Choosing to replace a roof window is one of the most cost-effective renovation jobs: in a single day you gain insulation, comfort, light and protection against leaks. Many roof windows installed in the 1980s and 1990s are now at the end of their life. Here is how to tell whether it is time to change yours, how to choose the new model and how the replacement works.
Signs that it is time to replace a roof window
- Condensation between the panes: the double glazing has lost its seal.
- Condensation on the inner pane in winter, a sign of poorly insulating glazing.
- Draughts around the sash, caused by hardened seals.
- A timber frame that is blackened, swollen or crumbling.
- Damp stains on the ceiling around the window.
- Difficult operation, a broken handle or hinge.
- Stifling heat in summer, for lack of solar protection.
A window more than twenty-five years old showing several of these signs deserves to be replaced. An isolated leak can sometimes come only from the flashing, which can be repaired without changing the window.
Same opening or a larger one?
In the vast majority of cases, the window is replaced in the same opening. The major manufacturers offer replacement sizes compatible with their older models, which avoids touching the roof structure and limits work on the covering. It is the quickest and most economical solution.
But it is also a chance to rethink the lighting of the room. A taller window, a wider window or a second window alongside can transform the feel of a loft. Enlarging means altering the rafters and fitting trimmers, but it remains routine work for a roofer.
Choosing your new roof window
The type of opening
- Centre-pivot, with the handle at the top: ideal when the window is placed high or behind furniture.
- Top-hung, opening outwards and clearing the view: ideal when the window is placed low.
- Balcony window or roof access window for special needs.
The frame
Varnished timber brings warmth to living spaces. Timber with a white polyurethane finish resists moisture and suits bathrooms, kitchens and children’s rooms.
The glazing
High-performance double glazing is the minimum. Triple glazing further improves thermal and acoustic insulation, welcome in the rain or near a busy road. Laminated glass on the inside improves safety.
Accessories
For summer comfort, an external roller shutter or an external awning blind is far more effective than an internal blind, because it stops the heat before it reaches the glass. Solar-powered versions can be fitted without wiring. A fly screen and a blackout blind complete the set-up for a bedroom.
The steps to replace a roof window
- Visit and measurements: identifying the old model, checking the flashing and the timbers.
- Ordering the window, the flashing suited to the covering and the accessories.
- Protecting the room and the floor.
- Removing the old window and its flashing.
- Fitting the new frame, with an insulating collar around it.
- Waterproof underfelt collar connected to the underlay, plus a drainage gutter.
- Flashing and refitting the tiles or slates.
- Interior finishes: vapour barrier, lining and painting if planned.
A replacement in the same opening generally takes half a day to a day. The room is never left open to the weather.
The flashing is the key
Most leaks around a roof window come from flashing that is unsuited to the type of covering or badly installed. Entrust the replacement to a roofer, who masters both the window and the roof around it.
How much does it cost to replace a roof window?
| Work | Indicative range excluding VAT |
|---|---|
| Replacement in the same opening, standard model | €900 to €1,600 |
| Enlarging the opening | €1,300 to €2,500 |
| Solar-powered external roller shutter | €500 to €900 extra |
| Full interior finishes | on quotation |
The price depends on the size, glazing, accessories and access. For a home more than ten years old, reduced VAT at 6% in principle applies to supply and installation.
Mistakes to avoid
- Choosing a window that is too small to save money: natural light is the main benefit of the job.
- Forgetting solar protection on a south- or west-facing window.
- Reusing the old flashing or fitting flashing unsuited to the covering.
- Neglecting the vapour barrier around the frame, a source of condensation.
- Hiring someone who does not master roof coverings.
Is planning permission needed?
Replacing a roof window with a model of the same size in the same place does not in principle require permission. Enlarging or adding a window may, depending on the region, the municipality and visibility from the street, require authorisation, especially in a conservation area. We check this with you.
Replace a roof window during other work
If you are planning to re-cover the roof, insulate it or convert the loft, schedule the replacement at the same time: the flashing is fitted in the best conditions, with no extra work on the covering. Our article on converting a loft gives advice on positioning windows.
Replacing for better energy efficiency
Old roof windows with single or first-generation double glazing are often the weakest point of an insulated roof. A modern model with high-performance glazing and an insulating collar noticeably reduces heat loss and avoids thermal bridges around the opening, helping to improve the energy rating of your home.
Frequently asked questions
Can you replace a roof window in winter?
Yes, in dry weather. As the job is short, the room is closed up again the same day. We simply avoid days of rain and strong wind.
Can the old flashing be kept?
No. The flashing is designed for a particular window model and ages with it. New flashing is essential to guarantee watertightness.
Can a new roof window reduce heat in summer?
Yes, especially with solar control glazing and external protection. The gain in comfort in a bedroom under the roof is often dramatic.
In summary
When you replace a roof window that is past its best, you improve insulation, remove the risk of leaks and make the loft more pleasant, summer and winter alike. The job is quick when the new window goes into the same opening. Our roofers work in Brussels, Walloon Brabant, Hainaut and Flanders: discover our roof window service and request a free visit.
