A casement window can look flawless from the street and still quietly leak water into the wall cavity for years. The component that keeps that from happening is small, easy to overlook, and almost never discussed in showrooms: the weep hole. For anyone buying aluminium windows, understanding what weep holes do and why they fail is one of the quickest ways to avoid mould, rotted framing, and an expensive reinstall later. That is what this guide covers.
Weep holes are small openings, usually narrow slots or short circular vents, cut into the bottom rail of a window frame on the exterior side. Their job is simple: they give water that gets inside the frame a way to escape to the outside. Aluminium window profiles are hollow extrusions, and rain that blows past the sash or condenses along the glass collects inside those hollow chambers. Without a drainage opening, that water pools at the bottom of the frame and eventually works its way into the building structure and along the interior sill.
The weep hole sits at the lowest point of the sill, and the slot geometry inside the extrusion is engineered so that collected water naturally flows toward the outlet. In a well-built window the drainage path is continuous, unobstructed, and angled so the water leaves the frame and runs onto the exterior cladding rather than back toward the occupants.
Water management in an aluminium casement window is really a multi-step path rather than a single hole. Understanding the full path makes it easier to spot product differences between different windows when you compare suppliers.
Not all weep holes are created equal. A window with a flat, obstacle-free drainage channel drains more reliably in heavy rain than one where water must climb over serrations or through a cramped path. This is where a window's engineering becomes visible during a storm.
Well-designed aluminium casement windows from a reputable manufacturer put real thought into this. For example, the aluminum casement window systems from ALPES use a flat-channel, obstacle-free drainage slot that releases water quickly and keeps the drainage path clear. On their outward-opening casement models, this design combines with soft drainage gaskets so that water exits smoothly while the interior of the frame stays dry. Equal attention is paid on the sliding door side, where a passive check valve accelerates water removal and prevents backflow during wind-driven rain.
Blocked slots. The most frequent cause of leaks is simply a blocked weep hole. Dust, debris, insect nests, and leftover sealant can clog the opening over time. The fix is routine maintenance: clear the slot with a small brush or a straightened paperclip, then flush it with clean water so you can see the water exit freely on the outside.
Sealed weep holes. A surprising number of leaks start because someone sealed the weep holes to "stop a draft" or hide them for cosmetic reasons. Sealing them traps water inside the frame, which accelerates gasket wear, can fog the double glazing, and eventually causes the very wall damage the sealant was meant to prevent. Weep holes are supposed to be visible from the exterior; if yours are hidden, that is a problem, not a sign of a cleaner install.
Covered openings. During renovation or landscaping, trim boards, cladding, or raised planters are sometimes installed directly over the weep openings. Because the holes sit low on the frame, they are easily overlooked. Check that no exterior finish material blocks the bottom rail of the window.
Clogged glazing rebate. On windows where the weep slots are inside the frame rebate rather than on the face, trapped dirt at the bottom of the glazing channel can prevent water from reaching the outlet. If you notice pooling at the base of the glass after rain even though the exterior slots look clean, inspect the bottom of the glazing rebate for debris.
Keeping the drainage system working takes only a few minutes twice a year:
If a window was caulked shut by a previous owner, have the drainage restored by a professional rather than leaving it sealed.
The demands on a window's drainage system depend heavily on where it is installed. In hot, humid coastal regions or locations with strong wind-driven rain, more water reaches the frame and it needs to be evacuated faster. In arid climates, the main risk is dust accumulating in the slots during long dry spells and then blocking drainage when the first heavy rain arrives.
ALPES has supplied aluminium windows and doors for high-end villa projects across the Middle East, Dubai, Saudi Arabia, and Australia, and their casement and sliding systems are designed to cope with both extremes. Their multi-chamber casement window profiles combine the thermal break for insulation with drainage chambers engineered for steady outflow, so the window stays dry inside whether it is facing desert dust storms or tropical downpours.
Weep holes are rarely listed on a datasheet, so ask the supplier directly:
A good supplier will be able to answer clearly. If you are comparing options, choose a casement window supplier that manufactures its own profiles and can explain exactly how water moves through the frame before it reaches the weep hole.
Weep holes look like a trivial detail, but they are the difference between an aluminium casement window that stays dry for decades and one that leaks quietly into your walls. Buy a window whose drainage system is engineered properly, keep the weep slots clear, and never seal them. Those three habits protect the frame, the glass, and the structure that surrounds the window.
If you are planning a project and want aluminium casement windows built with reliable, well-designed drainage, get in touch with ALPES for a quote tailored to your climate and opening requirements.
