How Window Drainage Works — and Why It Matters in a Monsoon City

Every window, however well sealed, lets a small amount of water past its outer defences during heavy, wind-driven rain — the gasket keeps most of it out, but not all of it. Window drainage is what happens to that water afterwards: small weep holes built into the frame track let it drain back outside instead of pooling on the sill or working its way into the wall.

It's easy to assume a well-sealed window doesn't need drainage at all, but that's backwards — the better the outer seal, the more important it is that any water which does get past it has somewhere to go. A perfectly sealed track with no drainage path just traps whatever gets in, which shows up months later as staining, a musty smell, or water finding its way inside through a path no one can see from outside.

In a properly engineered aluminium system, drainage is designed in from the start: the sill track is set at a slight fall toward the weep holes, and the holes themselves are positioned and sized to clear water faster than wind-driven rain can add it during a monsoon downpour. It's a small detail that's easy to skip on a site-fabricated window and easy to get wrong even when it's included.

This is also where drainage, sealing, and sound insulation overlap without being the same thing. EPDM gaskets do the work of keeping water and noise out at the sash edge; weep holes handle whatever gets past that first line of defence. A window can have excellent gasket sealing and still fail in heavy rain if the drainage path is undersized or blocked by debris — the two systems have to work together, not substitute for each other.

For a monsoon city like Mumbai, this is worth asking about directly rather than assuming it's handled: ask whether the window's frame profile has a tested drainage path, not just a sealed track, and whether that's been verified as part of the system's water-tightness testing rather than added as an afterthought during fabrication.