A bay window is a set of windows, usually three, built into a box that projects out from the wall of the house. The box has its own floor, its own ceiling and in most cases its own small roof. That box is the reason replacing a bay is a carpentry job with a window in the middle of it, and the reason quotes for one come back so far apart.
One trade guide puts the structural share of a bay window job at 40 to 60 percent of the total, and that matches what I see. So this guide starts with the structure. How a bay is held up and how to tell which method yours uses. Why you cannot swap one of the three units. The roof that fails before anything else. What the projection angle does to the seat. And what it takes to turn a bay back into a flat wall. We fit bay and bow window replacement across the St. Louis metro, from Kirkwood out to St. Charles.
What Is Actually Up There
Six parts, and knowing their names changes how a quote reads.
- The center unit is fixed. It does not open.
- The two side units are angled back to the wall and do open, usually casements or double hungs.
- The head board is the ceiling of the box.
- The seat board is the floor of it, the shelf people put plants on.
- The roof sits on top, normally a small hip or shed, flashed into the wall above.
- The skirt closes in underneath, and on many houses it is doing nothing structural at all.
A bow window is the same idea with 6 windows in a gentle curve instead of 3 windows at an angle. Everything below applies to both. Frames come in vinyl, fiberglass or wood, and the frame material matters far less here than the box it all sits in.
How Yours Is Held Up, and How to Tell
The support method separates a cheap bay job from an expensive one more than anything else does, and you can work yours out before anyone quotes.
The box weighs several hundred pounds with the glass in it, and it is hanging in mid air. It gets carried one of three ways.
- Cables from above. Steel cables run from the head board up through the wall into the framing over the opening, so the box hangs rather than sits. Look for two small capped holes in the head board, one near each front corner. This is the most common method on a factory bay.
- Knee braces from below. Angled brackets run from under the seat board back to the wall. Go outside and look up under the skirt. If you see two diagonal arms, that is what you have.
- A cantilevered floor. The floor joists themselves run out past the wall and the bay sits on them. You will not see cables or braces, and the skirt is just cladding. This is normal on a house built with the bay rather than one added later.
Why it matters: dropping a like for like unit into a cantilever is close to a flat window job. A cable-hung bay means opening the wall above to reach the anchors. And a new unit heavier than the old one forces the support to be re-done whichever type you have.
You Cannot Replace Just One of the Three
People ask this constantly and the answer is almost always no.
The three units are mulled together at the factory, which means joined into one assembly with a shared frame member between them and sealed as a single unit. There is no seam you can unpick. Take one out and you are cutting the assembly apart, and what goes back will not seal the way the original did.
There is one exception worth knowing. If a single pane has fogged because the seal between the glass layers failed, the insulated glass unit inside that sash can sometimes be swapped on its own. That is glass work, not window work, and a glass company is the right call rather than us. If the frame is sound and only the glass has gone cloudy, start there.
The Roof Is What Fails First
A bay carries a roof nobody ever looks at. It is small, it is low pitched, and it is flashed into the wall above. That flashing joint is the leak, and because the water runs down inside the wall you usually find out from the ceiling of the box rather than from outside.
Four things to check, from inside, with a flashlight:
- Stains on the head board, especially at the back corners where it meets the wall.
- A seat board that flexes when you press it. Water gets in at the roof and ends up in the floor of the box.
- Corners that have opened up between the center and side units. That is movement, and movement means the support has shifted.
- Condensation between the glass layers. That is a failed seal in a sash, and a different problem from a leak.
Find rot in the head board or the seat board and the job changes shape. The box comes apart, not just the glass. On a Webster Groves bay last winter the glass was perfect, and what we found under the seat board was pulp.
