Window Seal Repair London: How to Fix Draughts and Leaks

Draughts, water ingress and gaps around windows can make a property less comfortable and may indicate that a component needs attention. However, not every leak is caused by a failed rubber seal — the cause may involve window alignment, drainage, external joints or the condition of the surrounding frame.

What This Guide Covers

• Draughts are often caused by misalignment rather than worn seals — fixing the alignment must come first
• Water ingress can enter at a different point to where it becomes visible inside
• Applying silicone sealant in the wrong place can block drainage channels and make things worse
• Rubber seal profiles can be replaced without changing the frame if the seal dimensions are matched correctly
• Identify the source of the draught or leak before replacing any components

Introduction

London homeowners face unique challenges when it comes to glazing — from period properties with sash windows in conservation areas, to modern developments requiring structural glass installations. Getting the right advice before you commit can save you significant time and money.

At BuilderFix Glazing, we have completed thousands of glazing jobs across Greater London and Surrey — from emergency repairs in Kensington and Chelsea, to bespoke walk-on glass installations in Mayfair and Belgravia. This guide is based on real experience and the questions our customers ask us most often.

In Depth

Window rubber seals — also called gaskets, weatherseals or perimeter seals — are the strips of rubber or thermoplastic elastomer that run around the sash and frame, compressing against each other when the window closes. They create the weathertight line between inside and outside. New seals compress firmly and spring back reliably. Older seals flatten, crack or pull away from their channels, losing the ability to create consistent contact across the full perimeter of the window.

The key point is that a worn seal does not automatically cause a draught. For the seal to work, the sash must close squarely against it with enough pressure to compress the rubber along its full length. If the window has dropped on its hinges — even by 2–3mm — the sash closes unevenly. One corner of the seal compresses fully while the opposite corner barely touches. The draught comes through the gap at the corner, not through a failed seal. Replacing the seal in this situation makes no difference whatsoever. The hinge must be adjusted first.

Water ingress around windows is even more frequently misdiagnosed. Water may enter through a gap in the external sealant bead between the window frame and the surrounding masonry. It may come through a cracked or missing flashing above the head of the window frame. It may enter through a blocked drainage hole at the base of the frame — uPVC windows have small drainage slots that allow any water that gets past the outer seal to escape; if these block with debris, water backs up inside the frame and eventually finds a path inward. None of these faults involve the rubber seal on the sash.

External silicone sealant is appropriate for the joint between the window frame and the masonry — but only when applied correctly and only to a clean, dry surface. Applying a bead of silicone over an existing cracked bead without removing the old material creates a weak bond and usually fails quickly. Blocking a drainage slot with sealant — which happens surprisingly often — turns a small water ingress problem into a large one. Before any sealant is applied, the drainage slots should be checked and cleared.

When seal replacement is genuinely the correct repair — after alignment has been checked and drainage confirmed as clear — the new seal profile must match the existing channel dimensions. uPVC manufacturers each have their own proprietary seal profiles, and fitting a slightly wrong profile either leaves gaps at the corners or prevents the window from closing. The correct approach is to remove a short section of existing seal, measure it, and source the matching profile from a glazing hardware supplier.

What Does This Typically Cost in London?

Costs vary depending on the type and size of work, the glass specification, frame material, and access. As a general guide, always obtain a free written quote before committing — this protects you from unexpected charges.

At BuilderFix Glazing, we provide free, no-obligation written quotes for all work. Our prices are transparent and fixed before we start — no surprises on the invoice.

What to Ask When Getting a Quote

• Is the price fixed, or could it change once work starts?
• What glass specification is included?
• Are all fixings, sealants and finishing included?
• What compliance documentation will I receive?
• What happens if there is an issue after completion?