A wet reveal, a stained window board or a damp patch below a frame gets reported as “the window’s leaking”. Sometimes the perimeter sealant has failed. Often it hasn’t, and the water is coming through the frame, the glazing, the sill or the wall itself. For FMs and managing agents, a little diagnosis before booking work avoids wasted visits. This guide explains how to narrow it down.
Where water gets in
- The perimeter sealant: the joint between the frame and the wall. Gaps, splits or sealant that has pulled away let rain in.
- The frame and glazing: failed gaskets, blocked drainage slots or damaged frame joints.
- The sill and flashing: poorly lapped or missing sill details let water run back under the frame.
- The wall: porous brickwork, cracked render, failed copings or cavity problems above the window.
- Condensation: moisture forming on cold glass and frames inside, mistaken for a leak.
Clues that point to the sealant
The perimeter joint is a likely culprit if water shows at the edges of the reveal rather than below the glass, and if it appears during wind-driven rain on that elevation. Close inspection may show sealant that has split along its length, peeled off the frame or the wall, hardened and cracked, or has gaps at the corners. Our guide to sealant failure types explains what each kind of failure says about the cause.
Clues that point elsewhere
Water running down the inside of the glass or pooling at the bottom of the frame often means condensation or a glazing issue. Water that appears long after rain, or in patches well above the window, points to the wall or roof. Blocked drainage slots at the bottom of the frame can cause water to back up inside. Clear these before deciding the sealant is to blame. If the frame or glazing itself has failed, that needs a window specialist.
Need the work done? Mastic Doctor covers South East England, London and all of Scotland, with a 12-month guarantee on our workmanship. Send details for a fixed quote.
What a perimeter reseal fixes
Where the sealant is the problem, a reseal is normally straightforward. The old bead is cut out completely. The frame and the masonry or render are cleaned back to sound, dry surfaces, with primer used where the manufacturer specifies it. Backing rod controls the depth where the gap allows, and a low-modulus neutral-cure sealant is applied and tooled. Colour can be matched to the frame or the wall. Our window perimeter sealing page has more.
Before you book
Send photos of the inside and outside of the window, note which elevation it is on and what the weather was when the leak appeared. Say roughly how many windows are affected and how they are reached. On older buildings, check the asbestos register before old sealant is disturbed (see asbestos in old mastic). If you manage several blocks, a planned perimeter programme may make more sense than reacting to each leak.
Managing several buildings
For agents and estates teams with many windows to look after, a single leak is often a warning that others on the same elevation are close behind. Sealant on one face of a building tends to age at a similar rate. When a perimeter reseal is needed, consider pricing the whole elevation or block at the same time. Our guide for managing agents covers block-wide planning, and our mastic sealing page covers the service.
Quick answers
Will resealing stop every window leak?
Only leaks that come through the perimeter joint. If water is entering through the frame, glazing, sill or wall, a reseal won’t solve it. That is why it pays to look at the source first.
Can new sealant go over the old?
No. New sealant needs to bond to clean, sound faces. Sealing over old, failed material traps dirt and leaves a weak bond that soon fails.
Can you match the frame colour?
Usually. Sealants come in a wide range of standard colours, and we’ll confirm the colours available for the specified product with the quote.
Related services
- Facade and window perimeter sealing
- Mastic sealing for building owners and managing agents
- Coloured and colour-matched sealant
More advice
- Adhesive, Cohesive and Substrate Sealant Failure
- Sealant and Curtain Walling: General Guidance
- Sealant Planning for Managing Agents
- Weather and Sealant: When Work Can Go Ahead
- All advice
This guide is general information about sealant work. It isn’t legal, fire-safety or structural advice; for those, speak to a qualified professional.