A flat rooflight on a garage roof: what to check first

A garage roof was built for one job: keeping rain off a car. That shapes what it can carry around an opening.

House roofs are designed to take a ceiling, insulation and a loft hatch. A garage roof, particularly on a 1960s or 1970s estate garage, may be built with slender joists at wide spacing and a thin deck on top. Cutting a hole in it removes the support of the joists that run through the opening. So the first visit is a survey of the structure. We look at the joist size, the span, the direction they run and what rests on them.

The opening is then framed with new trimmers, so the load goes round the hole instead of through it. That detail is the same on any flat roof, and our page on why the joists need trimming for a flat rooflight describes it. On a light garage roof, the trimmers may need to be heavier than the existing timbers, and the ends need to bear on something sound.

Then there is the covering. A garage is often topped with felt, sometimes with a corrugated sheet, and occasionally with something rubber or fibreglass. The kind of covering decides how the new upstand is tied in, and there are separate notes on felt roofs and EPDM rubber roofs. Old cement-based corrugated sheeting from the mid twentieth century can contain asbestos. If you have any doubt about the material, have it identified by a specialist before anyone drills or cuts it.

The building's relationship to the house changes the rules, the fire detailing and how much insulation the new opening needs.

A rooflight in a detached garage that stays a garage is the simplest case. It lights a workbench or a gym corner, and it does not need to meet the thermal standard of a bedroom. The trouble arrives when the garage becomes a room. A conversion to a study, a playroom or a gym that is heated and lived in brings Building Regulations Part L for the new thermal element, along with Part F for ventilation. The rooflight then has to reach the standard of a rooflight in a house. Our notes on insulation around a flat rooflight explain what that looks like.

Part B matters near a boundary. A rooflight set close to a neighbouring property has to meet fire rules, which affect the glazing material and how near it can sit. How close a flat rooflight can go to a party wall has the detail, and it is always checked for the address.

The upstand is the raised frame the glass sits on. On a garage it has a slightly harder job than on an extension.

A low upstand looks neat on a garage, and it is tempting. But a garage roof often has a shallow fall, sometimes less than you would like, and the covering is likely to be older. A sensible height keeps the glass clear of any standing water and gives the roofing edge room to be dressed up the side of the upstand properly. Our page on rooflight upstands goes through height, insulation and finish.

The upstand is also where quiet begins. A flat garage roof is a thin shell, and rain on it can be loud inside a room that has no ceiling lining. If the garage is going to be a workroom, an office or a teenager's band practice space, the glazing build and the seals are worth specifying for that use, and they go into the written quiet spec that comes with every quote.

A garage is a place where things are kept. The glass needs to let light in without advertising what is underneath.

Laminated glass on the inner pane is the usual choice overhead. It holds together if struck, and it is harder to get through than toughened glass alone. In a garage that is useful for two reasons: a dropped tool or a stray football is less of an event, and the room is harder to enter from the roof. An obscure or frosted finish is an option if you want light without a clear look into the garage from a neighbouring window. The page on obscure glass for a flat rooflight covers it.

Where the glass is within reach of a person standing on the roof, overhead safety glazing matters, and that is a Part K consideration. We specify for it from the survey.

A garage job is usually quick, because there is rarely a ceiling to remake or a room to protect. The order still matters.

1. A survey of the joists, the covering and any boundary or party wall.
2. A written specification covering the size, the glass, the upstand and the trimming.
3. Protection inside, then the opening is cut and framed.
4. The upstand is fitted and dressed into the roof covering.
5. The rooflight is set, sealed and checked, then the lining is made good.

The whole sequence is set out in how a flat roof rooflight is installed. A garage with a lined ceiling below needs a lined reveal too, and lining the reveal under a flat rooflight shows the options. To talk through your own garage, use the quote form.

Do I need planning permission for a rooflight in a garage?

Often not, because rooflights in an outbuilding or a flat roof commonly fall within permitted development, but the rules differ for listed buildings, conservation areas and flats. It is checked for the address before we quote.

Does a garage rooflight need Building Control?

It depends on the building. A small detached garage has its own exemptions, while an attached or converted garage is treated more like part of the house. We handle the notification where the work is notifiable.

Can the garage roof take the weight of a glass rooflight?

Usually, once the opening is framed with trimmers sized for it. A glass rooflight weighs more than a dome, so the survey checks the joists and the bearing before we price it.

Would a dome suit a garage better than flat glass?

A dome is lighter and lower in cost, and flat glass looks better and lasts better. The comparison in flat glass or dome covers it, and you can return to the main flat roof rooflight installation page for the wider picture.