Replacing Your Old VELUX Roof Windows? Here’s What to Watch Out For

VELUX Roof Window Replacements: What Every Installer Needs to Watch Out For

Replacing old VELUX roof windows isn’t always as straightforward as swapping like-for like. Many of the windows now being replaced are 25+ years old, and back then both

installation methods and product design were different. That means you’ll often

uncover challenges on-site that, if not prepared for, can slow the job down or lead to an

imperfect finish.

Below are the most common issues you’ll run into – and how to deal with them like a

pro.

1. Brackets on Old VELUX Windows Often Sit Too Low

On older VELUX models (size codes purely numerical, such as GGL–1 to GGL–9 or 101–

810), installers sometimes misread the old instructions and fixed brackets too low.

• This usually means the window is sitting 25–40mm higher than it should.

• The giveaway? The window looks like it’s perched above the roofline, and

removing the side frame covers reveals gaps and exposed frame.

• The fix: before quoting, check bracket placement. Re-sitting the new window

correctly often requires an EL/EW replacement flashing (instead of EDL/EDW).

If lowering the window is tricky, consider using an ELX flashing to bridge the gap.

Tip: Always spot this at survey stage – it’s extra work you’ll want priced in.

2. Roofing Material Changed from Slate to Tile (or Vice Versa)

If the roof has been re-covered since the original window was installed, you’ll need to

double-check flashing compatibility.

• VELUX no longer manufactures flashings for pre-V21 windows (anything with all numeric size codes).

• EL/EW flashings won’t fit older windows – parts are simply too long or too short.

• In these cases, the only safe solution is to fit a new VELUX window when the

roof covering changes.

3. Tiles Cut Too Tight Against the Window

It’s common to find tiles butted right up to the sides, top or bottom of the old frame.

• For a proper replacement, these need to be trimmed back to leave the correct

expansion and drainage gaps.

4. Matching Replacement Tiles or Slates

A new window opening often disrupts the surrounding roof covering.

• If you need extra slates or tiles, take them from a less visible part of the roof

(e.g. rear elevation).

• Replace the donor spot with less tidy pieces so the finished window area looks

seamless.

5. Flashing Apron Adjustments

Another common problem: tiles or slates fixed hard against the old bottom flashing

apron.

• The new window usually sits lower in the roof than the old one, meaning you’ll

need to extend the opening slightly to accommodate the new apron.

• For windows 25+ years old, it’s often best to retain the old installation height.

Modern VELUX windows sit 27mm deeper, but older rafters rarely leave enough

tolerance for this.

• Exception: The GGL–5 (700mm wide). Its modern replacement FK06 is 40mm

slimmer, giving enough clearance to install at the lower height.

6. Don’t Reuse Old Brackets

Reusing brackets is asking for trouble – they’re rarely accurate, and with large windows

like SK08 or UK08, even a slight twist can throw the sash out of line.

• Always fit the new brackets supplied.

• For any long window (1400mm+), don’t forget the extra L-shaped support

brackets on either side. They prevent timber deflection and frame distortion over

time.

7. Old Screw Fixings vs. New Push-Fit System

On older windows, the screws used for the external covers can cause leaks and water

ingress.

• Modern VELUX windows use a push-fit system instead – much cleaner, much

safer, and no hidden damage risk.

8. Plasterboard Groove Mismatch on Smaller Windows

Older models like the GGL 102, 104, 206 had slimmer frames to maximise glass area.

• Their plasterboard grooves are 10mm narrower and 14mm shorter than today’s

equivalent models.

• The result? A 5mm gap on each side and 7mm at top and bottom.

• Solution: trim out with a thin bead or caulk for a neat finish.

Final Thoughts

VELUX replacements are rarely “plug and play.” Knowing where the pitfalls lie will save

you time, hassle, and callbacks.

• Always check bracket heights on the old install.

• Don’t assume flashings are available for older models.

• Allow for trimming tiles/slates and matching materials carefully.

• Never reuse old brackets – today’s systems are designed to last longer.

By spotting these issues before quoting, you’ll not only protect your margin but also

deliver a smarter, more professional job that keeps customers happy for the next 25

years.