographs taken from inside are more useful at this stage than a hole in the drywall.
What to check from the ladder
From the ground, or from a ladder placed where it is safe to stand, look at the edge where the bay roof meets the wall. Check that the head flashing extends up under the siding or house wrap and is not just resting on top of the trim. Check that the sides have counter flashing or a lapped joint, not only a bead of sealant. Look for shingles that have been cut around the flashing rather than lapped over it. That is a common shortcut when a roof was recovered without the flashing being replaced.
Look at the kickout at the lower end of the bay roof. If it is missing, or bent so that water runs past it into the siding, the siding below will feed water into the wall. On a frame wall, that water can sit in the sheathing for a long time before it shows up inside.
Do not climb onto the bay roof unless you have a secured ladder and someone holding the base. Bay roofs are small and steep, and the edge over the window can be slick after rain. Much of what you need can be seen from the ground with a phone camera held at the height of the window head.
What the repair should include
A bay roof leak repair that holds usually includes replacing or reworking the head flashing and the side flashing, not only the shingles. The shingles on the bay roof should be lapped over the new flashing so that water runs over each joint in order. If the bay roof is an older three-tab installation, the repair also has to answer whether the shingles can still be matched and sealed. Three-tab shingles that have lost their seal strip do not reseal well in cold weather, and a lifted tab on a bay roof is hard to see from the ground.
The repair should also say what was done to the trim and the sheathing. If the header was wet, the trim may need to come off, the wood may need time to dry before it is closed, and the framing may need to be checked for soft spots. A written scope should name each of those steps. It should also say whether the roofer will return after the next heavy rain to look at the joint under load, not only after the first dry week.
What it costs
The cost of a bay roof leak repair depends on which parts have failed. Reworking one head flashing and resealing the trim is a small job. Replacing the bay roof covering, the flashing on both sides, and the kickout, with the trim and sheathing repaired behind them, is a bigger one. A quote that gives one lump sum for the bay roof, without naming the flashing, the trim, the sheathing, and the shingles, is hard to compare with another quote. Ask each roofer to list those items on separate lines.
Ask also whether a second visit is included after the first heavy rain. Some bay roofs leak only under wind-driven rain, and a repair that has not been tested in that condition is still unproven. A second trip costs time, but a repair that is not finished can leave water in the wall through the next freeze.
What to ask
- Where exactly does the water enter, and how did you confirm it, by a controlled water test or by watching the joint during rain?
- Will the head flashing and the side flashing be replaced, or only sealed?
- What happens to the trim and sheathing if they are wet, and who checks them before the trim goes back on?
- Will the shingles on the bay roof be lapped over the new flashing, and what happens if they cannot be matched?
- Is a return visit included after the next heavy rain, and what is the process if the leak comes back?
- Will you put the scope, the photographs, and the measurements in writing before the work starts?
Keys Roofing documents the bay roof before any work starts, with photos and measurements, and gives you a written scope to keep. Keys attends as the contractor while you deal with your own insurer if an insurance claim is involved. Missouri law bars a roofing contractor from representing or negotiating a homeowner's insurance claim (RSMo 407.725), and from saying anything about paying, waiving, or covering a deductible. To have the bay roof looked at, call the office at (314) 220-2333.