Velux loft conversions: how a rooflight conversion works

Is there enough head height?

Measure from the top of the ceiling joists to the underside of the ridge. That figure then loses the depth of the new floor, which is usually deeper than the existing ceiling joists, and the insulation and plasterboard under the rafters. What is left has to give comfortable standing room over a useful part of the floor and over the top of the stair.

Steep roofs on older houses often pass easily. Shallow modern roofs often do not, which is when a dormer or a hip to gable conversion becomes the realistic option.

Planning and building regulations

Rooflights usually fall within permitted development for a house, with limits on how far they project from the roof plane and, on some elevations, requirements for obscure glazing. Conservation areas, listed buildings and flats change that, and the front roof slope is often more sensitive than the rear.

Building regulations apply in full: the new floor structure, fire safety and the escape route, insulation, ventilation and the stair. That applies to every loft conversion regardless of type.

What the build involves

Most of the work happens from inside, which is why a rooflight conversion is the quickest and least disruptive route.

• New floor joists, often spanning onto new or strengthened supports
• Rafters trimmed and doubled around each roof window opening
• Roof windows fitted with flashing kits to the tiles or slates
• Insulation between and under the rafters with ventilation kept clear
• A new stair and fire protected route to an exit

Trussed roofs and purlins

Houses built with factory trusses have diagonal timbers across the loft that cannot simply be cut out. They can still be converted, but the structure has to be redesigned by an engineer, often with steel or new timber beams. Traditional cut roofs with purlins and struts are usually easier to open up, though they still need a structural check.

Rooflight or dormer?

A rooflight conversion gives light and a room under the slope. A dormer adds full height floor area and a vertical window. If the roof passes the head height test, rooflights are usually the simpler choice. If it does not, a dormer is how you create the space. Our handbook comparison of dormer and Velux loft conversions sets out the trade offs side by side.

Light, heat and noise under rooflights

Roof windows let in more daylight than a vertical window of the same size, because they face the sky. The trade off is heat gain on sunny days and rain noise in heavy showers. Blinds with a reflective backing, glazing with a solar control coating and good insulation around the frame all help.

Placement matters too. Windows set low enough to see out of while standing, and positioned over the stair, desk or bed, make the room far more pleasant than windows placed only where the rafters happen to fall. Fire escape rules can also dictate the size and height of at least one window.

How we build loft conversions

We design and build loft conversions with our own team and have more than 40 years of building and carpentry experience. We start with a free site visit to measure head height, roof structure and stair position, then give a fixed-price written quote.

Common questions