WINDOW STYLES
Replacing a Bay or Bow Window
A projecting assembly is not a window in a hole. It is several units, a roof, a floor and a load path hanging off the side of a house — which is why its quote cannot be compared with a quote for a flat wall.
Three to six units joined at angled stiles · its own roof, seat board and support system
Who it's for
Houses that already have a projection and need it replaced, and rooms where the point of the window is the floor area, the seat and the light it adds rather than the glass alone.
What it actually is
Several units joined at angled stiles, plus a small roof, a seat board, a soffit and a support system. Only one of those six parts is a window, which is the whole reason this page exists.
Cost drivers
Whether the opening is altered, how the assembly is carried, who builds and flashes the roof, and who finishes inside and out. National figures for window replacement generally are on the cost guide.
What a projection adds, and what it commits you to
Everything in the right-hand column is a consequence of hanging a small structure off the side of a house.
What you gain
- Floor area and a seat that a flat-wall window cannot give a room
- Light from three or more directions instead of one
- A view along the wall as well as straight out from it
- Ventilation from the flankers while the center stays fixed glass
- An elevation feature that is genuinely difficult to reproduce any other way
What it commits you to
- A roof of your own, with its own flashing line into the siding
- A load path that has to be carried from below or hung from above
- A seat cavity and soffit that must be insulated and air-sealed
- Several joints between units, each of which is a seal
- A quote that cannot be compared with a flat-wall replacement
Why this is construction, not a window swap
Every other style in a replacement catalog sits inside the wall. This one hangs off it.
When a flat-wall window is replaced, the hole in the wall does not change and neither does anything holding the house up. The new unit is set into an opening whose header, jack studs and sill were sized long ago for exactly that job. A projecting assembly is a different animal. It extends past the plane of the wall, so its weight acts at a distance from the structure that carries it, and something has to resist that. It also has a top and a bottom that are outside the building envelope and have to be built, roofed, flashed, insulated and finished as though they were a small extension — because that is essentially what they are.
That difference is why two quotes for the same projecting window can be thousands apart and both be honest. One may have priced a matching assembly dropped into an unaltered opening with the existing support reused. The other may have priced a new header, a new support system, a rebuilt roof, new flashing into the siding, insulation of the seat cavity and interior trim by a finish carpenter. Those are not the same job and no amount of comparing bottom lines will reveal which one you are being sold. You have to ask.
The two editorial pages that rank for this term treat the decision as a choice of shape. The shape is the least consequential part. What matters is the load path, the roof and the opening, and a homeowner who walks into the first appointment already asking about those three is in a completely different conversation from one who walks in asking about angles.
What is the difference between a bay window and a bow window?
One is angular and one is an arc. Neither difference changes whether the job is structural.
| Where they differ | Bay | Bow |
|---|---|---|
| Number of units | Three: a wide center and two flankers | Four, five or six equal units |
| Geometry | Definite corners where the flankers turn back | An approximated arc of gentle angles |
| Depth of projection | Deeper for a given width, which is what makes a seat | Shallower for the same run of wall |
| Frame members in the view | Fewer verticals interrupting the glass | More verticals, and a softer line |
| Support required | More, because the load acts further from the wall | Less, but never none |
| Is it structural work? | Yes, in every version of the job | Yes, in every version of the job |
Whether the extra verticals of a bow read as elegant or as busy is genuinely a matter of taste, and it is one of the few decisions in this job that is.
What a projecting assembly is made of
Six parts, only one of which is the windows.
The head board and seat board
The horizontal panels that close the top and bottom of the projection. The seat board is the shelf people sit on and put plants on, and it has to be insulated underneath.
The support system
Either knee braces and brackets carrying the assembly from below, or cables running from the head of the unit up into the framing above. This is what stops the projection sagging.
The roof
A small roof of its own, with its own pitch, its own covering and its own flashing into the wall above. Every projecting assembly has one and every one of them is a place water can get in.
The flashing line
Where that little roof meets the house wall. It is the single most consequential detail in the whole job and the one most often blamed years later.
The units themselves
Usually a fixed center with operable flankers, joined at their angled stiles. Each joint between units is a seal, and the flankers are what give the assembly any ventilation at all.
