Skylight vs Roof Window: What's the Difference?

Skylight is the word people type into a search box. It covers anything glazed that lets daylight in from above. Roof window is the trade term for a window designed to sit in the plane of a sloping roof, with its own frame, an opening sash and metalwork that carries rain over the edges. Velux is a brand, yet plenty of households use it as a general word, much as they say hoover for any vacuum cleaner.

Does the name on the quote matter?

It matters more than it should. A quote that only says skylight might mean an opening roof window, a fixed pane or a dome on a flat roof. Those are three separate jobs, each with its own framing, its own waterproofing detail and its own cost drivers. When we survey, we note which product is going in, and the written price names it. If a quote says skylight and nothing else, ask what exactly is being supplied. Our page on the types of roof windows sets out the main families, and the choice usually turns on whether you need the window to open.

On site the test is simple. A roof window lies flush with a pitched slope, is built to open, and can be cleaned from the room below. Its frame is normally fitted between two of the sloping timbers beneath the tiles, then sealed to the covering with a purpose-made set of flashings.

What counts as a rooflight in the trade?

Rooflight is the wider word. It takes in a fixed pane in a pitched roof, a glass lantern, a plastic or glass dome, and a flat glass unit sitting on a kerb, which is a low raised frame built around the opening. Rooflights turn up on flat roofs, extensions and outbuildings, where the glazing stands proud of the surface instead of lying in the slope.

The practical difference shows in how water leaves. A roof window sheds rain down the slope in step with the tiles around it. A rooflight on a flat roof stands on its own upstand, so the roofer must dress the membrane up that upstand and finish the top edge with care. We cover that detail in our answer on a Velux in a flat roof.

Approved Document L, Volume 1, sets the energy standard when part of an existing house is added or replaced. Table 4.2 has a window row set at 1.4, with Band B of the WER scale, the rating system for windows, as the other route, and its note 8 says roof windows are included. Rooflights get a separate row, and the figure on it is 2.2.

Which U-value applies to my skylight?

That depends on what the product really is, and the manufacturer's data sheet should say so. Heat loss through a surface is written as a U-value, so a lower number keeps more warmth in. A roof window answers to the 1.4 or Band B row, while a rooflight answers to 2.2. Muddling the two is the commonest mistake on this subject, and it can leave a window specified to the wrong standard.

The 2021 edition applies until 24 March 2027, and the 2026 edition takes effect from that date. Ask the supplier which row a product is rated against and keep the sheet with your papers. Our answer on roof window U-values goes through the figures in more depth.

For a loft room under a pitched roof, an opening roof window is the usual answer. It gives a view, fresh air and daylight, and the outer pane can be cleaned from inside. A fixed pane gives light only, with no ventilation, so decide what the room needs before you order. For a flat-roofed kitchen extension, a rooflight is more likely, and it brings the upstand and membrane detail with it.

Can a homeowner fit either one safely?

Any hole cut in a roof lets weather in until the new frame is sealed, and the risky part is the waterproofing around the glass, not the glass itself. A slipped flashing can drip into the insulation for months before the ceiling shows it. We start every job with a survey, send you pictures of the slope or flat deck being worked on, and put a single written total on the product you choose.

If the house is listed, or stands within a conservation area, the consent route comes before the order. The main page on roof windows and skylights explains how we plan a fit from the first visit.