Flat rooflights on a garage conversion

A single garage is a long, narrow box, commonly around 2.5 metres wide by 5 metres deep. When it was a garage nobody cared that the back half was gloomy. As a home office, a spare bedroom, a gym or a playroom, that back half is where you actually sit, and it sits a long way from the only opening.

Adding wall windows rarely rescues it. The front wall faces the road, so a large window there trades privacy away and still only lights the first metre or two. The party wall to the neighbour cannot take an opening at all. The wall shared with the house is internal once the connecting door is knocked through. That leaves the roof, and on most garages the roof is flat or very close to it. A rooflight set into that deck throws light down into the middle of the plan, where a wall window never reaches, and it does it without giving anything up to the street.

This is why so many garage conversions across the south Essex semi belt end up lit from above. The 1930s and post-war stock here was built with attached or integral garages under shallow felt roofs, and the deep, single-aspect plan is the same from one street to the next. The roof is the answer the building was always going to give.

Before you choose a unit, look hard at the roof you are putting it into. A garage flat roof was built to keep a car dry, not to sit above a heated room, and there are three things to settle.

The first is the covering. If the existing felt or single-ply membrane is near the end of its life, the rooflight goes into a new covering, not an old one. Re-covering a tired flat roof is a roofing job, and it wants doing before or alongside the rooflight, never around it. We install into a sound, watertight deck. If yours is past its years, a roofer re-covers it first and we set the unit into the new surface.

The second is the build-up. Old garage roofs are usually a cold deck: insulation, if there is any, sits between the joists with a ventilated void above. A habitable room wants a warm deck, with the insulation laid over the deck so the structure stays warm and dry. This matters for the rooflight because the upstand it sits on has to be insulated to the same standard, otherwise you build a cold bridge right at the opening and invite condensation around the frame in winter.

The third is structure. Cutting a hole in a flat roof interrupts the joists that span it. A modest opening within a single joist bay may need little more than trimming, but anything wider means doubling the joists on either side of the hole and fitting trimmers across the ends to carry the load that the cut joists used to take. On a garage with a long clear span this is worth a structural check rather than a guess. None of it is difficult, but it is the difference between a rooflight that sits flush and dry for decades and one that dips as the deck sags around it.

A garage roof usually sits low, often only a little above head height inside once the ceiling is formed. That height, or the lack of it, steers the choice of unit.

A flush or low-profile flat rooflight sits on a shallow upstand and barely rises above the roof line. It reads as a clean pane of glass from the garden and takes up no headroom you can feel. It is the natural fit for a garage conversion and the workhorse of flat roof skylight installation. Sizes run from a single pane over a utility corner up to a run of units over a double garage.

A roof lantern is the raised, faceted option, and it can look right where the converted garage has become a kitchen or dining room and you want architecture overhead. The trade-off is height: a lantern stands proud of the roof and needs the room below to carry the visual weight, which a low single garage sometimes cannot. On a taller integral garage, or where the conversion opens into the main house, a lantern earns its place. On a shallow detached garage, flat glass is usually the honest answer.

Either way, the pitch matters. A flat rooflight is never dead flat. It is set to a fall of at least a few degrees, either by the unit’s own built-in slope or by tapering the upstand, so rain and dirt run off instead of pooling. A pane that ponds water will streak, hold algae and eventually let you down at the seals. The fall is not optional.

The mistake is to fit a rooflight that suits the roof rather than the room. A good rule of thumb for a habitable space is glazed roof area of roughly 15 to 20 per cent of the floor area if you want the room to feel genuinely bright rather than merely lit. A single garage of about 12.5 square metres wants somewhere near 2 square metres of glass to hit that. Below that it works but reads as dim towards the back; above it you gain light and lose a little winter warmth, which is a fair trade in a room used by day.

These are typical flat rooflight sizes and what they suit on a garage conversion. Sizes are the structural opening, and most ranges step in fixed increments, so your installer will land on the nearest available size to the hole your roof can take.

Two of the smaller units spread across the plan almost always beat one big pane in the middle, because they light the front and back of a deep room rather than pouring everything into the centre. On a double garage a single large rooflight or a matched run of two works well, since the room is wide enough to carry it.

Here is the part most quotes skip. A flat rooflight faces straight up, which means it collects sun from whichever way the garage happens to point, and it collects a lot. A pane lying flat takes the summer sun almost square-on at midday, so per square metre it gathers far more solar energy than a vertical window on the same wall ever could. On a garage with a south or west aspect, that flat pane is a solar collector sitting directly over a room you now want to work or sleep in.

The number that governs this is the G-value, the share of the sun’s energy that passes through the glass and turns into heat in the room. Clear double glazing runs a G-value around 0.6, so more than half of everything arriving comes through. On a 2 square metre rooflight taking strong afternoon sun, that is the heat output of a small electric heater running all afternoon, entering at ceiling height where hot air already wants to sit. In a converted garage, which usually has a hard floor and plasterboard walls with little thermal mass to soak up a peak, the room climbs fast and holds it.

