VELUX and Roof Window Installation: A Homeowner's Guide

Centre pivot windows rotate on a middle hinge. They are the standard choice, they are easy to clean from inside because they spin fully, and they suit rooms where the window sits above head height.

Top hung windows open outwards from the top, giving you an unobstructed view and headroom underneath. They suit loft bedrooms and offices where you stand at the window.

Flat roof windows and lanterns are a different product entirely, designed to sit on an upstand on a flat deck. If you are lighting the middle of a rear extension, that is what you want.

The rule of thumb is roughly one tenth of the floor area in glazing for a comfortable amount of daylight, and roof glazing gives you around twice the light of a vertical window of the same size.

Position is dictated partly by rafter spacing. Fitting between existing rafters avoids structural work and keeps the cost down. Anything wider means cutting a rafter and installing trimmers, which is perfectly routine but adds time.

Height matters for the view. A window with its bottom edge around 900mm to 1100mm above the floor gives you a view sitting down as well as standing, which is what makes a loft room feel like a room rather than a roof space.

The tiles or slates come off around the opening, the underlay is cut back and dressed properly, the timber trimmers are formed, and the frame is set square and level to the roof plane. Then the flashing kit goes on.

The flashing kit must match your covering. There are different kits for plain tile, profiled tile, slate and flat interlocking tile, and using the wrong one leaves a gap that no amount of sealant will fix long term. This is the single most common fault we find on windows fitted by general builders.

Internally the reveals are formed, insulated and plastered. Splaying the bottom reveal vertically and the top reveal horizontally spreads the light much better than square reveals, and it costs nothing extra to do it right.

For a straightforward window in an existing roof, the main regulation concerns are the structural opening, the thermal performance of the glazing and the fire escape requirement in habitable loft rooms.

A loft conversion bedroom generally needs an escape window with a minimum clear opening and a maximum sill height from the floor. Not every roof window meets that, so it has to be specified deliberately.

If your property is listed or in a conservation area, roof windows on a street-facing slope may need consent, and conservation style windows with a flush finish and central glazing bar are often the accepted answer.

Supply and fit for a standard sized roof window in an existing pitched roof generally falls between £900 and £1,800 depending on size, glazing spec and whether the internal finish is included.

Larger units, triple glazing, electric or solar operation, and anything needing a rafter cut and trimmers push it higher. Blinds fitted at the same time are cheaper than retrofitting them later.

Flat roof lanterns are a bigger job because of the upstand and the deck alteration, typically £1,800 to £4,000 fitted depending on size.

Leaks around a roof window are nearly always a flashing issue rather than a window issue. Wrong kit, kit fitted over rather than under the correct courses, or sealant used as a substitute for lead and soakers.

Condensation on the inside of the glass in winter usually means the room lacks ventilation rather than the window being faulty. Use the trickle vent, and in a bathroom or kitchen make sure extraction is adequate.

Draughts and cold spots at the reveals mean the gap between frame and structure was not insulated before plastering. It is a cheap step that gets skipped and a nuisance to fix afterwards.