What is a northlight rooflight?

Look at the roof of an old mill or a Victorian weaving shed and you will often see a row of asymmetric triangles, like the teeth of a saw. Each tooth has a long, gently sloping face and a short, steep one. The steep face is glazed and points north. The long face is clad and insulated. Repeat that profile across a wide floor and you can light a building the size of a warehouse from above, evenly, with no windows in the walls at all.

The form was worked out by mill engineers in the nineteenth century, before electric lighting was cheap or reliable. Textile work, printing and precision engineering all needed a lot of light and needed it consistent, because a shadow moving across a loom or a lathe over the course of a day is a nuisance and a hazard. The sawtooth solved that. It is still called a northlight roof, and a single glazed slope built to the same principle, dropped into a modern flat or low-pitched roof, is a northlight rooflight.

The modern version does not have to be a full sawtooth. Manufacturers make individual northlight units: a glazed face on a steep frame, with an insulated back and upstand, that sits on a flat roof and tips its glass towards the north. You can install one over a single room or run a bank of them across a larger span.

North-facing glazing in the northern hemisphere never sees direct sun. The sun tracks across the southern sky, so a slope tipped the other way collects only the light bouncing around the sky itself. That light is diffuse, soft and remarkably constant from mid-morning to mid-afternoon. It does not throw hard shadows and it does not swing in intensity as clouds pass in the way a sunlit room does.

The trade-off everyone worries about, and the thing this business spends most of its time on, is the opposite orientation. South-facing roof glazing collects heat all through the cooling season and can turn a room into a greenhouse, which is why we specify a low G-value for it. A northlight sidesteps that problem entirely. Because no direct sun reaches the glass, solar gain through it is small and glare is negligible, so you can use clear glazing with a high light transmittance and take all the daylight on offer. The G-value of the unit barely matters here, which is the reverse of the advice we give for a south-facing lantern. The number that matters on a northlight is the U-value, because a large area of glass that never earns any free solar warmth needs to hold heat in well on a cold night. Our guide on energy-efficient skylight glazing sets out how those two figures pull in different directions.

The northlight is a commercial and workshop answer first, and that is still where it fits best. Anywhere the work needs flat, shadow-free daylight over a wide area, the shape does something no ordinary rooflight can. That covers a lot of ground:

• Studios and workrooms. Artists, photographers, model makers and colour-critical trades want light that does not shift or cast a coloured tint. North light is the traditional studio choice for exactly that reason.
• Industrial units and warehouses. A sawtooth run lights a deep floor plate from above without any reliance on side windows, which is useful where the walls are racked out or shared with the next unit.
• Retail and gallery space. Even overhead daylight shows goods and pictures in a steady colour, and there is no patch of sun crawling across the floor to fade stock or dazzle a customer.
• Schools, sports halls and workshops. Consistent daylight with low glare suits rooms where people are reading, making or moving fast, and it cuts the hours the lights need to be on.

If you are fitting out a unit, a studio or a large open space and this is the effect you are after, it belongs in a conversation about commercial skylight installation rather than a domestic roof window. On houses the northlight is rarer, though it does turn up on a deep single-storey extension or a garden studio where the client wants gallery-quality light and does not want the summer heat that a south-facing rooflight would bring.

An old northlight roof was single-glazed and leaked heat badly, which is why so many were later boarded over. A modern one is a sealed insulated unit, so you keep the daylight without the running cost. A few points are worth pinning down before anyone quotes you:

• Pitch. The glazed face wants a decent angle, commonly somewhere around 60 degrees, so that it faces the bright part of the sky and sheds rain and dirt rather than letting it sit on the glass.
• Glazing. Clear high-transmittance double or triple glazing is usually right, because there is no solar gain to control. Put the effort into the U-value instead, from about 1.6 down towards 1.0 W/m²K depending on the build-up.
• Orientation. True north gives the purest, most even light. Rotating the glazing towards north-east or north-west lets a little low sun in at the ends of the day, which some rooms want and others do not.
• Ventilation. Opening units at the high point of the glazing let warm air escape and are worth designing in, particularly across a wide floor where heat collects at the ridge.

New or enlarged rooflights are notifiable under Building Regulations. Part L covers the thermal side, which on a northlight means the U-value in particular, and Part K covers safety glazing where the glass could be walked into or fallen against. We make the Building Control notification to the relevant local authority as part of the job.

Work out first whether you actually want north light or simply a lot of light. If you need even, shadow-free, cool daylight over a working space, a northlight is the right shape and worth the design effort. If you just want a bright room and the roof happens to face a helpful direction, an ordinary rooflight or lantern with the correct glass for its orientation will cost less and do the job.

Once you know which it is, tell us the roof you are working with, the way it faces and the rough area you want glazed, and we will come back with a specification and a price. Ring 01702 898232 or request a quote, and we will look at the light you want in the room before we talk about the hole in the roof.