Solar-control glass is the fix, and the choice follows the aspect rather than the fashion. These are industry-typical figures for the glazing you would actually specify:

• Clear double, argon filled: U-value around 1.3 W/m²K, G-value around 0.60. Right for a north-facing garage roof, or any room that runs cold and would welcome the free heat.
• Solar-control double: U-value around 1.2, G-value around 0.35. The sensible default for a south or west aspect, cutting the summer heat roughly in half while keeping the room bright.
• High-selectivity solar-control double: U-value around 1.1, G-value around 0.28. For a large glazed area on a sunny aspect, where you want to hold the light transmittance up near 0.6 and take the heat right down.

The point is not that lower G-value is always better. A north-facing garden office wants the light and the warmth, so clear glass is the correct call there. It is that the glass has to be chosen for the way the roof faces, and a flat rooflight faces up into the strongest part of the sky. Choosing it well is exactly what solar-control glazing is for. Ask for the G-value in writing alongside the U-value, and if the answer is vague, the specification is not finished.

Converting a garage into living space is a change of use, and the whole job is notifiable under Building Regulations, the rooflight included. Several parts bear on the glass overhead.

Part L covers thermal performance and sets a maximum U-value for a new rooflight in the conversion. Any unit worth fitting beats that limit, and the warm-roof build-up around it is part of meeting it. Part K covers safety glazing and guarding: anything glazed overhead needs laminated glass on the inner pane, so that if it ever breaks the interlayer holds the fragments rather than dropping them into the room. Toughened outer panes are standard on rooflights, and the laminated inner is the layer Part K is really about. Part F covers ventilation, which for a habitable room means both background ventilation and a means of purging stale or humid air, and an opening rooflight is often the neatest way to provide the purge.

There is a gap in the rules worth understanding. Part L limits the U-value but sets no G-value limit for a rooflight in an existing building, and Part O, which does tackle summer overheating, applies to new homes rather than to a garage you are converting on a house you already own. So nothing in the regulations forces anyone to think about solar gain here at all. That is precisely why so many garage conversions end up with compliant glazing over an unusable summer room. The regulations pass and the afternoon still bakes.

New rooflights are also, for the most part, permitted development on a house, provided they project no more than 150mm above the roof plane and sit below the ridge, which a flat rooflight on a garage comfortably does. Flats, maisonettes, listed buildings, conservation areas and any street under an Article 4 Direction are the exceptions, and in Leigh’s conservation areas an Article 4 Direction removes the usual permitted-development rights for roof changes, so a planning application is needed where elsewhere it would not be. Treat all of that as general guidance and confirm your own position with the local authority before work starts. On every installation we make the Building Control notification to Southend-on-Sea City Council, or the relevant authority for your address, on your behalf.

Solar-control glass answers the summer. Winter has its own demands, and a garage conversion is more exposed to them than a room inside the original footprint, because it started life as an unheated, single-skin box.

Condensation is the risk. Warm, moist air from the new room rises and meets the coldest surface it can find, and in a poorly specified conversion that surface is the glass or the frame of the rooflight. A good U-value keeps the inner face warm enough to stay clear, and an insulated upstand keeps the frame surround out of the cold-bridge zone. The two work together: the best glass in the world will still weep at the edges if the kerb around it is left uninsulated.

Ventilation is the other half. An opening rooflight sits at the highest point of the room, which is exactly where you want to release stale air and where the summer heat collects. Vent from the top and the whole room clears far faster than it would through a window at head height. On a low garage roof you can usually reach a manual unit with a pole, but where the ceiling is higher, or the conversion has opened into the house, an electric opening rooflight with a rain sensor takes the chore away and shuts itself when the weather turns. Glass controls what the sun brings in. Opening controls what the room lets out. A comfortable converted garage wants both.

As a rough industry guide, a good-quality fixed flat rooflight supplied and installed into a garage conversion, with the upstand formed and the covering made good, typically falls somewhere between 900 and 2,500 pounds depending on size, glass specification and access. An opening unit adds a few hundred pounds, and an electric opening rooflight with a sensor more again. A run of units or a lantern over a double garage sits above that range. Solar-control glass carries a modest premium over clear, and on a sunny aspect it is the cheapest comfort you will ever buy, because the alternative is running a fan or an air-conditioning unit against the sun every afternoon for the life of the room.

When you get a quote for the glass over a garage conversion, four questions settle most of it. Which way does the roof face, and has the glass been chosen for it? What is the G-value, and is it written down? Is the U-value quoted whole-unit rather than centre-pane, so it is the honest figure Building Control will want? And is the upstand being insulated as part of the price, or left as a cold bridge waiting to catch condensation? A clear answer to all four tells you the room has been thought about, not just measured.

If you are converting a garage in south Essex and want the light without the afternoon heat, we survey the roof, work out its aspect and specify the glass to match before anyone talks about a unit. Ask us for a specification for your roof with the G-value written on it alongside the U-value, and you will know exactly what is going in over your new room.