Flat rooflight glazing for a south-facing extension

Every other rooflight on a house tilts. A pitched-roof window lies on the slope, so the summer sun strikes it at an angle and glances off a good share of the energy. A vertical gable window is steeper still. A flat rooflight does neither. It sits horizontal, or near enough, and points at the whole sky.

That matters most in the months you least want it to. Southend sits near 51.5 degrees north, and at midsummer noon the sun climbs to about 62 degrees up. Land that beam on a horizontal pane and it is arriving barely 28 degrees off square, so almost its full strength drives down through the glass. A horizontal unit also takes in light from the whole vault of the sky rather than the half a wall window faces, which is why a flat rooflight keeps loading heat off a bright overcast sky long after the direct sun has gone behind cloud. On a clear July afternoon the roof of a rear extension can be taking somewhere near 800 watts for every square metre of glass, dropping in from almost directly above, into the top of the room where the warm air is already gathering.

Put a south-facing garden under all that and the picture is complete. The rear extension on a 1930s semi, and south Essex has a very large stock of them, is typically a single-storey flat-roofed box over a knocked-through kitchen and dining space, opening onto a garden that faces the sun. The rooflight in the middle of that roof is a solar collector with a dining table under it. None of this is an argument against fitting one. It is the reason the glass has to be chosen for the job rather than picked off the cheapest line in the catalogue.

Two figures govern how any glazed unit behaves once it is in the roof, and they pull in opposite seasons.

The U-value is the rate of heat loss: watts per square metre of glass for every degree of temperature difference between inside and out, written W/m²K. Lower is better. It decides your winter heat loss and whether the inside face runs cold enough to gather condensation, and it is the figure Building Regulations set a limit on. Flat rooflights on the market sit between roughly 1.6 W/m²K for a plain double-glazed unit and about 0.8 for a good triple.

The G-value, or solar factor, is the share of the sun’s energy landing on the outside of the glass that finishes up as heat indoors. It sits on a scale from 0 to 1. Clear double glazing is around 0.6, so six parts in ten of the solar energy come through. A solar-control unit pulls that towards 0.3 or below, turning the near-infrared band back at the outer face before it can enter the glass at all. Tinting works differently and worse. A tinted pane soaks the energy up, heats through, and then sheds a fair part of that warmth on into the room, so you lose the view without gaining much cool. A reflective coating rejects the heat at the surface and still passes the daylight.

On a flat south-facing rooflight the G-value is the number that decides comfort. Winter heat loss through a horizontal pane is a small, steady drain that a decent U-value handles. Summer solar gain through that same pane is many times larger in watts, it lands when the room is already warm, and only the G-value touches it. Ask any installer for both figures. If only the U-value comes back, half the specification is missing.

The values below are typical published data for flat rooflight glazing units. Individual products differ, and a figure quoted centre-of-pane will always read a little better than the same unit measured whole, frame included, so take the table as a picture of what is available rather than a fixed catalogue. It is the spread between the columns that tells the story.

Run your eye down the two performance columns. The U-value barely moves: 1.6 at the worst to 0.8 at the best, a factor of two on a quantity worth a few tens of watts on a hot day. The G-value swings from 0.60 to 0.26, and it is governing several hundred watts on that same day. Take a 1.5 square metre flat rooflight receiving 800 watts per square metre at midday, which is 1,200 watts landing on the glass. At a G-value of 0.60 about 720 watts pass into the room. At 0.27 about 320 watts do. The 400 watt gap between those two panes is more heat than four people would give off sitting under it, pouring in all afternoon at ceiling height. That is the whole argument for choosing the glass to suit the aspect, and it is why we specify the glazing before the unit rather than after.

The body-tinted row is worth a second look. Its G-value beats clear glass by only a modest margin, but its light transmittance has crashed to 0.42, which means it robs the room of nearly as much brightness as it does warmth. The high-selectivity unit holds transmittance up near 0.60 while cutting the G-value to 0.27. One leaves you a bright, cool room; the other leaves you a gloomy one that is barely cooler. Keeping the light while shedding the heat is the entire job that solar-control glazing is engineered for, and it is why the two are not the same purchase.

A flat rooflight is not laid dead level. Standing water leaves dirt rings, streaks the glass and, over years, works at the seals, so manufacturers of glass flat rooflights specify a minimum fall, commonly around 5 degrees, so rain sheds and washes the surface as it goes. That fall is built into the frame or the timber upstand it sits on, using tapered firrings under the roof covering so the whole assembly tips a little towards the outlet.

The upstand itself, the kerb the unit sits on, does two jobs. It lifts the glass clear of any water sitting on the flat roof, and it keeps the rooflight above the level that permitted development allows, which on a house generally means projecting no more than 150mm above the surrounding roof surface. On a warm-deck flat roof the kerb is usually a treated timber box built up off the deck and dressed into the waterproof layer, whether that is single-ply membrane, felt or a liquid coating. This is the detail that governs whether a flat rooflight stays watertight for its life, and it is fitted as one continuous assembly with the roof covering rather than dropped in afterwards.

That small built-in slope has a quiet effect on solar gain too. A few degrees of pitch does almost nothing to soften the near-vertical summer sun, so a nominally flat rooflight still behaves, for heat purposes, like a horizontal one. It does not buy you out of needing solar-control glass. What it buys you is a clean pane and a dry seal, which are reasons enough.

