Leaking Window

A leaking window needs a water-path investigation, not simply a draught adjustment. Start with the conditions: does water appear during wind-driven rain, after prolonged rainfall, or on dry mornings? Rain entering around a frame, moisture condensing on indoor glass and condensation trapped between panes are different problems with different remedies.

Photograph the first wet area before wiping it away, then note which direction the rain is coming from. Record whether water collects in the lower frame channel, runs from an upper reveal or soaks the plaster beside the cill. The place where water becomes visible is not always its entry point; it may have travelled behind finishes or through a wall junction.

Drainage slots need a clear route out

Many uPVC window systems manage small amounts of water that enter their outer channels by draining it back outside. If outlets become blocked with dirt, debris or misplaced sealant, water may accumulate and overflow towards the interior. The relevant channels and exits depend on the profile, so identify the system before assuming every hole has the same purpose.

Remove accessible loose debris carefully and follow the manufacturer’s maintenance guidance. Do not drill extra holes, push sharp tools deep into the frame or seal an opening simply because it looks untidy. A drainage route may also be obstructed by an incorrectly fitted external cill or finishing material, which requires a fitting assessment rather than repeated surface cleaning.

External sealant and mastic joints

The perimeter joint between frame and masonry is another common area to investigate. Weathered mastic can split, lose adhesion or pull away where materials move. Look from a safe position for visible separation, missing sections and water staining. Defects higher up may feed a wet patch lower down, so a bead beside the stain may not be the relevant joint.

A durable repair needs a suitable joint design, compatible materials and sound surfaces. Smearing fresh sealant onto damp, loose or dirty old material often hides the problem without restoring the weather barrier. Interior silicone alone is not a reliable cure for rain entering outside: it can leave water trapped within the surrounding construction.

Cills, ends and the wall below

An external cill should direct water away from the wall, with its discharge edge and drainage connections working as intended. Cracks, poor end details, unsuitable falls or gaps beneath the frame can let water track into places that should stay dry. A damaged end cap may be a clue, but the underlying assembly must be checked rather than blaming the cap automatically.

Water staining beneath a window can also originate in surrounding masonry, pointing or junctions above. If damp persists after a frame repair, widen the investigation instead of applying more mastic. The relationship between the window, cill, wall and any cavity drainage detail may need attention. Avoid disturbing these components without understanding how water is meant to escape.

Bay windows and leaks from above

Bay windows bring roof coverings, flashings, frame connections and sometimes gutters into the diagnosis. Water appearing at a bay head or ceiling may come from the small roof above, rather than through the opening window. Wind direction can affect which junction is exposed, explaining why a bay stays dry in some storms but leaks in others.

Keep photographs of any staining across the ceiling and adjoining wall, not just the wet frame. Checks at height should be left to someone with suitable access equipment. A bay-roof or flashing defect needs the appropriate repair; changing the window’s rubber gasket will not stop water arriving from above. Where different trades are needed, establish the suspected route before commissioning disconnected repairs.

What to do while arranging an assessment

Protect nearby furnishings, collect drips where safe and dry accessible surfaces to limit damage. Keep clear if water is near sockets or electrical fittings, and seek appropriate electrical advice rather than touching wet equipment. Do not open up plaster or remove glazing to investigate an active leak yourself.

An inspection can consider drainage, gaskets, glazing details and installation joints alongside the surrounding building. Any controlled water testing should be methodical, working through suspected areas rather than flooding the whole elevation. Once the entry route is repaired, allow affected materials to dry before redecorating. Fresh paint over damp plaster can hide continuing evidence without resolving the cause.