Symptoms
- Visible dip or sag in the bay's bottom seat/shelf
- Gaps or drafts where the bay meets the wall
- Cracks in interior trim or drywall near the bay
- Doors or windows in the bay area harder to open than before
Likely causes
- CommonInadequate support under the bay windowBay windows extend past the foundation and rely on brackets or knee braces that can weaken over time.
- CommonWorn weatherstripping and caulk at the multiple jointsBay windows have more seams than a flat window, giving more places for seals to fail.
- PossibleWater intrusion softening the support structureLeaks at the roof cap or seams above the bay can rot the framing underneath.
- Less commonOriginal installation without adequate structural supportSome older or poorly installed bays were undersized for the load from the start.
What to do
- Check underneath the bay window from outside or in the basement/crawlspace for visible support brackets or knee braces.
- Look for daylight or drafts along every seam where the bay's side panels meet the main wall.
- Reseal any cracked exterior caulk at these seams with a quality exterior caulk.
- Replace worn interior weatherstripping around each operable sash in the bay.
- Inspect the small roof or cap above the bay for damaged shingles or flashing that could be letting water into the structure.
- If sagging is more than very minor, do not add heavy items to the bay's seat until it's evaluated.
- Have a contractor assess whether additional support brackets are needed if the sag is structural rather than cosmetic.
Tools and parts
- Caulk gun
- Flashlight
Exterior window caulkSearch: “paintable exterior window caulk”
When to call a pro
Call a carpenter or structural contractor if the sag is visible to the eye or doors/windows nearby are binding, since that points to a support problem, not just sealing.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.