Waterproofing windows, drip caps and sills: how to stop rain leaks

Waterproofing windows, drip caps and sills is one of the most common causes of rain leaks in homes and apartment buildings: water doesn’t pass through the glass—it gets in through poorly detailed junctions (perimeter joints, subframe, drip cap/sill, cracks in the render) and ends up staining the interior.

How to tell if the leak comes from the window (and not the wall)

Before sealing at random, identify the entry point:

• Stains at the upper sides of the opening or in a corner: usually the frame-to-masonry junction or a crack in the exterior finish.
• Drips under the window or on the front edge of the sill: often a drip cap/sill without a drip groove or with insufficient slope.
• Intermittent damp only with wind-driven rain: typical of degraded joints or missing overlaps.

If it only happens with wind-driven rain, it’s almost always a detailing/junction issue—not “rising damp”.

Critical points: subframe, joints, drip cap and sill

Perimeter joint (frame–wall)

The most frequent weak spot. Typical failures:

• Aged or debonded sealant.
• No backer rod (no joint backing), leading to cracking.
• Surface-only sealing with no continuity to the façade finish.

Subframe and fixings

In refurbishments, water often finds a path through:

• Unsealed screws/anchors.
• Voids where the subframe meets the render.

Drip cap / exterior sill

A correct detail should include:

• Outward slope.
• A drip groove underneath to prevent water running back.
• Properly finished sides (end dams/returns or sealed junctions).

Interior sill

Once water gets inside, the sill can spread the moisture. The goal is to stop entry outside, not to “paint over” the stain inside.

Fixes that work (and when to use them)

1) Re-do the perimeter seal with the right joint build-up

Best when the window unit is otherwise in good condition.

• Remove old sealant.
• Clean and dry.
• Install a backer rod to control sealant depth.
• Apply an exterior-grade elastic sealant (PU or MS polymer) compatible with the substrate.

2) Waterproof the junction with an elastic tape/band + compatible finish

When there are cracks or discontinuity between the frame and the wall finish:

• Elastic band/tape around the perimeter.
• Compatible skim/reinforcement and final finish.

3) Correct or replace the drip cap / sill

If there’s no drip groove or the slope is wrong:

• Cut/add a drip groove (when feasible).
• Replace with a unit that has projection and a drip groove.
• Seal side and top junctions correctly (without trapping water).

4) Repair cracks in the façade finish around the opening

If water enters through nearby hairline cracks:

• Open and prepare the crack.
• Use an elastic sealant or suitable repair mortar.
• Finish with a compatible façade coating (breathable where required).

Common mistakes when “sealing a window”

• Using household silicone outdoors: short lifespan and poor adhesion.
• Sealing only from the inside: hides the symptom, doesn’t stop the water.
• Applying a very thin bead with no backer rod: cracks quickly.
• Blocking the window’s drainage/weep paths.
• Ignoring the drip cap: without a drip groove, water can run back to the wall.

When it’s a building-wide issue (not just one apartment)

In apartment buildings, the source is often in common elements:

• Cracked façade finish around openings.
• Repeatedly poor drip-cap details along an entire elevation.
• Junctions at parapets, slab edges or exposed façade areas.

In Madrid, intense rain combined with wind makes these defects show up fast. If multiple units are affected on the same orientation, it’s worth arranging a façade and junction inspection and fixing it as a coordinated job.

Quick checklist before calling a specialist

• Does it happen only with wind-driven rain?
• Any cracks around the opening?
• Does the exterior sill have slope and a drip groove?
• Is the perimeter joint cracked or debonded?
• Are other neighbours reporting the same issue?

We can help you stop the leak without unnecessary works

At Impermyna we pinpoint the entry point and propose the most effective fix (technical resealing, elastic tapes/bands, drip-cap corrections, or façade junction repairs). If you’re in Madrid or Toledo and dealing with window rain leaks, contact us and we’ll review it with you.