Bay & Bow Windows

A bay window is a three-unit assembly: a center fixed (picture) unit parallel to the house wall, flanked by two angled units (typically 30, 45, or 90 degrees). The mullions between units are where the structural mullion covers carry vertical load. A bow window is a curved assembly of four, five, or six units (usually casements at 10-15 degrees each) that creates a softer arc. Bows don't have the strong vertical mullion lines of a bay, which means they distribute load differently and usually need an additional support strategy — typically support cables from the head jamb to the king studs above.

Bays project further than bows for the same wall opening width because of the geometry. A 90-degree bay (rare but specifiable) projects almost the full width of the side units. A 30-degree bay projects roughly half that. A bow with five 10-degree units projects only 8-10 inches but spans a wider opening and looks more elegant on a Victorian or Craftsman elevation.

We quote both options for any project where the homeowner is undecided. Bays tend to suit traditional and colonial homes; bows suit Victorians, Queen Annes, and homes with curved interior features. Both work fine on contemporary architecture if the trim detail is appropriate.

The header above a bay window has to carry the same roof load it carried before plus any new load introduced by the projection. We size headers per the IRC R602.7 prescriptive tables for typical spans and per the manufacturer's recommended header for the specific unit (Andersen, Marvin, and Pella all publish required header specs in their installation manuals). For a typical 8-foot wide bay carrying a 28-foot tributary roof span, we're looking at a double 2x12 SPF or LVL header — often LVL (1.75x11.875 or 1.75x14) to keep the depth manageable in retrofit framing.

When we remove an existing window and convert to a bay, we re-check the header. Original headers sized for a 4-foot double-hung are rarely adequate for a 7- or 8-foot bay rough opening. We size up to LVL and add king studs and full-height jack studs to either side. On second-floor installs we also check the floor framing below for adequate point load transfer — sometimes adding a doubled joist or a beam under the projection.

The projection itself needs a seat board (the horizontal platform at the bottom of the bay) sized to span without deflection. Standard 3/4 inch ply over 2x6 framing is the minimum we use. For deeper projections (20+ inches) we use 2x8 framing with 5/4 plywood or torsion-box construction. The seat board ties back to the original wall framing with metal straps or lag bolts into the king studs.

Three ways to support a bay or bow projection against gravity and snow load: knee braces from below the projection back to the house wall (the traditional method, visible from outside, common on 1920s Tudor and Craftsman homes), tension cables from the head jamb up to the framing above (invisible from outside, used by Andersen and Marvin as standard on most modern bays), and full cantilever where the floor framing is extended to support the projection without any other bracing (only feasible during new construction or major remodel where the floor system is open).

Cable systems are now the dominant approach for retrofit and new install. Andersen ships their bays with factory-tensioned support cables that anchor to two-by lumber in the framing above the head jamb. We add a second redundant cable on any bay over 8 feet wide or over 18 inches projection. The cable doesn't carry the full load — it controls deflection so the unit doesn't sag over time. The seat board and structural mullions carry the dead load to the wall.

On historic homes where the original look had knee braces, we replicate them in milled cedar or fiber cement, tied through the sheathing into blocking. The braces are partly structural and partly aesthetic — we engineer the cables to do the actual work and treat the braces as a finish detail.

Every bay and bow needs its own roof. Stock units from Andersen and Marvin ship with a factory-built copper or aluminum roof at a 30 or 45 degree pitch that ties into the house wall above the unit. For custom or oversized bays we build the roof on site — standing seam metal or shingles over ice-and-water shield over 5/8 ply, with a step flashing where the bay roof meets the house wall.

The kickout flashing where the bay roof terminates at each end is the single most common failure point. If the kickout isn't installed (or is installed wrong), water sheeting down the house wall above the bay runs off the end of the bay roof and behind the side siding, then sits in the wall cavity and rots out the king stud. We install a fabricated copper or aluminum kickout at every bay-roof-to-house-wall junction. Code (IRC R903.2.1) requires it. Most original installs don't have it.

The head flashing above the unit needs to extend behind the house wall sheathing at least 4 inches and weather-lap over the unit's nailing fin. We use either rigid metal flashing or self-adhered membrane (Grace Vycor Plus, Henry Blueskin) tied into the WRB. Below the unit, on the seat board, we install a sloped sill pan that drains any incidental water back to the exterior — no flat horizontal surfaces inside the flashing assembly.

The two big air-sealing failures on bay windows are the underside of the seat board (between the bay floor and the house wall below it) and the top of the head jamb (between the bay roof and the house wall above it). Both are concealed once the trim goes on, and both are where you'll find ice damage and condensation if they're done poorly.

Under the seat board we install 2 inches of closed-cell spray foam directly to the underside, then build a soffit with R-19 batt and 1/2 inch ply enclosure. The spray foam is the air seal; the batt is the thermal. Above the head, we spray foam the cavity between the bay roof framing and the wall framing, then insulate the bay roof with closed-cell foam between rafters (R-25+ minimum for our climate zone).

Done right, the bay performs at or above the rest of the wall. Done wrong, it's a 30 sq ft cold spot in your living room that drips condensation in February. We've removed and rebuilt enough failed bays from the 1990s-era big-box renovations to know what doesn't work.