Guide · Flat Roof Rooflight Installation
Flat rooflights on a garage conversion
A garage conversion has a daylight problem built into it. The garage door goes, and whatever replaces it, usually a wall with a modest window in it, admits far less light than the opening it took over. The room is long, narrow and lit from one end. Every other surface is a party wall, an internal wall or a flank wall with a neighbour four metres away.
Which leaves the roof, and on most Essex garages that roof is flat. A rooflight in it lights the middle of the room, which is precisely where the daylight from the front never reaches. This page covers what the existing structure can take, how the roof has to change to serve a habitable room, and how to size and place the glass. It sits under flat roof rooflight installation.
Why the middle of a converted garage stays dark
Daylight from a vertical window falls away roughly with the square of the distance from it. The working rule in daylight design is that useful light from a side window reaches about two and a half times the head height of the window into the room.
A typical single garage is 5m to 6m deep with a 2.3m ceiling. A window at the front with a head at 2.1m therefore lights the first 5m at best, and that is with an unobstructed view out. Put a wall and a driveway and a neighbour’s hedge in front of it and the useful depth drops considerably. The back third of the room, which is where people put the desk or the bed, is the part with no daylight at all.
What is usually holding the roof up
Garage roofs are built to a lower standard than house roofs and were designed to keep a car dry, not to be altered.
On interwar and postwar Essex garages the common arrangement is timber joists at 47mm by 100mm or 47mm by 125mm, at 400mm centres, spanning the short way across the garage width, with a chipboard or softwood boarded deck and a felt covering over it. Later garages, on the estates at Great Notley and South Woodham Ferrers, tend towards 47mm by 150mm joists and an OSB deck. Some are asbestos cement sheet on a light steel or timber frame, which is a separate matter entirely and needs identifying before anybody starts.
Whether that structure will take an opening
Cutting a rooflight opening into a light garage roof is not the same as cutting one into an extension roof designed for it.
Removing one joist and trimming across it with doubled joists either side is usually within the capacity of a 47 x 125 deck spanning 2.5m, because the trimmers are short and the load being redistributed is modest. Removing two or three, or cutting an opening within a metre of a bearing where the joist end is doing its work, wants checking. Where the roof is already at the limit of its span, and plenty of older garage roofs deflect visibly in the middle, the honest answer is often that the deck is being replaced anyway, which makes the whole question easier.
Why the roof usually gets rebuilt anyway
This is the point most people have not reached when they start asking about rooflights, and it changes the shape of the project.
A garage roof is uninsulated. Converting the space to a habitable room brings it under Part L of the Building Regulations as a change of use, which means the roof has to achieve a U-value of around 0.18 W/m²K for a new element in a conversion. There is no way to get there without a substantial thickness of insulation. Doing that above the existing deck, as a warm roof, is the technically preferable route and is covered in detail on the main flat roof page. Once that is happening, the deck and covering are new, the kerb is built as part of it, and the rooflight stops being an alteration and becomes part of the design.
Headroom, and the constraint nobody expects
Garages are low. That is the constraint that decides more garage conversions than any structural question.
A single garage often has 2.2m to 2.4m from slab to the underside of the joists. Building Regulations do not set a minimum ceiling height for a habitable room in England, but 2.3m is the usual comfortable target and anything under about 2.1m feels oppressive. Insulating below the joists as a cold roof eats 100mm to 150mm of that. Insulating above, as a warm roof, costs nothing internally but raises the roof line by 150mm or more, which then has to be checked against the eaves of the house, the neighbour’s boundary and any planning constraint. The floor build-up takes another 100mm off the bottom. The height sums are done before anything else.
How a rooflight changes the daylight calculation
Overhead glass is worth roughly two to three times a vertical window of the same area, because it faces the whole dome of the sky and is not shaded by the fence, the neighbouring house or the tree at the end of the garden.
In a converted garage that ratio matters more than usual, because the vertical window is at the shaded end of the multiplier rather than the generous one. A 1000 x 1000 rooflight over the middle of the room can deliver more usable daylight to the back half than the entire front window does, and it does it without any privacy trade-off, because there is nothing above to look in.
Sizing to a narrow roof
Width is the binding constraint. A single garage is typically 2.4m to 2.8m internally, and once the kerb, the covering upstand and a sensible margin to the parapet or the edge are taken off each side, the practical maximum for a single unit is around 1.4m to 1.8m.
Length is easier, because the long dimension of a rooflight can run with the joists rather than across them, and a unit running with the joists cuts nothing. That geometry favours a long narrow rooflight, or a run of square ones, over one big square. It also happens to suit the room, which is long and narrow itself.
One long unit or several square ones
The decision is worth taking deliberately rather than by default, because the two give different rooms.
| Arrangement | What it gives | What it costs |
|---|---|---|
| One long rooflight down the centre | A continuous line of sky, dramatic, even along the length | Bigger glass, heavier lift, more structure |
| Two or three square units evenly spaced | Even light, each opening inside its own joist bay, easy handling | Reads as three openings rather than one gesture |
| One unit over the far end only | Fixes the dark end for the least money | The middle can still read as a corridor |
For most garage conversions two square units at 900 x 900 or 1000 x 1000, spaced evenly along the length, is the sensible balance. Every opening stays inside a single joist bay or needs one joist trimmed, the units are light enough to handle without equipment, and the light lands in the two places the front window never reaches.
