Rooflights on a garage conversion roof

What the roof usually is

Attached garages typically have either a shallow pitched roof tied into the house, or a flat roof, and the flat ones are often felt on a cold deck with minimal insulation.

The condition matters more than usual. Garage roofs are frequently the least maintained part of a property, and a covering that was adequate over a car may not be sound enough to cut a new opening into.

The conversion is the moment to do it

Converting a garage to habitable space triggers Building Regulations, and the roof has to meet the thermal standard for a habitable room. That almost always means upgrading the insulation, which means opening the roof up.

Doing the rooflight at that moment costs a fraction of doing it afterwards, because the covering is being replaced anyway. Adding one later means cutting into a roof you have just paid to make good.

Warm roof conversion

Where the roof is being upgraded, converting a cold deck to a warm roof is usually the right move: insulation above the deck, vapour control below, waterproofing above. It performs better and it makes the rooflight kerb detailing simpler and more reliable.

How many, and where

Garages are typically long and narrow, and one rooflight in the middle lights the middle. Two units spaced along the length light the room.

Position them away from the party wall with the house and away from any valley where the garage roof meets the main roof, because valleys carry a lot of water.

Head height

Worth checking early. Garage floors are often lower than the house and ceilings are often low, and adding insulation above the deck reduces head height further. A splayed reveal around the rooflight gains a little back and throws more light into the room.

Why a garage conversion needs a rooflight more than most rooms

A garage has one large opening at the front and frequently nothing else. Fill that opening with a wall and a window, and you have a long narrow room lit from one end.

The far end, which is usually where the room joins the house, ends up permanently dark. That is the single most common complaint about converted garages, and it is not a lighting problem so much as a daylight one.

A rooflight over the rear half of the room fixes it, and on a single storey garage it is one of the cheaper installations we do.

Establishing what you are working with

Garages come in three roof types and the answer differs for each.

Pitched, tiled. Usually trussed rafters on anything from the 1970s onward, at 600mm centres, with concrete interlocking tiles. A unit narrow enough to sit between two trusses needs no structural work at all, which is the quick job. Anything wider needs a designed trimmer.

Flat, felt or single ply. The same rules as any flat roof: a kerb at least 150mm above the finished surface, the deck falling away from it, insulation on the outer face and a vapour control layer sealed to the frame.

Garage flat roofs are frequently older and were laid with minimal fall, so correcting the fall locally around the kerb is more likely here than on a recent extension.

Corrugated fibre cement sheet, on older detached garages. Sheets of that age may contain asbestos. Where we suspect it, work stops and it is tested rather than guessed at. Removal is a licensed activity and not something we do.

The regulations change when the use changes

This is the part that catches people, and it is worth being clear.

An unheated detached garage is exempt from most of Part L, so the thermal requirements that apply to a dwelling do not bite in the same way.

A garage conversion is a material change of use. Once the space becomes habitable, the full requirements apply: Part L for thermal performance, Part F for ventilation, Part B for fire safety, and Part A wherever structure is cut.

So a rooflight going into a garage that is being converted is being specified to dwelling standards rather than outbuilding ones, and the whole conversion is notifiable regardless of whether planning permission is needed.

That is not a burden. It is the reason the finished room will be comfortable rather than merely enclosed.

Insulating a garage roof, and where the rooflight sits in it

Most garages are uninsulated, so the conversion involves creating an insulated envelope where there was none.

On a pitched roof that usually means insulating at ceiling level with a ventilated void above, or at rafter level to create a vaulted ceiling. The rooflight detail follows from which.

At ceiling level, the tube of the reveal through the roof space needs treating like any rooflight in a cold roof: insulation collar around the frame, vapour control layer sealed, and the loft ventilation not obstructed.

At rafter level, the rooflight sits in a warm roof and the insulation and vapour control layer both have to be maintained continuously up to the frame.

On a flat roof, the same choice between a warm build up above the deck and a cold one between the joists, with the kerb height following from the insulation thickness.

