Can you put a skylight in a flat roof?

No flat roof is truly flat. A well-built one carries a fall of at least 1 in 80, and often 1 in 40, so rainwater runs off towards an outlet rather than pooling. A rooflight has to respect the same logic. Lay a flat pane of glass horizontally and rain will sit on it, dry to a ring of dirt, and leave you looking up at a grimy square within a season. So the glass never goes in dead level.

There are two ways round it, and the unit you choose decides which one you get. A flat glass rooflight is made with a slight pitch designed into the frame, usually somewhere between 5 and 15 degrees, so the surface sheds water and grime on its own. A domed or pyramid rooflight throws the water off its curved or angled faces without needing any pitch at all. Both work. What you must not accept is a plain flat pane bedded straight onto a level roof with no fall built in anywhere, because that is the one arrangement that ponds.

A rooflight in a flat roof does not sit on the roof surface. It sits on an upstand, a raised timber or insulated kerb that lifts the frame 150 to 300mm above the finished roof. The roof covering, whether that is felt, EPDM rubber, GRP fibreglass or a single-ply membrane, is dressed up the sides of that upstand and turned over the top. The rooflight then clamps down over it.

That height is not decorative. It keeps the glazed unit clear of any standing water, driven rain and the odd blocked outlet, and it gives the roofer enough vertical face to lap the waterproofing properly. A skimped upstand, or a rooflight screwed flat to the deck, is where most flat-roof leaks begin. When a unit is being replaced, the upstand is usually already in place and sound, which makes swapping the glass a straightforward job: the opening and the kerb are there, and the only real decision left is what glass goes back in.

This is where a flat roof behaves differently from a pitched one, and where it needs more thought, not less. A pitched rooflight faces a compass direction. A flat one faces almost straight up, at the part of the sky the summer sun crosses. In June and July the midday sun in south Essex climbs to around 60 degrees above the horizon, which means it strikes horizontal glass close to square on, at the very hours it is strongest. Turning the house does not help you. A flat rooflight over a kitchen collects sun from mid-morning until late afternoon regardless of which way the front door faces.

The result is heat. Clear double glazing lets roughly 60 per cent of the sun’s energy through as warmth, and a couple of square metres of it over a room that already makes its own heat from cooking will turn that room into a greenhouse by early afternoon. This is the exact problem this business is built around, because Southend faces south across the estuary and the rear extensions here catch the full run of the sun. The fix is the glass specification. Ask for a solar-control coating with a G-value around 0.3 or below, which halves the heat coming through while still letting the room read as bright. It also takes the edge off glare, which matters near the water where light bounces up off the estuary as well as down from the sky. Our flat roof skylight installation work always starts with orientation and glass before it gets to the frame, and on a flat roof the orientation answer is settled before you begin: it faces up, so specify for summer sun.

A flat roof will take almost any style of rooflight, which is part of why it is such a good candidate. The choice comes down to the look you want and the span you need to cover.

• Flat glass rooflight. A single fixed or opening pane with a slim frame and a slight built-in pitch. It gives the cleanest modern look from inside and out, sits low against the roofline, and suits contemporary extensions. Sizes run from a small 600 by 600mm up to large single panes over two metres.
• Domed or pyramid rooflight. The traditional flat-roof choice, in glass or polycarbonate. The shape sheds water and debris well and the polycarbonate versions are light and inexpensive, though they let more heat through and yellow over the years. A polycarbonate rooflight is a reasonable pick for a garage or utility, less so over a living space.
• Roof lantern. A raised, multi-faceted glazed structure that stands proud of a flat roof and reads as a feature. It suits larger extensions and lets a lot of daylight into a deep-plan room. Because it presents so much glass, the G-value question matters even more here. See our roof lantern installation page for the detail.

For a deep kitchen extension a common and sensible arrangement is a large fixed pane for daylight paired with one opening unit at the highest point, so hot air can escape at the top of the room on a still day. Venting the heat out is worth as much on a flat roof as blocking it at the glass.

On most houses a new rooflight in a flat roof falls under permitted development, as long as it projects no more than 150mm above the roof plane and sits below the highest part of the roof. A low-profile flat glass unit clears that easily. A tall roof lantern may not, so it is worth checking the projection before you commit to a raised design. Flats and maisonettes, listed buildings, conservation areas and any property under an Article 4 Direction sit outside permitted development, and in Leigh the two conservation areas carry an Article 4 that removes those rights for roofing and window changes, so a planning application is needed there where it would not be elsewhere. Treat all of this as general guidance and confirm your own position with the local authority before ordering.

Building Regulations apply either way. A new or enlarged rooflight is notifiable: Part L sets a ceiling on how much heat the unit may lose in winter, and Part K covers the safety glazing needed in an overhead pane so that a cracked unit holds together rather than falling into the room. This is a genuine consideration on a flat roof, where the glass is directly overhead and anyone standing beneath it. On every installation we make the Building Control notification to the council on your behalf, so the paperwork is handled as part of the job rather than left with you.

Start by looking at what your flat roof is made of and how old it is. If the covering is a sound modern membrane or GRP with years of life left, a rooflight drops in cleanly. If the felt is old and tired, the sensible order is to sort the roof covering first with a roofer and then bring us in for the rooflight, so the new upstand is lapped into fresh waterproofing rather than into something on its way out. We install and replace rooflights; we do not carry out roof covering work itself.

Then think about the room below and which way it faces the sun. If it is a rear extension that already runs warm, tell us that up front, because it changes the glass we specify. Measure the rough opening you want, note whether you would like it to open, and ring 01702 898232 or request a quote. Give us the orientation and the size and we will come back with a unit, a glass specification and a price for the whole thing, upstand included.