Projection Angle, and What It Does to the Seat
Bays come at 30, 45 and 90 degrees to the wall. The angle is not a style choice, it decides how far the box sticks out and therefore how deep the seat is.
| Angle | Projection, with 24 inch side units | What the seat is good for |
|---|---|---|
| 30 degrees | about 12 inches | Plants, a shelf, not sitting |
| 45 degrees | about 17 inches | A cushion, a cat, a child |
| 90 degrees, a box bay | about 24 inches | A real window seat |
The arithmetic is just the width of the side unit multiplied by the sine of the angle, which is why a shallow bay can never give you a seat however wide it is. Overall widths commonly run from about 4 feet to 10 feet, and past that you are usually looking at a bow with more units rather than one enormous bay.
One more thing the brochures skip. The head board and the seat board are the coldest parts of the whole assembly, because they are thin panels with the weather on the other side. Low-E glass and argon fill do nothing for either of them, and a good U-factor on the glass will not save a bay with an empty seat board under it. If your bay has always felt drafty, that is usually where it is coming from, not the glass. Ask what insulation goes into both when you get a quote.
Turning a Bay Back Into a Flat Wall
A real option, and more common than people expect. It is worth it when the box has rotted, when the support has failed, or when the bay sits on a wall that never suited one.
It is not a window job though. The crew strips the roof, then the box. They frame the opening and set a header over it. They sheathe, insulate and close the wall on both sides. Then siding outside, finish inside, and somewhere in the middle of all that a flat window goes in.
Two things people underestimate. The siding will not match, because the rest of the wall has weathered for twenty years. And you lose the light: three angled units gather far more of the day than one flat one, which is the whole reason bays exist.
Bay or Bow, If You Are Choosing
| If this is true | Go with |
|---|---|
| You want a usable seat | Bay, at 45 or 90 degrees |
| You want the softer, rounded look | Bow |
| You want the most opening glass | Bow, more units means more that open |
| The wall is narrow | Bay, a bow needs more width to curve |
| The house is Victorian or period | Bow reads older, bay reads mid century |
Our bay versus bow comparison puts the two side by side, and the bow windows guide takes the bow on its own.
What Moves the Price
Bays are quoted per job, never off a rate card. When I price a bay the first thing I want to see is the head board, because the spread between two quotes on the same house usually comes down to these six things. You can check every one of them yourself first.
- Which support method you have, and whether the new unit can reuse it.
- Whether the new unit is heavier than the old one. Triple glazing and bigger units change the sums.
- The state of the head board and seat board. Rot found on the day is the classic overrun.
- The roof. Reusing it, re-flashing it, or building a new one are three different prices.
- Patching in and out, meaning siding, trim and the interior finish around the box.
- Height off the ground, because a second story bay is a scaffold job.
Make each quote say which of the six it has allowed for. Two totals with nothing itemized behind them tell you nothing, and on bays the distance between them is wider than on any other window.
The units are ProVia. Platinum dealer, with a showroom, according to ProVia rather than to us: it is on their own locator.
When This Is Not Our Job
- One fogged pane in an otherwise sound bay. That is a sealed unit swap and a glass company will do it for a fraction of what a window costs. You do not need us for that.
- A bay that has dropped because the support failed. The structure gets fixed first. A new window set on a failed support is money thrown away.
- Rebuilding the box you have. New units are what we do. Carpentry repairs to an old box belong to a different trade.
- Commercial buildings. Residential only, which is what these units are built for.
And the honest limit. Everything above tells you what to look at and what to ask. It cannot tell you what your bay will cost. What sits behind the head board decides most of that, and nobody knows what sits there until somebody opens it. Finished work is in our photo gallery.
Common Questions
Getting a Quote That Means Something
Work out which support you have, look at the head board and seat board with a flashlight, and decide whether you are keeping the bay or losing it. Turn up to the conversation with those three answers and the quotes stop being guesses.
The ranges we carry are on our bay and bow window replacement page, with a 10 year guarantee behind the workmanship. Somebody can come out and look inside the box. That visit costs nothing: book an estimate, or call 314-277-5952.
