Sills, Slopes and Storms: Judging Windows by How They Shed Water

Most window comparisons start indoors: U-values, glass thickness, frame colours, maybe a clever handle. All useful stuff. But the weather does not read the brochure. In the UK, wind-driven rain arrives at an angle, often from the south-west, and it is very good at finding the small decisions that never made it onto the showroom display. If you want a window that stays dry and draught-free through a British winter, you need to compare the parts that actually manage water.

Why the weather conversation starts outside the frame

A window is not just a frame with glass in it. It is a small drainage system bolted into a wall. Rain hits the glass, runs down the frame, and has to be guided away from the building. If that route is unclear, or relies on a bead of sealant doing all the work, you are asking for trouble. This is where two quotes that look identical on paper can behave very differently after a few months of real weather.

The sill is not just a shelf

A sill, or cill, is the first line of defence. It needs enough projection and fall to throw water clear of the wall below, and a drip detail underneath to stop water tracking back towards the brickwork. A flush sill can look sharp on a modern house, but on an exposed elevation it may be the wrong tool for the job. When comparing quotes, ask what the sill projection is, whether it has a drip groove, and how it joins the frame. A generous sill with a poor seal is no better than a neat one with a proper upstand and drainage path.

Follow the water: drainage and drip lines

Water should have an obvious route: off the glass, into the frame drainage channel, out through weep holes or a sill outlet, and away. If that route is blocked, pinched or depends on gravity doing something it cannot do, water will eventually find another way. Compare how the frame drains, where the outlets sit, and whether they can be cleaned. On sliding sashes and tilt-and-turn windows, drainage design matters more because there are more ledges and channels for water to sit in. Ask your installer to show you the drainage path on the actual product, not a generic diagram in a brochure.

Exposure is local, not a postcode

Two houses on the same street can have very different weather. A gable end facing open fields, a coastal wall with salt-laden wind, or a hillside spot that catches the prevailing south-westerlies will punish a window far more than a sheltered terrace. Conservation areas and older walls add another layer: the window may need to sit back, keep a traditional profile, or work with a deep reveal. None of that rules out modern performance, but it does mean the installation detail should be designed for the spot. If your installer has not asked about orientation and exposure, that is a conversation worth having before you sign.

Seals, tapes and the bits behind the frame

Weathertightness is a team effort. The frame, glass, gaskets, drainage and the seal between frame and wall all have to work together. A common weak point is the junction between the window and the wall, especially on older properties where the reveal is uneven. Expanding foam alone is not a weather seal; it needs a proper backing and a flexible sealant or airtight tape in the right place. Compare what each installer proposes at this junction. The cheapest quote may be skipping the details that keep driven rain out.

A practical checklist before you sign

Before you compare prices, ask each installer:

• What is the sill projection and does it have a drip groove?
• How does the frame drain, and where are the outlets?
• How will the window and wall junction be sealed, and with what?
• Have you allowed for the exposure of this elevation?
• What happens to water that gets behind the frame?
• Are the drainage outlets accessible for cleaning?
• Will you show me a sample of the sill and frame joint?
• What guarantee covers water ingress, and what does it exclude?

The boring details are the weatherproof ones

A window that looks right in a showroom can still disappoint when the rain comes sideways. The good news is that the answers are not secret. They are in the sill detail, the drainage path and the seal at the wall. Compare those with the same care you give to glass and frames, and you are much more likely to get a window that keeps the weather where it belongs: outside.