Quick answer

A uPVC window can leak in heavy rain when its drainage slots are blocked, drainage caps or air holes are missing, the sash is not closing evenly against its gaskets, or the frame-to-wall perimeter has failed. First identify exactly where the water appears. Clean only accessible drainage paths, check that caps and seals are intact, and record the wind and rain conditions. If water enters from the head, wall joint, glazing edge or concealed frame area, arrange a professional inspection instead of sealing random openings—some slots are intentional drainage routes and must remain open.

Start by locating the first visible water

Do not begin by applying silicone around every joint. Dry the sill and track, then observe where water first appears during rain. Water rising inside the lower track points toward a drainage problem. Drops at a sash corner may indicate a closing, gasket or hardware issue. Dampness around the plaster, reveal or frame perimeter suggests that the frame-to-wall junction or the wall above needs inspection. Water at the top should also prompt a check of the head and facade because the visible exit point may not be the entry point.

Record the window, floor, opening type, wind direction and whether the leak occurs only with wind-driven rain. Safe indoor photographs and a short video give the service team useful evidence after the opening has dried.

Check the designed drainage route

A window is not necessarily defective because some rainwater reaches its external drainage chamber. The system collects incidental water and discharges it outdoors. Deceuninck's user guidance explains that purpose-made channels and correctly located air holes assist this flow. It also warns that blocked drainage slots can lead to internal water ingress.

Remove loose dust, leaves or insect debris from accessible tracks and drain openings with a soft brush or vacuum. Deceuninck India advises cleaning drain holes with an air pump or blower and refitting the caps, which assist drainage during windy rain. Do not enlarge holes, drill new routes or push sharp tools into the profile. Drainage layouts vary by system, and unapproved alterations can cause damage.

Inspect caps, gaskets and sash closing

Confirm that external drainage caps are present, undamaged and not buried behind plaster or sealant. The manufacturer's guidance says outlets should remain above adjacent plaster or masonry so water can discharge away from the building envelope. Upper-floor exterior checks require trained personnel and suitable access.

Inspect visible gaskets for dirt, displacement or cuts. Close and lock the sash normally and look for an uneven gap or incomplete handle movement. Do not force the handle or adjust locking points by trial and error. Hinges, keeps and rollers affect sash alignment and compression; incorrect adjustment can create a new leak. Ask the fabricator to assess a sash that rubs, drops or closes unevenly.

Separate a window leak from a perimeter leak

If the drainage path is clear but the wall or reveal becomes wet, inspect the junction between the uPVC frame and the building opening. Cracked, detached or incomplete perimeter sealant, poorly finished plaster, an unprotected head joint or water tracking through the facade can bypass the window's own drainage system. Replacing only an internal bead of sealant may hide the stain temporarily while moisture continues entering the wall.

A competent inspection should review the outside and inside perimeter, sill support, head condition, adjacent facade and any sealant that blocks an intended drainage path. The repair material and joint preparation must be compatible with the frame, substrate and movement expected at the opening. Where the source is uncertain, a staged water test can isolate one zone at a time. Avoid spraying the whole window with a pressure washer; that does not reproduce normal exposure and makes the entry path harder to diagnose.

Rule out condensation before ordering a repair

Moisture on the room-side glass or frame is not always rain leakage. Condensation forms when humid indoor air meets a colder surface, especially in air-conditioned rooms, kitchens, bathrooms or poorly ventilated spaces. A rain leak normally follows rainfall and a specific entry path; condensation relates more closely to indoor humidity and surface temperature.

Check whether moisture appears during dry weather or covers a broad area of glass. Ventilate where practical and review indoor moisture sources. Water or fog trapped between panes of an insulated glass unit is different again and normally requires glazing-unit assessment.

Know when to call the fabricator

Request professional service when water enters from the head or glazing line, drainage remains slow after safe cleaning, a cap or gasket is missing, the sash is misaligned, the handle will not lock correctly, or the frame-to-wall joint is cracked. Treat repeated leakage, damaged plaster, electrical proximity or water from an upper facade as urgent inspection items. Do not lean outside or dismantle glass, beads, hardware or fixed-frame covers.

Send the service team the opening dimensions, room and floor, photographs, opening style, approximate installation date and a video showing the first point of entry. Mention whether the event involved ordinary rain or strong wind-driven rain and whether nearby windows behaved differently. SSS Builds fabricates and installs Deceuninck uPVC systems; a useful service request lets the team distinguish maintenance, hardware adjustment, seal repair and building-envelope work before proposing action.

Use a pre-monsoon routine

Before the wet season, clean accessible tracks and drainage openings, confirm that drainage caps are present, wipe gaskets with a soft damp cloth and operate each sash and lock. Deceuninck India recommends periodic frame cleaning and inspection of seals, hinges, locking parts and screws. Use mild soap and water on profiles rather than abrasive pads, harsh detergents or solvents.

After the first strong shower, inspect the lower track and surrounding wall while the evidence is fresh. Early attention to slow drainage, loose hardware or a separating perimeter joint is simpler than repairing damaged paint and plaster later. For new work, ask the supplier to define the selected system, opening style, reinforcement, glass, hardware, drainage arrangement, installation interface and service scope in the quotation. Water performance belongs to the complete fabricated and installed assembly—not to the profile name alone.

Discuss your requirement

Move from research to the right specification.

Share the application, quantity or opening details with the SSS Builds team for a product-specific discussion.

Explore Deceuninck uPVC Systems

Frequently asked questions

Direct answers

Should I seal the drainage holes in a uPVC window?

No. They are intentional outlets that allow collected water to drain outdoors. Blocking them can cause water to accumulate and enter the room.

Why does my uPVC window leak only during windy rain?

Wind changes the pressure and direction of water at the opening. Blocked drainage, missing caps, uneven sash compression or a weak perimeter joint may show up only under wind-driven rain and should be inspected.

Can I clean uPVC drainage holes myself?

You can remove loose dirt from safely accessible tracks and openings with a soft brush, vacuum or gentle air. Do not drill, enlarge slots, use sharp tools deep inside the profile or lean outside; call the fabricator if drainage remains slow.

How can I tell condensation from rain leakage?

Condensation may appear on broad cold surfaces or during dry weather and relates to indoor humidity. Rain leakage usually repeats with rainfall and has a traceable entry point at the track, sash, glazing or frame-to-wall junction.