Can rooflights be fitted to a garage roof?

Flat garage roofs and pitched garage roofs

Garage roofs come in three broad arrangements and each is a different installation. A separate flat roof, felt or single-ply with a shallow fall, is the most common on attached and integral garages. A lean-to is one shallow slope running down from the house wall. The third is a continuation of the main house roof carrying on over the garage at the same pitch.

The flat roof takes a kerb-mounted unit. The continuation takes an ordinary roof window with the same flashing kit as the house, because it is the same roof. The lean-to needs checking rather than assuming, because every roof window has a stated pitch range from its manufacturer and some garage lean-tos sit below the shallow end of it. Where they do, a kerb-mounted unit is the honest answer rather than forcing a pitched product onto a slope that will not carry it. Which arrangement you have is answered from the drive in a couple of minutes.

The upstand does the work on a flat roof

A kerb-mounted unit does not sit on the deck. It sits on an upstand built above the finished surface, and the height of that upstand keeps the frame clear of everything a flat roof collects: standing water, lying snow, silt and leaf debris.

Building it properly is the job. The kerb is framed and insulated, the corners are formed rather than folded, and the covering is carried up the sides and over the top so that the waterproofing terminates above the water line rather than at it. Done this way there is no route for water to reach the timber, and the perimeter is what decides whether a flat-roof installation stays dry for decades.

Before any of that we want to know about the deck itself: which covering is on it, whether the falls run to the outlet or to a low spot, where the water leaves, and whether the timber underneath is sound. Better established at survey than after the covering is opened.

An unheated store is the simpler job

If the garage is staying a garage, a workshop or a store, the work is about as straightforward as rooflight installation gets. There is no thermal envelope to maintain, no vapour control layer to keep continuous and no plastered reveal to form. The unit goes onto its kerb or into the slope, the covering is detailed around it, and that is the extent of it.

It is also a genuinely useful thing to do. A garage lit only by a strip light and whatever comes through the door is an unpleasant place to work, and glass in the roof puts daylight over the whole floor rather than at one end.

Access helps too. A garage roof is low, reachable without a full scaffold to a second storey, and the work is contained away from the house, so the disruption inside is close to nothing.

Conversion brings insulation and Building Regulations in

Turning a garage into habitable space is a different proposition. A garage was built unheated: the walls may be single skin or an unfilled cavity, and the roof will normally have no insulation at all. A thermal envelope has to be created where none existed.

The insulation itself is straightforward on paper. The difficulty is at the junctions, where new insulation meets existing structure, and that is where condensation shows up. Warm moist air from the new room reaches a cold part of the construction, meets a surface below dew point and turns to water. Because the rooflight is often the coldest visible thing in the room, the owner assumes the unit is the cause. Usually it is not.

What prevents it is continuity. The vapour control layer on the warm side runs unbroken and is sealed to the frame, and the insulation line is carried across the reveal, which is worth splaying in a low room so the opening widens as it comes through the ceiling. Headroom is tight in a converted garage, and glass in the roof takes no useful wall or head height at all.

The abutment along the top edge

A lean-to garage roof runs down from the house wall, which gives it an abutment along its whole top edge, and that detail is doing serious work. Water running down the wall arrives at it, and the junction has to throw everything clear rather than let it find a way behind the covering. On a properly built abutment there is a flashing chased into the brickwork or stepped into the joints, dressed down over the covering with enough cover to hold in wind.

Where a rooflight sits in relation to that abutment is a specification question rather than a matter of preference. The higher up a shallow slope a unit goes, the less room there is between its head and the flashing above. Manufacturers set minimum distances between a unit and an obstruction, and on a short lean-to those distances can decide the layout. If the wall above carries a downpipe or a hopper, water is concentrated at one point on that roof, and a unit in that path works harder than one clear of it.

Notifiable work, and who tells Building Control

Forming a new opening in a roof is structural work, and where a garage is being converted to habitable use the thermal element and ventilation come into scope alongside it. That makes the work notifiable, and the notification is ours to make as part of the job.

Where the garage is staying an unheated store the position is lighter, and we say plainly which case yours falls into rather than leaving it vague. The wider status of a conversion, particularly where an annexe is intended for somebody to live in, is a question for your Building Control body and in some cases the planning authority, and we do not present ourselves as the people who decide it.

Garage conversions are steady work for us across the villages north of the city, and there is more on how they run on our page for rooflight installation in Yarnton. We install VELUX, Fakro, Keylite and Roto units, new installations and full replacements only. Our workmanship carries a ten-year guarantee and we have been installing rooflights for fifteen years. Tell us what the space will be used for before you tell us which unit you have seen.