Does rooflight orientation matter?

A rooflight does three jobs at once: it lets daylight in, it lets solar heat in, and on a cold night it lets warmth leak back out. Orientation barely touches that last one, which is set by the glass build-up and the way the unit is fitted. It has a heavy hand on the first two.

The sun tracks across the sky from east to west and sits low in the south for most of the English year. A slope facing that arc collects direct sun for hours at a stretch. A slope turned away from it sees mostly diffuse light bouncing off the sky and the clouds, which is softer, steadier and much cooler. So the same 1.2 by 1.2 metre roof window can be a gentle daylight source on one roof and a heat trap on another, purely because of the compass reading.

The knock-on effects are worth spelling out. Direct sun brings glare, the harsh contrast that makes a screen unreadable and a white worktop dazzle. It brings load, the heat that builds through the afternoon and lingers after the sun has moved off. And it brings movement of that heat through the day, so a room can be comfortable at breakfast and stifling by mid-afternoon. Diffuse light brings none of those in any real quantity. That is the whole reason orientation is not a detail but the starting point.

North is the easy one. A north-facing slope never sees the sun square on, so it delivers cool, even daylight with almost no heat penalty. Artists have wanted north light in their studios for centuries for exactly this reason. Standard clear double glazing suits it, and the modest warmth it does catch in winter is a bonus rather than a burden.

East catches the sun first thing. The light is strong and low, and it drives straight into the room, but it arrives when the building has cooled overnight, so it rarely builds into a real overheating problem. A bedroom under an east-facing rooflight will wake you early unless you plan for a blind, which is worth knowing before the glass goes in rather than after.

West is the awkward one. The sun arrives in the afternoon, when the fabric of the house is already warm from the day, so the heat stacks on top of heat that is already there. A west slope and a south slope can show the same figure on paper and behave differently in the room, with the west one feeling worse in late summer because of the timing.

South is where the money and the mistakes both live. A south-facing slope is in sun from mid-morning to late afternoon through the whole warm half of the year. That makes it the best possible position for a well-specified rooflight and the worst possible position for a badly specified one. With clear glass it overheats. With glass chosen for the job it can be the brightest, most usable room in the house. This is the point at which the number to ask for, the G-value, stops being optional. We set out sensible targets by orientation in our guide to a good G-value for a skylight.

All of the above assumes a pitched roof that leans one way. Level the glass off and the rules change. A flat rooflight, a roof lantern or a walk-on panel points at the sky rather than at any point of the compass, so it sees the high summer sun almost square on regardless of how the house is turned. You cannot orient your way out of the heat, because there is no orientation to work with.

This matters because so many extensions built in the last twenty years have a flat roof with glass in it. A three or four metre lantern over a kitchen island is a wonderful thing in April and a genuine problem in July if the glass was chosen on looks alone. On flat roof skylight installations the sensible approach is to treat the glazing as if it were south-facing whatever the house does, and to lean on solar-control glass and an opening unit near the high point to let hot air escape. A pitched rooflight at least gives you a compass reading to work with, which is one reason pitched roof installations have a little more room to breathe on the glass specification.

Southend-on-Sea faces south across the Thames Estuary, which is unusual for an English seaside city and is the reason orientation carries more weight here than it does inland. Two local things stack on top of the general physics.

The first is the open horizon. Inland, a south-facing roof usually spends part of the day shaded by the house opposite or by trees. Along the estuary the southern sky is clear, so a south slope collects sun with nothing to interrupt it. The second is glare off the water, which lifts the brightness on a fine day and can make a seafront room feel dazzling even when the sun is not directly on the glass.

The housing stock lines up with this in an unfortunate way. Across the 1930s semis of Westcliff, Thorpe Bay and out towards Hadleigh, a great many gardens sit at the back of the house, and a great many of those backs face south. Put a rear extension on one of those, glaze the roof for daylight, and you have aimed glass at the sun over the room that already gets hot from cooking. Getting the orientation reasoning and the glass right on that job is the whole basis of our energy-efficient skylight installation work.

Start with a compass. Stand in the room, work out which way the roof slope faces, and note whether it is pitched or flat. That single reading tells you more about how the finished rooflight will feel than the brand on the frame or the depth of the frame ever will.

If the slope faces north or east, you have room to keep the glass simple. If it faces south or west, or if the roof is flat, treat the glass specification as the important decision and ask for the G-value in writing before you commit. And if you would rather hand that part over, tell us the orientation and the rough opening size and we will come back with a glass specification and a price. Ring 01702 898232 or request a quote, and we will look at which way the roof faces before we look at anything else.