Where the Rain Is Supposed to Go: A Bristol Guide to Sills, Seals and Drainage

When you stand on the pavement and admire a new window, your eye goes to the frame colour, the glazing bars, maybe the handle. Fair enough. But the part that decides whether your wall stays dry is often the part you cannot see from there: the external sill, the sealant line beneath it, and the little drainage route that takes water away from the frame. In a city that gets its fair share of wind-driven rain, that unseen half of the job deserves a closer look.

Bristol homes are a mixed bag. You might have a Victorian terrace with a stone sill, a 1930s semi with a concrete cill, or a newer flat where the window sits close to rendered wall. Each has its own way of shedding water. The principle is the same, though: water should land, run, and leave. It should not sit, soak, or find a gap.

The route water is meant to take

A well-detailed window has a few quiet defences. The external sill should slope gently away from the building, so water runs off the front edge rather than back towards the wall. Underneath the sill, there should be a drip groove or drip line — a small channel that encourages water to fall clear instead of clinging to the underside and tracking back to the masonry. Around the frame, the sealant line should be continuous and neat, but not doing the job of a dam. And inside the frame, the drainage track and weep holes should let any water that gets past the outer seals escape to the outside.

Think of it as a chain. If one link fails, the water finds another way. A sill with no fall is a shelf. Sealant smeared over a drainage slot is a plug. A weep hole blocked with grit and pollen is a forgotten exit.

What a badly fitted sill looks like

You do not need a ladder and a moisture meter to spot trouble. Some signs are visible from ground level. Look for green algae or dark streaks on the render below a window. Look for a white, powdery bloom on brickwork — efflorescence — which often means water is moving through the wall and drying out on the surface. Look for paint or render that has blown, or a tidemark just under the sill.

On the sill itself, check the slope. A sill that looks flat, or worse, tips back towards the glass, is asking for trouble. Sealant that bridges the gap between sill and wall in a thick, uneven bead may look reassuring, but it can trap water instead of shedding it. If the sealant has split, gone black, or pulled away, that is a maintenance job rather than a reason to panic — but it should not be left through another winter.

Bay windows deserve a special mention. They have more external joints, more sill length, and often more opportunities for water to sit. On older Bristol bays, the original stone or timber sill may have been patched over the years. If a new window is fitted without checking the sill beneath, you can end up with a smart frame sitting on a tired base.

The drainage track is not a gutter

Modern uPVC and aluminium windows usually have a drainage system built into the frame. Water that passes the outer weather seal lands in a small track and should exit through slots or weep holes at the front or sides. These are small by design. They block easily.

You can check the visible slots from inside and outside when the window is open — gently. Do not poke them with a screwdriver, and do not seal them up because they look like gaps. If you can see caps or covers, make sure they are the ventilated type and not painted over. A quick brush or vacuum with a soft nozzle can clear loose dust, but if water is sitting in the track, it is worth getting someone to look at it rather than drilling extra holes.

Blocked drainage often shows up as water appearing on the internal sill after heavy rain, or as a faint musty smell around the frame. It can also cause the glass unit to mist between the panes over time, although that is not the only cause. The point is simple: the drainage path should be clear before the next downpour.

Questions to ask before installation

When you are comparing quotes, the conversation often sticks to glass ratings and frame colours. Add a few questions about the unseen work. A good installer will not mind.

Ask how they will check the existing sill and wall condition before fitting. Ask what fall they will set on the new sill, and how they will make sure water clears the wall below. Ask what type of sealant they use and whether they will leave a drainage gap rather than a fully sealed bead where one is needed. Ask how the frame drainage works and where the water exits. Ask whether they will replace or repair the sill if it is cracked, hollow, or sitting at the wrong angle. And ask what happens to the render or plaster around the window if making good is needed.

It also helps to ask about the drip line in plain English: 'If water runs off the sill, where does it go?' If the answer is vague, keep asking.

Repair first, sometimes

Not every damp patch means a new window. If the frame is sound and the glass is fine, a failed sealant line or a poorly draining sill can often be put right on its own. That is usually cheaper and less disruptive than a full replacement. On older properties, especially in conservation areas, repairing the existing sill and re-detailing the sealant can be the better move. The window has probably done its job for decades; it may just need the water route cleared and the edges looked after.

A small check before you sign off

When the job is finished, take five minutes before the fitter leaves. Look at the sill from outside if you can do so safely. Does it slope away? Is the sealant neat without blocking any drainage slots? Are the weep holes visible and clear? Inside, check that the frame is not sitting in a puddle of sealant and that the window opens and closes without rubbing. Then, after the first heavy rain, have a look at the wall below. A dry wall is the quiet proof that the unseen half of the job was done properly.

Windows are judged by how they look, but they earn their keep by how they handle water. In Bristol, that is not a small thing.