The short answer
Yes. Most garage roofs can take a rooflight, flat or pitched, provided the timbers around the opening are made good and the unit is waterproofed properly. A new rooflight in an existing dwelling has a thermal limit of 2.2 W/(m²·K), and on a flat roof the upstand, the raised edge around the opening, needs as much care as the glass itself.
Daylight in a garage turns it into a workshop, a studio or a tidy utility space. The opening is the easy part; keeping it dry for decades takes the detail.
Which garage roofs suit a rooflight?
A rooflight is a glazed unit set into the roof, and it can go into most garage roofs. What changes is how the opening is formed. In a pitched roof the unit sits between rafters, the sloping timbers, and is flashed into the covering. In a flat roof it sits on a raised frame, with the waterproof layer turned up the sides.
Can an asbestos cement roof take one?
That is the awkward case. Cutting into asbestos cement sheets is exactly the kind of breaking up that HSE guidance warns about, and an AC roof is always a fragile one. So a hole is never sawn into an old sheet roof. Where a garage has one, the sheets come off carefully and the new roof is built with the rooflight designed in from the start. Our page on asbestos garage roof removal covers that route, and the garage and asbestos roofs hub lists the other options.
Clear panels moulded into corrugated sheets are another way to bring in light. They suit an outbuilding that stays cold, but they are a different product from a rooflight, so the figures below do not describe them.
The 2.2 figure and what it means
Approved Document L covers energy and fuel, and its Table 4.2 sets limits on new or replacement parts fitted to an existing dwelling. The limit is a U-value, a measure of heat loss in which a lower number means a warmer unit. For a rooflight the table says 2.2 W/(m²·K), and that is the figure to put on your order.
Is a roof window judged by the same number?
No. A roof window, which opens and is usually fitted in a pitched slope, follows the window row of that table, which reads 1.4, with Band B as the alternative measure for window energy ratings. The 2.2 limit is for rooflights only, and the two get muddled easily, so ask which type is being priced and check it is the type you want.
Does any of this matter if the garage stays unheated? It matters most when the space is lived in or later converted. A rooflight bought with a poor figure can fail an inspection once the garage becomes a room, so choosing a compliant unit now avoids replacing glass later. Our guide to a garage conversion roof shows where that leads.
Upstands, kerbs and keeping the water out
On a flat garage roof the weak point is the junction between glass and membrane. The rooflight sits on a kerb, a low timber box built around the opening, and the roof covering is turned up its sides. That raised edge is the upstand. Water stays on the roof surface longer than people expect, so the upstand must stand clear of it with a proper height of dry edge.
What does Part L say about upstands?
Table 4.2 also covers the kerb: an upstand made on site is held to 0.35 W/(m²·K) at most, so it is insulated rather than left as bare timber. A cold kerb sweats and stains the ceiling beside the glass, which people often blame on a leak.
Pitched roofs rely on flashing instead: strips of lead or aluminium that step up the sides of the unit and sit under the covering above it. Done well, a rooflight will outlast the covering around it. Done badly, it leaks within the first winter, which is why we set the unit in at the same time as the covering, not as an afterthought once the roof is finished.
Permission, structure and what to check first
Before anything is cut, three checks are worth making. First, the structure: a rooflight opening interrupts a rafter or joist, and the timbers either side need trimming, which means doubled-up timber around the hole. The second is the regulations: ask your building control body whether the work needs a notice, and Somerset Building Control on 0300 303 7790 can answer for Taunton addresses.
Does a garage rooflight need planning permission?
On a house, Class C of the permitted development order covers roof windows that do not protrude more than 0.15 metres beyond the slope, and a roof window on a side slope has to be obscure-glazed. A garage is not a house, so the planners are the people to ask, particularly in a conservation area, or inside Exmoor, where the Park Authority decides. Dormers are a separate matter and are never permitted development in those places.
The third check is air movement in the space below, since fresh glazing can bring condensation with it. Our garage roof ventilation answer explains how that is handled. Send us a photo of the garage from the drive, and we come out, check the roof, talk you through the findings and put one total in writing.
More on garage daylight
Will a rooflight make my garage colder in winter?
Glass loses heat faster than an insulated roof, so a larger rooflight means a cooler space. A unit that meets the 2.2 figure with a well-insulated kerb loses far less than an old single-skin sheet. If you plan to heat the garage, mention it when we visit, and the unit gets chosen with that in mind.
Can I fit a rooflight myself on a flat garage roof?
Cutting the opening is simple; making it watertight is not. The kerb, the upstand and the way the membrane turns up the sides all need to be right the first time. A roofer should also check the deck and joists around the hole, since a wet joist is far costlier than the unit.
The full picture: Garage and asbestos roof replacement in Taunton · More short answers