Where to place them along the length
Not in the centre of the room, which is the instinctive answer and usually the wrong one.
The front of the room already has light from the window, so glass there is largely wasted. Weighting the rooflights towards the rear two thirds evens the room out. Where the garage is being split into two spaces, a room and a utility for instance, each space wants its own unit rather than one shared across a new dividing wall. And the position wants checking against what is already in the ceiling void: on an integral garage there may be a soil pipe, a boiler flue or a run of cabling from the house that nobody knew about until the deck came off.
Integral, attached and detached garages
The three types behave differently and the differences are practical rather than academic.
An integral garage sits under the first floor of the house, so it has no roof of its own and a rooflight is not available. Daylight there comes from the front and from any side wall. An attached garage, the common arrangement on interwar Essex semis, has its own flat or shallow pitched roof abutting the house flank wall, and that abutment is where drift snow collects and where the flashing detail concentrates. A detached garage has the freest roof of the three and the most straightforward rooflight installation, though it also has the most demanding thermal envelope because every surface is external.
The abutment against the house wall
On an attached garage the junction between the flat roof and the house flank wall is the detail that decides whether the conversion stays dry, and a rooflight near it interacts with it.
The covering is dressed up the wall to a proper height and terminated into a chase or under a flashing, and the roof falls away from the wall rather than towards it. A rooflight sited tight against that abutment sits in the zone where water and snow concentrate, and it leaves no room to work on either detail properly. Keeping at least 400mm to 500mm of clear roof between the kerb and the wall makes both jobs straightforward.
Planning permission and permitted development
Converting a garage into a habitable room is normally permitted development where the work is internal and does not enlarge the building, and rooflights are generally permitted where they project no more than 150mm above the existing roof plane and sit no higher than the highest part of the roof.
The exceptions are worth checking rather than assuming. Article 4 directions in conservation areas remove the rights, and the older cores of Maldon, Saffron Walden, Coggeshall and Thaxted all carry protections. Many newer estates carry a planning condition requiring the garage to be retained for parking, which is a common one across the developments around Chelmsford and Braintree. And where a warm roof raises the roof line, the height change itself may need considering. Checking takes a phone call to the local authority and is done before anything is ordered.
Building Control, and what a conversion is assessed on
A garage conversion is a material change of use and is fully notifiable, so Building Control is involved from the start rather than at the end.
The assessment covers the thermal elements, the floor, walls and roof, against Part L. It covers ventilation under Part F, means of escape and fire separation under Part B, which matters where the room adjoins the house, and the electrical work under Part P. The rooflight sits inside that: its U-value contributes to the roof calculation, and where it opens it may contribute to the ventilation provision. Sunspire handles the notification for the rooflight work as part of the job, which matters years later when a buyer’s solicitor asks for the certificates.
Ventilation in a room with one external wall
Converted garages ventilate badly, because there is usually one opening wall and it faces a driveway.
Part F wants background ventilation, typically trickle vents, plus purge ventilation capable of clearing the room quickly. An opening rooflight covers the purge duty well, because it sits at the highest point and warm stale air leaves under its own buoyancy while cooler air is drawn in through the front window. That stack effect works in a long low room better than people expect. Where the reach is beyond about two and a half metres, and in a garage with a raised warm roof it often is, an electric or solar opener is far more likely to be used than a pole.
Damp, the floor and the surfaces around the opening
Garages are frequently damp before conversion, and the damp is usually coming from the floor rather than the roof.
A garage slab often has no damp proof membrane, and its level is normally set below the house floor so that water runs out rather than in. Both get addressed in the conversion, with a new floor build-up over a membrane, tied into the wall damp proof course. It matters to the rooflight discussion because a room that was damp before conversion has a moisture load in its fabric that takes months to dry, and during that period condensation will appear on the coldest surface in the room. If the glass is well specified and the reveal is properly insulated, that surface will not be the rooflight.
Insulating and lining the reveal
The reveal between the ceiling and the glass is where a garage conversion most often loses its thermal detail, because the roof build-up is deep and the kerb is tall.
Insulation runs continuously from the roof deck up the inside face of the kerb to the underside of the rooflight frame, with no gap left at the top, and the vapour control layer is carried up and sealed to the frame. Where insulation stops short, that line becomes the coldest strip in the room and shows as a dark mark round the top of the reveal within a winter. Splaying the lining outwards then spreads the light across the ceiling, which in a long narrow room roughly doubles the apparent size of the opening for the cost of a little more plastering. It is the cheapest improvement on the whole job. What a sensible range looks like is set out under costs.
In a garage conversion the rooflight is not decoration, it is the only daylight the back half of the room will ever get.
Keep reading
More on flat roof rooflight installation
Flat rooflight falls, drainage and ponding
Read the guide →Walk-on rooflights and where they make sense
Read the guide →Single-ply, felt and GRP roofs around a rooflight
Read the guide →Flat rooflights on a cold roof build-up
Read the guide →How many rooflights a flat roof extension needs
Read the guide →
Get a quote
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
- Surveyed before it is priced
- 10-year workmanship guarantee
- Building Control notification handled
- New installations and replacements