Last reviewed October 31, 2026.
The big picture window in the living room has fogged between the panes, and somebody at dinner says it out loud: if that window is coming out anyway, why not push it out into a bay.
That one idea changes the job. A flat window sits inside the wall. A bay or a bow hangs off it, so it needs something to hold it up, a small roof of its own, and sometimes a trip to the building department.
Telling a bay from a bow
Andersen defines a bay as three or more windows combined into one unit at various angles. The two side windows meet a center picture window at 30, 45 or 90 degrees, and the 90 degree version, with square corners, is called a box bay. A bow is four or more windows arranged in an arc. Andersen’s bows use at least four casements set at 10 degree angles. That is why a bow reads as a gentle curve and a bay shows its corners.
Pella’s 2026 Architectural Design Manual adds two specs worth knowing. A factory-assembled bay or bow combination has to be less than 144 inches wide, and a head board and an insulated seat board come standard. Andersen describes that inside shelf as a favorite perch for cats and a home for potted plants.
What keeps the unit from sagging
The whole unit cantilevers past the wall, and its weight has to land somewhere. Pella’s manual says its support cables must be attached to structural members capable of supporting 1,300 pounds. When the framing cannot carry that, knee braces are required in addition to the cables.
Pella’s drawings show two ways to anchor those cables: a cross bracing mount and a header mount method. The header mount method requires a projection of less than 18 inches. Andersen goes another way and says its factory-joined bays are supported by brackets underneath.
Ask the installer which method the quote uses, and whether anyone has checked that the framing above the opening can take the load.
The small roof over a bay window
Pella states plainly that its bay and bow units are not designed to support roof structure. So the little roof on top is its own piece of framing, covered with shingles or with metal such as copper.
Copper is sold by weight. Patrick Sheeley, writing in Fine Homebuilding in November 2007, explains that 16 ounce and 20 ounce refer to the approximate weight of one square foot of sheet, and that the lighter 16 ounce is the one used most in residential work.
For a flat-seam bay roof, Sheeley says each panel should not exceed 24 by 18 inches before the locks are bent, and that flat seams on a roof with no pitch must be soldered. Out on Shelter Island, Professional Roofing reported in December 2019 that TRM Enterprises of Bridgehampton covered four angled bay windows with 300 square feet of 20 ounce locked-seam copper over plywood and self-adhering underlayment.
The joint where that roof meets the wall needs flashing like any other roof-to-wall joint. Our roof flashing guide covers those details, and the metal roof cost guide has Long Island prices for a copper bay roof.
When a bay needs a building permit
Swapping an old bay for a new one the same size is one filing question. Cutting a flat wall open and pushing a bay out of it is another.
The Town of Huntington’s filing guidelines, revised September 1, 2021, say no building permit is required for roofing, siding and window or door work “if replacing in-kind and no structural changes,” and ask owners to contact the Building Department before working on structures. A new bay hanging off framing that never carried one, or an opening that gets wider, is hard to call in kind. The details are on our Huntington permit guide.
Islip’s town sheet draws the line at whether headers and studs are disturbed, and our Islip page quotes it. Villages keep their own building departments, so call the one that covers your address before the unit is ordered.
Bay and bow prices from Long Island companies
| Unit | Installed price | Source |
|---|---|---|
| Bay, bow or specialty | $1,800 to $4,500 each | EcoView Windows of Long Island, Islandia (Aug 2026) |
| Composite bay or bow | $2,800 to $5,500 | Long Island Windows and Siding, Setauket (Oct 2025) |
EcoView labels its figures as estimates for planning only. Neither page itemizes the support hardware, the roof over the unit or the permit filing, so ask for each as its own line on the quote. Our window replacement cost guide compares bays with other window styles and covers the lead paint rule for pre-1978 houses.
Questions to settle before you sign
- Is it a bay or a bow, how many units, and what are the angles?
- Which support method is included, cables, brackets or knee braces?
- Who builds the roof over it, and in shingles or copper of what weight?
- Does the opening change size, and who files the permit if one is needed?
- Are the head board and seat board insulated, and who finishes the interior trim?
The window replacement page has the rest of what to compare between window quotes.
