Repair a roof window.

Whether you are dealing with a persistent leak, a "blown" double-glazing unit, or a mechanism that no longer opens, addressing these issues early is vital. Timely repair or replacement prevents structural timber rot and protects your home's internal finishes from water damage.

What Does the Work Involve?

• Initial Assessment: A thorough inspection of the window, including the frame, glazing, and external flashing, to determine if a repair is viable or if a full replacement is necessary.
• Access Setup: Setting up safe access, which may involve ladders, roof ladders, or a mobile scaffold tower depending on the height and pitch of the roof.
• Stripping Back: For replacements or flashing repairs, the surrounding roof tiles or slates are carefully removed to expose the window's "envelope."
• Unit Removal/Repair: The old window sash and frame are removed, or specific components like the EPDM seals and lead flashing are replaced.
• Weatherproofing: Installing a new underfelt collar and vapour barrier to ensure the window is completely airtight and watertight.
• Internal Finishing: Re-fitting the internal lining or making good the plasterboard and insulation around the new frame.
• Testing: Verifying the operation of the opening mechanism, checking for draughts, and clearing all debris from the site.

Typical Costs

The cost of roof window work depends heavily on the size of the unit, the type of glazing (standard, noise-reduction, or triple-glazed), and the ease of access to the roof. Prices for 2025/26 generally include VAT and reflect standard UK labour rates.

Note: Replacing an existing window is usually cheaper than a "new cut" installation, as the structural opening already exists. However, if the internal timber has rotted, costs will increase for structural repairs.

How Long Does It Take?

• Glass pane replacement: 1–2 hours. This is a straightforward swap that usually doesn't require disturbing the roof tiles.
• Flashing or seal repair: 2–4 hours. Requires removing surrounding tiles to reseal the junction between the window and the roof.
• Full unit replacement (like-for-like): 4–6 hours. This is the most common scenario for older Velux windows that have reached the end of their lifespan.
• Multiple window replacements: 1–2 days. Often more cost-effective as the scaffold and labour costs are spread across several units.

DIY or Professional?

While a confident DIYer might tackle a simple gasket replacement, full roof window replacement is a job for professionals. Working at height carries significant safety risks, and an incorrectly installed flashing kit can lead to devastating leaks inside your roof structure.

Professional installation ensures the window meets thermal efficiency standards and remains covered by the manufacturer's warranty (often 10 years).

Furthermore, if you are replacing a window, the work must comply with UK Building Regulations regarding thermal performance. A FENSA or CERTASS registered installer can self-certify the work, saving you the hassle of a local authority inspection.

Choosing the Right Tradesperson

• Check Certifications: Look for installers who are part of a Competent Person Scheme (FENSA/CERTASS) or are "Certified Installers" for brands like Velux or Fakro.
• Insurance: Ensure they hold valid Public Liability Insurance (minimum £2 million) and are insured for "working at height."
• Scaffolding: Ask if they provide their own tower or if you need to hire an external scaffolding firm.
• Internal Works: Clarify if the quote includes "making good" the internal plasterboard or if you will need a separate decorator.

Questions to ask:

• Is the flashing kit included in the price?
• Will the new window meet current U-value (thermal efficiency) requirements?
• How do you plan to protect my furniture and flooring from dust and debris?
• Do you provide a written guarantee for the installation work as well as the product?

UK Regulations

• Building Regulations (Part L): Replacement windows must meet specific U-values to ensure they are energy efficient.
• Safety Glass (Part K): Any roof window must use toughened or laminated glass to prevent injury if the pane breaks.
• Planning Permission: Usually not required for like-for-like replacements under Permitted Development, unless you live in a Conservation Area or a Listed Building.
• FENSA/CERTASS: You should receive a certificate within 30 days of completion to prove the installation complies with Building Regs.

Common Problems

• Condensation between panes: This indicates the "blown" seal has failed, and the unit is no longer providing thermal insulation.
• Leaking from the top: Often caused by blocked "gutters" in the flashing kit where leaves and moss build up.
• Draughts: Usually caused by perished rubber gaskets or a frame that has warped over decades of sun exposure.
• Stiff operation: Friction hinges require regular lubrication; if left, they can snap or damage the timber frame.
• Internal damp patches: Often mistaken for a window leak, this can actually be caused by a failure in the vapour barrier around the window.

Frequently Asked Questions