A roof window (janela de sótão or janela de telhado) is not a facade window: it is fitted into a roof, with flashings and tiles around it. “Velux” is a brand, but many people use the name for any roof window. In this guide I only read pages from Velux Portugal, so the product and warranty points are about that brand. For the law, I read the official text.
When to replace
Velux Portugal says that when roof windows are 25 to 30 years old it is time to think about replacing them. Signs it lists: cracks that cannot be repaired, leaks that damage the timber, worn seals with draughts and misted glass, which the brand links to spent gas in the glass. If you are repairing the roof, it is a good moment to replace the windows. These are the maker’s statements, not an independent body’s. Mist between the panes has its own guide.
Replacing a window with another of the same size: Velux says it takes a few hours or half a day. Cutting or enlarging the opening takes one or two days. The work it describes is removing the old window and the tiles around it, fitting the insulating frame and the outer frame, connecting it to the tiles, mounting the sash and redoing the interior finish.
Licence and the câmara
Here the sources do not agree.
- Velux says, on one page, that replacing with a window of the same size needs no permit or authorisation, and that enlarging the opening or adding new windows may need a licence. On another page it says no licence is needed if the windows take up less than 20% of the roof. It does not name the law.
- The law. In Decreto-Lei 108/2026 (Diário da República, 29 May 2026; in force since 1 October 2026), “alteration works” are those that change the physical or aesthetic characteristics of the building (article 2(d)). Exempt from licence and prior notice, among others, are replacing roofing materials with ones with an identical exterior finish, and replacing the frames of openings with the same geometry and an identical finish (article 6-A(1)(h) and (j)). The text I read says nothing about roof windows or a 20% rule. Whether a like-for-like replacement fits the exemption for “frames of openings”, I could not confirm.
- Listed buildings. The exemption does not apply (article 6-A(2)) and works need a licence (article 4(2)(d)).
- Municipal regulations can also classify other works as exempt (article 6-A(1)(i)).
Still, ask the câmara if you will open a new window, enlarge the opening or change the look of the roof. If in doubt, say which window, what size and what colour.
If there is a condominium
The Civil Code says the roof is a common part of the building, even when it serves a single flat (article 1421(1)(b)). If you live on the top floor and want a roof window, talk to the administrator. Works that change the architectural line or the aesthetic arrangement need the assembly’s prior authorisation by two thirds of the building’s value (article 1422(3)). If it is an “innovation”, article 1425 applies. Which of the two is your case, I cannot say. See the guide on replacing windows, which has a letter for the administrator.
Safety
Working on a roof is dangerous: the European Agency for Safety and Health at Work says it needs strict safety rules, even for short jobs (fact sheet 49, 2004). Velux recommends a specialised team, with brand installers. The Velux warranty stops covering defects that result from installation contrary to its instructions.
Warranty
The Velux Portugal page gives 10 years for the roof window, 5 for external blinds and 3 for curtains. The document I read (edition of 1 February 2022) includes the glass in the 10 years and replacement glass at 5, and excludes condensation, installation outside the recommended areas and lack of maintenance. That edition may no longer be the one in force: ask for the current one.
Condensation and comfort
Velux says condensation on the glass is a sign to ventilate more: at least 5 minutes, 3 to 4 times a day, without drying clothes indoors. See ventilation and the condensation calculator. For summer heat, see the solar factor. I found no comparable insulation values from Velux to give here: ask for the data sheet of the model and read the glass guide.
How to order the right window
- The identification plate is on the upper part of the frame (on windows made after 1968, in the top right corner, behind the ventilation membrane). It shows type, size, variant and production code.
- Velux suggests the window should have at least 20% of the usable floor area of the room. This percentage has nothing to do with the licence one.
- Ask the quote to include the insulating frame, the flashing and the interior finish. Use the quote checker.
What I could not confirm
- Whether the law requires a licence or notice to replace or open a roof window, and Velux’s 20% rule.
- Whether a like-for-like replacement counts as an exempt frame replacement.
- Insulation values (Uw) and prices of roof windows.
- Whether the window grants cover roof windows: see the grants guide.
- Velux’s current warranty, beyond what the page says.
Sources
- Velux Portugal: find an installer, how to replace a roof window and replacing roof windows (maker): age, time, steps, licence statements. Read on 23 October 2026.
- Velux Portugal: warranty and the document “Garantia dos produtos VELUX” (edition V-P 01022022). Read on 23 October 2026.
- Velux Portugal: care and maintenance and identification plate. Read on 23 October 2026.
- Ordem dos Arquitectos: Decreto-Lei 108/2026 of 29 May (copy of the Diário da República): articles 2, 4, 6-A. Read on 23 October 2026.
- Mapfre: Civil Code, horizontal property: articles 1421, 1422, 1425 (wording of Decreto-Lei 267/94). Read on 23 October 2026.
- EU-OSHA: Fact sheet 49, Safe work on roofs, 2004 (Portuguese version). Read on 23 October 2026.
What to do next
- If the building has other old windows, see replacing old windows.
- Ask before you hire: questions for the installer.
- Could there be a grant? Take the grants quiz.