The skirt and soffit
The finished underside and the wall below the projection, inside and out. Insulating and air-sealing this cavity is why a poorly built assembly makes a room cold.
Three versions of this job
- Matching unit, unaltered opening. Same width, height and projection; the header stays and the existing brackets or cables are sound and reused. Ask specifically whether the quote assumes this, because it depends on a replacement existing in the exact geometry of an assembly that may be decades old.
- New assembly, modified opening. The opening is widened, narrowed or raised, which means dealing with the header above it and making good the siding and the interior wall either side.
- A projection where there was none. A new opening, a new header, a new support system, a new roof and a new section of building envelope. Treat it as a small construction project that happens to involve windows.
The flat-wall styles
A sagging assembly, a seat board that has dropped at the outer edge or a door-like gap at a flanker all say the support has moved — worth diagnosing before you decide anything, because sometimes the units are fine and the support is not. The repair-or-replace guide works through that call generally, and the hub sets both shapes against the styles that stay inside the wall.
The roof is where these fail
Ask about the flashing before you ask about the glass.
A projecting assembly has a roof measured in feet rather than in squares, and small roofs are unforgiving. The water that lands on it has to be led away from the wall it butts into, which means step flashing worked into the siding, an apron at the head of the assembly and a continuous path for water to leave. Get that wrong and the water does not appear where it entered — it runs down inside the wall and shows up as a stain on the ceiling below or as rot in the framing under the seat.
In cold regions there is a second mechanism. Heat leaking from the room into a poorly insulated seat cavity or roof cavity melts snow on that little roof, the meltwater refreezes at the cold eave, and the ice that forms holds the next thaw against the flashing. The fix is not a better gutter, it is insulation and air sealing in the cavity, which is why the insulation line in a quote matters as much as the window line. If a company describes the roof and the seat cavity as detail work and moves on, that is the moment to press.
How the work actually runs
Five stages. Ask which of them each quote includes before you compare any two of them.
Survey and structure
Someone establishes how the existing assembly is carried, what the header does, and whether the replacement matches the opening. This is where a bay job is won or lost.
Order and lead time
A projecting assembly is made to order. Nothing else can be scheduled honestly until the delivery date is known.
Removal and framing
The old assembly comes out, the opening is exposed, and any framing or header work happens with the wall open to the weather.
Set, support and roof
The units are set and leveled, the support system is fixed, the roof is built and flashed, and the cavity is insulated before anything is closed up.
Finish inside and out
Seat board, interior trim, exterior skirt, siding repair and paint. Confirm before signing which of these are in the quote and which are a separate trade.
Materials, and why wood turns up here
The seat board is furniture, and it is the part of the assembly people touch.
Projecting assemblies attract wood interiors more than any other style, for the plain reason that a seat board is a piece of furniture in the room. A stained hardwood seat under a stained interior frame reads as built-in joinery; the same seat in a molded synthetic finish rarely does. That pushes many of these projects toward clad units, where the exterior takes a durable factory-applied skin and the interior stays timber for finishing on site. It is the one style where the interior finish is often the deciding factor rather than an afterthought, and what wood and clad-wood frames actually ask of an owner is worth reading before committing to the look.
The other materials all exist in this format and all work. What changes with material is upkeep, the width of the frame members between units, and how the assembly handles the condensation that a window seat full of plants and cushions tends to generate.
Eight questions that separate the quotes
None of them is about the window. All of them change the price.
- Is the existing opening being kept exactly as it is, or altered in any dimension?
- How is the assembly supported — from below on brackets, or from above on cables?
- Who builds and flashes the roof, and is it the same crew that sets the windows?
- Is the insulation of the seat cavity and the soffit itemized, or assumed?
- Who repairs the siding and the interior wall where the old assembly met them?
- Is the seat board included, and in what material and finish?
- Who is pulling the permit if the building department requires one?
- What happens to the schedule if the framing behind the old assembly turns out to be rotten?
Ask every company the same eight, in the same order, and write down the answers. Two quotes that looked incomparable usually become comparable once you know which of these eight items each of them contains.
Projecting-window questions
Settle the scope before you settle on a shape
Establish how the assembly is carried and who builds the roof, then ask for estimates.