The temptation on a flat roof is to glaze as much of it as will fit, because the structure allows it and the daylight is lovely. On a south-facing extension that is exactly the wrong instinct without the glass to back it up. The heat a rooflight admits scales directly with its area, so doubling the glass doubles the solar load on a room whose floor area has not changed.

A useful way to think about it is the ratio of glazed area to floor area of the room below. A flat rooflight covering a tenth of the floor of a north-lit utility room is a gentle wash of light. The same ratio in glass over a south-facing kitchen-diner is a serious heat input, and once you push past roughly a fifth of the floor area in south-facing horizontal glass, the G-value stops being a nicety and becomes the thing that decides whether the room is usable in summer. Larger apertures, and runs of several units over an open-plan space, are where the high-selectivity glazing at the bottom of the table earns its keep.

Room fabric plays in as well. A room with a hard tiled floor and plasterboard walls has little thermal mass to soak up a midday peak, so it heats fast and the glass has to hold the input down. A room with an exposed masonry wall or a solid floor rides the peak more gently. None of this changes the specification on its own, but it moves where on the G-value scale a given room needs to land, and it is the sort of thing a proper survey weighs up rather than guesses at.

The specification decides what comes in through the glass. It has no say over the heat already sitting in the room, and hot air rises and settles, so on a single-storey extension the warmest air of the day pools in a band right under the ceiling, exactly where a flat rooflight is. That is the best possible place to be able to let it out.

An opening flat rooflight empties that pooled layer quicker than a window down at head height can, because it lifts the hottest air straight up and releases it at the top of the room. Open a door or a low window somewhere else and you set up a through-draught: cool air drawn in at floor level, hot air pushed out at the ceiling. On the high or awkward ceilings an open-plan extension tends to have, a hand-cranked unit ends up never touched, so an electric opening rooflight on a switch and a rain sensor is the one that actually gets used when it counts. Plan the venting and the glazing as a pair. The low G-value glass holds the incoming peak down; the opening sheds whatever heat is left. Skimp on either and the room turns against you on the warmest afternoons.

A flat rooflight on an ordinary house is usually permitted development, meaning no planning application, provided it projects no more than 150mm above the roof plane, sits below the highest part of the roof, and, on a side elevation, uses obscure glazing with any opening restricted below 1.7m. Flats and maisonettes, listed buildings and conservation areas are the exceptions. In Southend that matters in specific places: the Leigh Cliff and Leigh conservation areas carry an Article 4 Direction that removes permitted-development rights for changes to roofing and windows, so a planning application is needed there where it would not be on a similar house a street away. Clifftown and Leigh Old Town are conservation areas too. Treat all of this as the general position and confirm your own address with Southend-on-Sea City Council, or the relevant local authority if you are outside the city boundary, before ordering anything.

Building Control is separate and applies far more widely. A new or enlarged rooflight is notifiable under the Building Regulations whether or not planning is involved. Part L covers the thermal side and sets the maximum U-value the glazing has to beat, and a good flat rooflight beats it comfortably. Part K covers safety: overhead glazing has to use a laminated inner pane, so that a break leaves the glass hanging in its interlayer inside the frame instead of coming down into the room, with the outer pane toughened as standard. On every job we carry out, that notification to Building Control is made on your behalf, so the paperwork is part of the installation rather than something left with you.

There is a gap in the rules worth understanding. Part L caps the U-value but attaches no G-value limit to a rooflight going into a house that already stands. Part O, the part written to curb summer overheating, bites on new-build dwellings and not on an extension bolted to a home you have owned for years. For the bulk of extension work, then, the regulations ask nobody to consider solar gain at all. That is exactly why it drops off so many quotations, and how a roof full of paperwork-perfect glazing can still hand you a room that is no use from early to late afternoon on a hot day.

As a rough guide to the market, a fixed flat glass rooflight in a common size, say around one metre by one and a half, supplied on its own, tends to run somewhere between £500 and £1,300 depending on the glass specification, with solar-control units towards the upper half of that band. An electrically opening unit of the same size sits higher again, often £1,200 to £2,500 supplied, because of the actuator, controls and rain sensor. Large bespoke units and runs of several rooflights over an open-plan space are priced individually. Installation on top depends on the roof build-up, the kerb work and access, and forming a new opening in an existing flat roof is more involved than dropping a unit into a roof being built from scratch. Our own cost guidance and the flat roof skylight installation service page set out how a real quote is built up for a specific roof.

Whatever you are quoted, four questions settle whether the glass is right for a south-facing extension:

1. What G-value does this glass carry? A figure should come back without hesitation. “It is energy efficient” or “it is A-rated” will not do, because a window energy rating in this country leans on winter behaviour and describes summer solar gain not at all.
2. Is that U-value the centre-pane or the whole-unit figure? Centre-pane numbers leave out the spacer and the frame and flatter the product. The whole-unit figure is the fair one to compare, and it is the one Building Control reckons against.
3. And the light transmittance? Set it beside the G-value. The target is a low G-value paired with transmittance still around 0.6, not a low G-value won by darkening the pane.
4. May I see the glass data sheet? Manufacturers all publish one, values worked to BS EN 410. The request is free, and the willingness to hand it over says a good deal about the firm quoting.

Fifteen years and more of fitting skylights and rooflights around south Essex sits behind this, with a ten-year guarantee on the workmanship of every job, and on a south-facing extension the aspect and the glass are the first things the survey settles, not an afterthought. Before you sign for anything, get the figures that apply to your own roof: ask us for a specification that puts the G-value in writing next to the U-value for the exact unit going in over your kitchen.