Do rooflights work on a north-facing slope?

Diffuse sky light against direct sun, and why the difference shows overhead

A south facing rooflight receives two things: the beam of the sun, and light scattered off the whole dome of the sky. A north facing one receives the second only, for almost all of the year. At Gloucester’s latitude the sun sits low enough through winter that a north slope sees none of it at all, and only around midsummer, when the sun rises well north of east and sets well north of west, does a shallow north slope catch a little direct light early and late in the day.

That sounds like a large loss and it is smaller than it reads. Overhead glazing sees an open hemisphere of sky where a wall window sees a fence, a hedge and the house opposite, so a roof window collects far more of the diffuse component than a vertical window of the same area. Take the beam away and a north facing unit is still doing something a wall window on any elevation cannot.

The steadiness is the point, rather than the peak

Gloucestershire skies are overcast a great deal of the time, and under a genuinely overcast sky the whole dome is the light source, so orientation makes almost no difference to how bright a room is. On a grey afternoon in February the gap between a north facing roof window and a south facing one of the same size is close to nothing.

The gap opens on a clear day, and it opens in both directions. The south unit is dramatically brighter for three or four hours, and for two of them the room is uncomfortable and the blind is down, at which point the useful daylight is lower than the north room next door has had all day without anybody touching anything. A north facing unit has no peak and no trough. Somebody working at a screen, drawing, sewing or cooking notices the absence of glare long before they notice the absence of sun.

Sizing up, because there is no overheating penalty to pay

On a south or west slope the glazed area gets trimmed to keep the room usable in summer. On a north slope that constraint disappears, so the size question is decided by the structure and by the room instead. The starting rule of thumb for a habitable room is a glazed area around a fifth of the floor area, and on a north slope there is no reason not to go past it.

What limits you is the roof: rafter centres, the position of a purlin, the run of slope available, and whether a wider unit means cutting a second rafter. The size codes and the arithmetic behind all of that are set out on the roof window installation page, and the same rules apply whichever way the slope points. Two units spread along a north slope beat one large one in the middle, because the second fills the shadow the first casts and the light across the floor evens out.

Glass specification changes when there is no solar gain to control

A solar control coating on a north facing unit is money spent on nothing. It works by rejecting a share of the incoming solar energy, and on a north slope there is almost none arriving, so all it does is cut the daylight transmission and give the glass a faint tint. The g-value, which matters enormously on a west facing loft, is close to irrelevant here.

Spend the budget on the U-value instead. A north facing unit loses heat all winter and gains nothing back, so it is the one orientation where triple glazing, a krypton or argon fill and a warm edge spacer earn their place most clearly. A low emissivity coating on the inner pane still does its job of reflecting long wave heat back into the room, and light transmission is worth protecting, so a high transmission glass with a soft coat low emissivity layer is usually the right build. The numbers are worked through on the energy efficient rooflights page.

The colder outer pane, and dew that forms on the outside of the glass

The first genuine drawback. A well insulated unit lets very little heat out through the glass, so the outer pane runs close to the outdoor air temperature. On a clear still night it radiates to a cold sky and drops below the dew point, and dew forms on the outside. On a south slope the morning sun clears it within minutes. On a north slope there is no sun to clear it, so it sits there until the air warms up, sometimes until mid morning in autumn.

This is the unit working exactly as designed, not a fault. It is more noticeable on triple glazing than double for the same reason. An anti-dew coating on the outer pane raises the surface energy so the moisture forms as a thin film rather than droplets, and the view through it stays usable. On a north facing bedroom window with a view worth having, it is worth specifying at the order stage.

Moss, algae and a slope that stays damp until eleven in the morning

The second drawback is about the roof around the unit rather than the unit itself. North slopes across the vale, the Stroud valleys and anywhere with tree cover stay wet longer, and they grow moss and algae on the covering far faster than the south slope of the same house. Moss breaks down into grit, grit travels down the slope with the rain, and it collects against the first horizontal obstruction it meets, which is the head flashing of your new roof window.

Two things follow. The head lap gets taken beyond the manufacturer’s minimum as a matter of course on a north slope, because a flashing that is holding a bank of wet moss against it is being asked to do more than the standard dimension assumes. And a self cleaning coating, which relies on ultraviolet light to break down the film of dirt and then on rain to sheet it away, gets much less ultraviolet on a north slope and works correspondingly less well. Clearing the head of the flashing once a year takes ten minutes and prevents nearly all of it.

North facing rear slopes across Gloucester, Cheltenham and the vale

Whole streets in this county face the same way. Gloucester’s Victorian terraces on the north side of an east to west street have their rear addition roof facing north, at 22 to 30 degrees, over a kitchen that is already the darkest room in the house. That is the single most common north facing job in the city, and it is a good one: the room needs the light, the light it needs is even and shadowless, and there is no summer overheating problem to design around.

The interwar hipped semis at Longlevens, Hucclecote and Warden Hill often present a north facing rear hip, where the shorter run of slope matters more than the orientation does. And in the Stroud valleys a north slope frequently faces a wooded bank as well, which is the case where orientation stops being the useful question. How much open sky the unit can see matters more than which way it points. A north facing window with clear sky above it beats a south facing one under a mature beech, every day of the year.

Rooms where north facing is the right answer, and the one where it is not

A home office with a screen in it is the clearest case: no glare across the monitor, no blind to keep adjusting, no reflected sun at four in the afternoon. A workroom, a studio or a sewing room wants the same thing. A kitchen benefits because the cooking side of the room generates plenty of heat already. A bedroom facing north is easier to sleep in during June, when a south facing loft is light at half past four in the morning and hot by ten.

The room where north is the weaker answer is a loft sitting room where somebody actively wants to feel the sun through the glass. If that room has a south or west slope available as well, split the glazing: a larger north unit for the working daylight and a smaller south one for the sun, with solar control glass on the south one only. That combination gives a better room than either orientation on its own.

Working out which of your slopes to use

The decision is made from photographs and a compass bearing before anybody climbs anything. Email info@severnskylightsgloucester.co.uk with a picture of the roof from the garden, a picture of the room from inside, which way the slope faces, and what is standing in front of the sky above it: trees, a bank, a neighbouring gable. That is enough for an indicative range and, more usefully, an honest opinion about which slope the daylight should come from. The survey then confirms the pitch, the rafter centres and the run of slope, and the fixed price follows. The costs page explains what moves the figure, and the areas pages cover the thirty miles by road around Gloucester.