Below the Frame: The Drainage Details That Decide Whether Your New Window Keeps Rain Out

A window quote is a funny document. It will tell you the glass is A-rated, argon-filled and made with a warm-edge spacer, then it will say 'make good' and expect you to nod. But the part that decides whether you spend the next decade looking at a brown tidemark above the skirting is often one short line: 'supply and fit sill'. That's it. No material, no fall, no mention of a drip groove. Nothing about where the water is supposed to go.

That is the tell. If a quote is detailed about the frame and vague about the bottom edge, it has likely been written for the showroom, not for a British winter.

The sill is doing more work than the glass

The glass gets the headlines, but the sill is the part that actually manages rain. A correctly detailed sill throws water clear of the wall, drips it away from the brickwork and seals the joint between the frame and the building so that driving rain does not track sideways into the reveal. In the trade it is often spelled 'cill', which is a useful reminder that it is a small civil engineering project, not a shelf.

When it goes wrong, it goes wrong slowly. First a patch of damp plaster under the window. Then bubbling paint. Then a musty smell in the corner of the room and a bit of wallpaper that lifts when you touch it. By the time you can see it inside, the water has usually been finding a path for months.

What a proper sill detail actually includes

A quote that has thought about drainage will usually mention, or at least be able to explain, a handful of things:

• The sill itself — material, colour, dimensions and whether it is a stub sill, a full sill or a sill with end caps. 'Standard white' is not a specification.
• The fall — a slight slope away from the frame so water runs off rather than sitting in a puddle against the sealant.
• The overhang and drip groove — a bit of projection past the wall and a groove on the underside that stops water tracking back to the brickwork.
• The seals — proper sealant at the frame-to-sill joint and at the ends, not a smear of mastic over a gap.
• The cavity closer or tray — the detail at the reveal that stops water getting into the cavity and finding its way inside.
• The weep holes — the drainage slots in the bottom of the frame. They need to be there, they need to be clear, and they should not be siliconed over by someone in a hurry.

You do not need to become an expert in all of this. You just need to notice whether the person quoting you can talk about it without reaching for the brochure.

The words that should make you pause

Some phrases are not necessarily bad, but they are doing a lot of heavy lifting. 'Make good' is the classic. Does it mean re-rendering the wall under the sill, or does it mean a bead of sealant and a shrug? 'Existing sill retained' is another one. Sometimes that is fine — a sound concrete or stone sill can stay. But if the old sill is cracked, hollow or sitting at the wrong fall, keeping it is just moving the problem six inches lower.

Also look out for a quote that lists 'sill' without saying whether the old one is being removed. Removing a sill can mean disturbing render, brickwork and the damp proof course. That is not a reason to avoid it, but it is a reason to have it priced and described rather than discovered on the day.

Five questions to ask before you sign

1. What sill are you fitting, and does it have a drip groove and end caps?
2. Is the old sill coming out, and what happens to the render or brickwork underneath?
3. How is the sill sealed to the frame and to the wall, and what sealant is being used?
4. Are the frame's drainage slots left clear, and is a cavity closer or tray included at the reveal?
5. Does 'making good' include internal plaster and external render, or just a sealant bead?

Ask these in a normal voice. A good fitter will answer them in plain English. A poor one will tell you not to worry about it, which is your cue to worry about it.

The cheap detail that costs the most later

The frustrating thing about sill and drainage work is that it is invisible when it is done well. Nobody stands in the garden admiring a drip groove. So it is tempting to shave a few pounds off the job by accepting a low-profile sill, a reused old sill, or a 'we'll seal it on the day' approach.

But fixing a failed sill later is not a small job. You may be looking at removing the window, cutting back render, replacing plaster inside, and paying for scaffolding or access again. The quote that looked £200 cheaper can turn into a four-figure repair, plus the cost of redecorating the room you have just finished. Rain has a lot of patience. It will find the weak point eventually.

None of this means you need the most expensive sill on the market. It means you need a quote that has actually specified one. A sensible mid-range sill, fitted with a fall, a drip groove and a proper seal, will outperform a fancy frame sitting on a badly detailed base.

The bottom line

When you are reading a 2026 window quote, start at the bottom. The glass and the frame matter, but the sill, the weep holes and the cavity detail are what separate a dry room from a damp one. If those things are missing from the paperwork, they are probably missing from the plan. Ask now, while the window is still in the workshop and the wallpaper is still on the wall.