Repairing a Cracked Velux Window: Replacement and Repair Options

A cracked Velux window — whether from storm impact, thermal stress, or accidental damage — leaves your roof exposed to water ingress and heat loss. The first decision is whether to replace only the glazing unit, the whole sash, or the entire window frame, and the right answer depends on what exactly has cracked, how old the window is, and whether your home insurance will contribute to the cost.

Key points

• Velux glazing units — the sealed insulating glass panel — can often be replaced without removing the window frame, using a product reference number printed on the inside of the frame.
• A cracked double-glazed unit loses its thermal performance immediately: the argon or krypton gas fill escapes, and the U-value degrades significantly.
• Part L of the Building Regulations requires replacement glazing in an existing dwelling to meet a whole-window U-value (Uw) of 1.6 W/m²K or better.
• FENSA-registered installers can self-certify glazing replacements as Building Regulations compliant without a separate Building Control application.
• Buildings insurance may cover cracked roof windows caused by a sudden event — check your policy schedule before paying out of pocket.

What exactly is cracked?

Not all cracked Velux windows represent the same problem. Before calling anyone, identify what has broken:

Replacing only the glazing unit

For most Velux windows manufactured in the last 20–25 years, replacement insulating glass units (IGUs) are available directly from Velux or through specialist glazing suppliers. This is the most cost-effective repair where only the glass has failed.

How to identify and order the correct unit

1. Open the window fully and look for the white product label on the inside top edge of the frame.
2. Note the product reference (for example, GGL CK04) and the serial number.
3. Use the Velux UK spare parts service or contact a Velux-approved installer to order the matching IGU.
4. Confirm the glazing specification — standard double-glazed, triple-glazed, or laminated inner pane — to ensure the replacement meets current Part L requirements.

Older Velux windows, particularly those installed before 2000, may use non-standard glass sizes that require bespoke manufacturing. Lead times for bespoke units can be 3–6 weeks.

Who should fit the replacement glass?

Glazing unit replacement on a roof window at height requires:

• Suitable access equipment — a ladder alone is insufficient for safe glazing work on a pitched roof.
• Experience with Velux glazing bead removal — incorrect technique risks cracking the new unit during fitting.
• Appropriate disposal of the broken glass — laminated and toughened glass requires separate disposal from standard glazing waste.

A FENSA-registered glazier or a Velux-approved installer is the appropriate professional for this work. FENSA registration means the installer can self-certify compliance with Building Regulations, avoiding the need for a separate Building Control application.

When full window replacement makes more sense

A glazing unit replacement is not always the most economical route. Consider a full window replacement if:

• The frame is more than 20 years old and the timber or PVC seals show clear signs of degradation.
• The sash is warped or the opening mechanism is faulty — addressing multiple faults simultaneously is usually more cost-effective.
• The existing window predates the 2002 Part L revisions, meaning the overall unit may remain thermally poor even with a new IGU.
• The installer's quoted labour cost for a repair approaches 80% or more of the combined cost of a new window and installation.

Deciding what to do: a decision tree

• Is the crack limited to the glass pane only, with no damage to the frame or flashing? → Order a replacement glazing unit using your product reference number.
• Is the window more than 20 years old with other signs of wear? → Obtain quotes for both glazing-unit repair and full window replacement before deciding.
• Is the frame cracked, warped, or no longer sealing correctly? → Full window replacement is likely the better option; seek quotes from a Velux installer or equivalent.
• Was the damage caused by a sudden event such as a storm or impact? → Check your buildings insurance policy before authorising any paid repair.
• Is there water ingress around the frame as well as through the broken glass? → The flashing may also be damaged — ask a roofer to inspect before ordering glazing.

Costs to expect

Indicative UK costs, last reviewed 2026-05-12:

• Replacement glazing unit (supply only): £80–£300+ depending on size, specification (double or triple glazed), and whether bespoke manufacture is required.
• Glazing unit fitting (labour): £80–£200 depending on access and complexity. Access equipment such as scaffold is additional where required.
• Full Velux window replacement (supply and fit): £500–£1,500+ depending on size, specification, and access conditions.

Always obtain at least two quotes; prices vary considerably by region and access difficulty.

What to ask before accepting a quote

• Does the quote cover both supply of the glazing unit and all labour?
• Is VAT included in the stated price?
• What glazing specification will the replacement unit carry — double, triple, or laminated inner pane?
• Will the installer self-certify compliance via FENSA, or do I need to notify Building Control separately?
• Is there a workmanship guarantee, and for how long does it run?
• What access equipment is required, and is its cost included?
• Will the flashing be inspected during the same visit?

When to get professional help

Working on a roof at any meaningful height must be carried out by a qualified professional. The Working at Height Regulations 2005 apply to anyone working at a height from which they could fall and sustain injury — this includes the majority of roof windows on first-floor and higher pitched roofs.

Red flags indicating you should call a professional without delay:

• The crack extends into the frame or sash, not just the glazing.
• There is visible water staining or damp on the ceiling or lining below the window.
• The window no longer closes or latches properly following the damage.
• The roofing material immediately around the window has been disturbed or dislodged.

How Housey can help

For glazing unit replacement or full window replacement on a roof window, window and door installers on Housey can provide competitive quotes from FENSA-registered installers. If you also suspect damage to the surrounding flashing or roof covering, roofers on Housey can inspect and repair the adjacent roof at the same time, avoiding the need for two separate site visits.

Frequently asked questions

Can I replace just the glass in a Velux window myself?

Velux glazing units are designed to be replaceable, but working at roof height to remove and refit glass requires correct access equipment and careful technique to avoid cracking the new unit during installation. For a domestic pitched roof, this work should be carried out by a professional for both safety and warranty reasons.

How do I find the right replacement glass unit for my Velux window?

Open the window and look for the white product label on the inside top edge of the frame. The product reference — for example, GGL CK04 — identifies the exact model. Velux's UK spare parts service and most specialist glazing suppliers can cross-reference this number to the correct insulating glass unit (IGU).

Will buildings insurance cover a cracked Velux window?

Buildings insurance typically covers sudden accidental damage or storm damage to fixed glass. Damage from gradual deterioration or wear is usually excluded. Check your policy schedule and speak to your insurer before commissioning repairs — they may require an approved repairer to carry out the work.

Does a replacement glazing unit need Building Regulations approval?

Yes — replacement glazing in an existing dwelling falls under Part L of the Building Regulations and must meet a whole-window U-value of 1.6 W/m²K or better. A FENSA-registered installer can self-certify compliance; otherwise a Building Control application is required before work begins.

Sources and further reading

• Velux UK spare parts and accessories — VELUX Group
• FENSA — glazing registration and self-certification — FENSA
• Approved Document L — Conservation of fuel and power — GOV.UK
• Working at Height Regulations 2005 — legislation.gov.uk