Bay windows are a beautiful feature; they add light, space, and character. But they’re also one of the most common places we see problems caused by historic bad installs or corners being cut.
If you’re worried about sagging bays, cracks, dropped window boards and historic bad installs, you’re not overthinking it. And if you’re concerned that previous installers didn’t support the weight properly, that concern is valid too because a bay window isn’t just a “window job”. It’s a structural opening that often carries real load.
This is a behind-the-scenes look at how we approach bay window replacements at Finesse Windows to keep everything supported, aligned, and safe; and most importantly without causing sagging, movement, or cracked brickwork..
Why bay windows are different (and why they go wrong)
A standard window sits in a straight opening, usually supported by a lintel above. A bay window, on the other hand, projects out from the wall and often relies on a combination of:
• The existing frame and internal structure
• The bay roof (if present)
• The window board and internal supports
• The load path above (brickwork, cavity wall, or upper windows
On top of that, a lot of the long-term strength in a bay comes from what you don’t see; the subframe.
At Finesse Windows, we install an aluminium subframe as standard on bay replacements. This provides an additional layer of structural support and helps the bay hold its shape and alignment over time.
When bays fail, it’s usually because the weight above wasn’t properly supported during installation, or because the original build didn’t have the right structural support in the first place. Another common issue we see is bays that were fitted without a proper subframe system, meaning the bay is relying too heavily on the window frames and trims to do a job they were never designed to do.
Common warning signs we’re called out to fix include:
• A bay that looks like it’s “dropping” at the front
• Cracks in the brickwork above or around the bay
• Gaps opening up around frames
• A window board that’s sloping or separating from the wall
• Doors or windows above the bay becoming stiff or misaligned
Step one: we check what’s really holding the weight
Before we remove anything, we inspect the bay and the surrounding structure. This is where many problems are either prevented or discovered.
• Lintels (and whether they’re present, adequate, and in good condition)
• Signs of movement or historic settlement
• How the bay is currently supported (or not supported)
• Whether the load is being carried correctly to the sides of the opening
• Whether there’s an existing subframe system, and if it’s doing what it should
Why does this matter? Because if the opening above relies on the existing frame for support, removing that frame without a plan can cause movement, and that’s when you get cracking brickwork, dropped bays, or internal damage.
Step two: we support the bay before the old frame comes out
Our goal during replacement is simple: keep the structure stable while the old frame is removed and the new one goes in. Depending on the bay design, we may use temporary supports to hold the load safely while we work. That can include:
• Supporting the head of the opening so weight isn’t transferred onto the bay during removal
• Stabilising the bay projection so it doesn’t drop forward
• Protecting the brickwork and internal finishes from movement
This is the part homeowners never see, but it’s the part that protects your property.
Step three: we build in strength with an aluminium subframe (standard at Finesse)
This is a key difference in how we approach bay replacements.
A properly installed aluminium subframe adds strength and stability to the bay structure. It helps the bay stay square, helps distribute load more reliably, and reduces reliance on the window frames alone.
It’s also one of those details that not every installer includes, and it’s often the difference between a bay that stays true for years and one that slowly starts to move.
In simple terms: the subframe helps the bay hold its shape and resist the gradual “creep” that leads to dropped fronts, opening gaps, and sloping boards.
Step four: we install frames designed to stay rigid
Bay windows are more prone to flex than straight runs. That’s why we use steel reinforcement in our window frames where it’s required. Steel reinforcement matters because:
• It helps the frame stay rigid under load
• It reduces the risk of bowing or twisting over time
• It helps keep sightlines straight and sashes operating smoothly
• It supports the structure so the bay doesn’t gradually move
In plain terms: a reinforced frame is far less likely to move, and movement is what causes cracks, gaps, and dropped boards.
Step five: we install frames designed to stay rigid
A good bay replacement isn’t just swapping out old for new. It’s making sure the bay works as a complete system:
• Correct alignment and fixing points
• Proper load transfer to the structure
• Secure joints and stable corners
• A finish that doesn’t hide problems - it prevents them
That’s also why the aluminium subframe matters: it’s part of building the bay as a stable structure, not just fitting frames into an opening.
If we see signs that the bay has been historically installed badly, we don’t just “make it look neat”. We address the cause because cosmetic fixes don’t stop structural movement.
What this means for you
If you’ve been putting off replacing a bay window because you’re worried it will sag, crack, or cause damage; you’re asking the right questions. At Finesse Windows, we take bay replacements seriously because we’ve seen what happens when installers don’t. So if you’re looking at a bay and thinking:
• “Has this dropped over time?”
• “Are those cracks getting worse?”
• “Did the last installer actually support this properly?”
…we’ll walk you through what we find, explain what’s supporting the opening now, and make sure it’s properly supported (including the right subframe system) before we replace anything.
Want us to take a look?
If you’re unsure whether your bay has the right structural support, or you’ve had a bad experience with a previous install, get in touch with Finesse Windows.
We’ll assess the bay, check for lintels and correct structural support, and explain the safest approach to replacement so your new windows look great and stay that way