Repair window seals.

Repairing or replacing seals is a cost-effective alternative to full window replacement. It prevents moisture from entering your wall cavity and stops the growth of unsightly black mould around your frames.

What Does the Work Involve?

• Inspection: Identifying whether the failure is in the external mastic, the rubber gaskets, or the internal "blown" glazing unit.
• Preparation: Stripping away brittle, cracked, or mouldy old sealant and cleaning the joints with solvent.
• Resealing: Applying fresh, high-modulus silicone or polymer sealant between the frame and the masonry.
• Gasket Replacement: Pulling out shrunk rubber seals from the opening sashes and pushing in new, weather-tight gaskets.
• Drainage Check: Ensuring the "weep holes" in the bottom of the frame are clear to allow water to escape.
• Glass Replacement: If the seal inside the double-glazed unit has failed (causing internal misting), the entire glass unit is measured and replaced.

Typical Costs

Costs are primarily driven by access requirements and the number of windows. Working on ground-floor windows is significantly cheaper than repairs requiring specialist ladders or scaffolding for high-reach areas.

How Long Does It Take?

• Single window reseal: 1–2 hours.
• Full house gasket replacement: 1 full day for a standard semi-detached home.
• Blown unit replacement: 30–60 minutes per window (following a 7-day lead time for glass manufacturing).
• Complex repairs: 4–6 hours if frames require structural adjustment or hardware repairs.

DIY or Professional?

Replacing external mastic is a viable DIY task if you have a steady hand and a good sealant gun. However, if your windows are "blown" (condensation between the panes), this requires a professional to measure and fit a new Integrated Glass Unit (IGU).

Note: Professional repairs often come with a guarantee, which is essential if you plan to sell your home in the near future.

Choosing the Right Tradesperson

• Check Accreditations: Look for FENSA or CERTASS registration, or membership in the GGF (Glass and Glazing Federation).
• Insurance: Ensure they hold valid Public Liability Insurance (minimum £1m).
• Warranty: Ask if they provide a 5 or 10-year guarantee on both the labour and the new glass units.
• Questions to ask: "Will you check the window hinges and handles while you are here?" and "Is waste disposal of old glass included in the price?"

UK Regulations

• Part L Building Regs: Replacement glass must meet specific thermal efficiency (U-value) standards.
• Safety Glass: Any glass below 800mm from the floor or in doors must be toughened safety glass by law.
• FENSA/CERTASS: While minor seal repairs don't usually require a certificate, full frame or glass replacements should be logged with a competent person scheme.

Common Problems

• Sealing over old silicone: New sealant will not bond to old material, leading to leaks within months.
• Blocked weep holes: If a tradesperson seals the drainage holes at the bottom of the frame, water will back up and rot your sills.
• Incorrect sealant: Using cheap "bathroom" silicone externally, which degrades quickly under UV sunlight.
• Ignoring the root cause: Sometimes a "failed seal" is actually a dropped window sash that simply needs realigning.

Frequently Asked Questions