The water on your windowsill is coming in through the holes your frame is trying to drain

Ask why water runs inside past a sealed-looking aluminium slider after a hard storm and the answer is almost never the glass, the brush seals or the frame joints: it is the tiny slots along the bottom of the outer track, designed to dump wind-driven rain back outside, doing it at maybe ten percent of their capacity because the rain has packed them with harmattan dust, paint overspray and the mortar crumbs of the fitting. Unblock those five minutes of work and the window returns to the design it was sold with; leave them and every heavier rain tests the same weak point harder.

The system in one honest paragraph

Sliding frames expect water in. Storm gusts force rain past the sashes into the track by design - the track is a gutter, and the weep holes are its downpipes, usually at the outer bottom edge of the frame near each end, sometimes under little snap caps or a slotted cover. When the holes run clear, the track drains within minutes of the rain easing. When they clog, the track fills to the lip and finds the path nobody designed: over the end weep, past the end plugs, under the sash and onto the sill - which is why the leak seems to be at the corner, where the standing water simply reached the corner first.

The clear, and the two ways to botch it

• Tool set: a cocktail stick or fence pick, an old brush, a cup of water. Open the sliding leaf fully, clear each hole from inside and outside, brush the channel, then pour and watch: clean weeps drink the cup within a minute and show it leaving outside. The whole job is two minutes per window; do the whole house before the first hard storm of the rains and again after harmattan settles.
• The silicone botch: the single most common way windows flood is the fitting's own tradesman sealing the frame to the wall and running the same bead over the weep holes "for a clean finish" - or a paint job glazing them shut. Any weep hole you find under paint or silicone is opened; the wall joint is where the seal belongs, the holes stay as born.
• Pressure-washer botch: the compound crew hosing the facade at the closed window will drive a barrel of water past every seal in seconds. The squeegee-and-cloth version reaching the track is how facade cleaning and dry rooms coexist.

The trade-off nobody warns you about: the holes work both ways

Open drains are open doors - to mosquitoes at dusk and to the small, fast cockroaches at night. The answer is not to close them but to screen them: narrow mesh offcuts or brush-seal strips pressed into the channel behind the holes, sold for exactly this, cut to span the slot rather than seal it. One honest housekeeping note: a screened weep needs the same twice-a-year clear or it clogs twice as fast - the mesh is doing its job by catching what the window would otherwise pass.

When clean weeps still end with a wet sill

Three suspects remain and each belongs to a different page of this desk: the ground or tile lip outside sits level or falls toward the frame, so the wall itself becomes the bath - regrade or add a drip channel to the outer sill; the water is a guest from above, a roof or canopy leak running down the cavity to appear at the window (the water-travels principle of this desk's ceiling-leak guide - chase it up, not in); or the sash brushes and end plugs are past their life, which is a parts job for the aluminium man rather than this page. The order of reading the bill, though, stays: holes first, because they are free, then falls, then parts.

The desk's standing rule

Two appointments a year and a pick: pre-rains clear, post-harmattan clear, cup test each one; never paint or silicone over a weep hole, never screen one without scheduling it into the same appointment. Every flooded sill this desk has seen had a working drainage design that nobody maintained - the window did exactly what it was built to do; the house stopped helping it.