Get the insulation right and a rooflight is a small area of glazing in a well insulated roof. Get it wrong and the rooflight becomes the visible symptom of a poorly insulated conversion, because it is where the condensation shows.

The floor level problem

Worth flagging because it affects rooflight position.

A garage floor is usually lower than the house floor and it has to be raised, insulated and damp proofed as part of a conversion. That reduces the ceiling height, sometimes considerably.

Where the resulting room is low, a rooflight does two useful things: it adds a sense of height at the point where the ceiling opens up into the reveal, and on a vaulted ceiling it lets light into the highest part of the space.

It also means any escape window height requirement is measured against the new finished floor rather than the old slab, which matters where the room is being used as a bedroom.

What the room is for

A bedroom. Means of escape applies, which means the rooflight may need to satisfy a minimum clear openable area, minimum opening dimensions and a maximum sill height above the finished floor. A top hung unit that swings clear generally satisfies it where a centre pivot does not.

It also means a blackout blind, ordered with the window so it clips into the frame channels rather than being fitted to the reveal.

A home office or a studio. The strongest case for a rooflight, and a north facing one is the best working light there is: even, cool and glare free, with no moving patch of sun across a screen.

A utility or a shower room. Ventilation matters most. An opening unit at the highest point, plus an extract fan ducted to outside.

A gym or a playroom. Ventilation and overheating are the considerations. An opening unit with a rain sensor, and solar control glass if the roof faces south or west.

Access, which is the cheap part

A garage is single storey, so a mobile tower reaches the roof on almost any plot. No scaffold, no highway licence, and no working at height premium.

That is why a rooflight in a garage conversion is one of the better value pieces of work in this trade, and it is worth taking advantage of by doing it as part of the conversion rather than as an afterthought.

Do it while the room is open

The strongest advice on this page.

During a conversion the ceiling is off, the insulation is going in and the plastering is being done. Forming a rooflight opening at that stage costs a fraction of retrofitting one into a finished room, because the making good is happening anyway.

Run a capped electrical cable to the rooflight position at the same time, even if you fit a manual unit. Twenty minutes then, and it covers a future motor, blind or rain sensor. Photograph where it is before the plasterboard goes on.

And if you are undecided about whether the room needs one, form the opening. A converted garage that turns out to be dark is a much harder problem to solve afterwards.

What changes when the garage becomes a room

Everything about the roof specification.

An unheated garage does not care about thermal bridging, condensation, or the U value of anything. A habitable room does, and the requirements that apply are those for a change of use.

The roof needs insulating to current standards, which on a flat garage roof usually means a warm roof build up with insulation above the deck. The rooflight has to meet the thermal requirement for the work being done.

And the condensation risk that was irrelevant becomes the main design issue.

The build up decision

Most garage roofs are either a shallow pitch with a light covering, or a flat roof with felt or a single ply membrane, built to a lower specification than a house roof because they carried nothing.

Converting means the covering is frequently replaced anyway, which is the moment to move to a warm roof. Insulation above the deck removes the ventilated void entirely, and with it the whole class of cold roof problems around a rooflight kerb.

Where the covering is being kept, and the roof is a cold roof, the kerb interrupts the ventilation path and air has to be routed around it deliberately.

The kerb, and the height problem

A warm roof build up raises the roof level by the thickness of the insulation, which on a garage with a low eaves or an abutment against the house can be a genuine constraint.

Check what that height rise does to any door or window above, and to the flashing into the house wall, before committing to the build up.

Structure

Garage roofs are built to carry the roof and nothing else. An opening needs trimming, and the existing timbers may be lighter than a house roof would use.

Beyond a modest opening that means designed work, and on a conversion it is being looked at by building control anyway.

Security, which matters more here

Garages are frequently accessible from a side passage or a boundary, and a rooflight on a low single storey roof is a plausible entry point.

Laminated glass rather than toughened. It holds together when attacked rather than clearing the opening, and it is a modest cost at order that cannot be added afterwards.

The daylight opportunity

A converted garage usually has one small window in the old door opening and solid walls elsewhere. A rooflight is frequently the only way to make the space feel like a room rather than a garage with a floor in it.