Do rooflights work on a north-facing slope?

North light and the reason studios were built to face it

Drawing offices, artists’ studios and workshops were glazed to the north long before anyone wrote a daylight standard, and the reason was practical. Direct sun moves. It throws hard shadows across a bench, it changes colour through the day, and for an hour at a time it makes a surface impossible to read.

Sky light does none of that. It arrives from the whole visible dome rather than a point, so it wraps round an object instead of cutting it, and it holds roughly the same character from ten in the morning to four in the afternoon. In a room used for close work, a desk, a hob, a sewing table, that stability is worth more than peak intensity.

The same argument holds for rooms with screens in them. A south rooflight over a home office is a nuisance for part of every clear day. A north one never is.

The honest loss in daylight, and where it shows

A north slope receives sky light and reflected light from surrounding surfaces, and nothing else. Compared with the south slope of the same roof it loses the direct beam entirely, which is the larger component on a clear day and a significant part of the annual total.

What that means in the room is less dramatic than the numbers suggest, because a rooflight in a ceiling already outperforms a window of the same area by a wide margin. The sky it sees is unobstructed, and it is looking at the brightest part of the sky rather than the dimmest. A north rooflight will still be several times better than a north window on the wall below it.

Where the loss shows is at the edges of the day and the year. December afternoons go flat earlier. A north-lit room needs its artificial lighting on sooner than a south-lit one, and the difference is roughly the last hour at each end.

Rooms that suit a north slope without any compromise

Kitchens with an island under the glass, home offices, studios, north-facing bedrooms and any room with a television in it are all better served by diffuse light. So are rooms that already run warm: a converted loft under a dark covering will gain enough heat through the tiles without adding a south rooflight to it.

Bathrooms are a case of their own. The glazed area is usually small, the room is used for short periods, and the priority is ventilation rather than daylight quantity. A north slope is entirely adequate.

The glazing specification changes on a north roof

On a south slope the specification argument is about controlling solar gain: a low g-value, sometimes a solar control coating, sometimes an external blind or awning. On a north slope that whole line of thinking is redundant, and specifying for it actively hurts you.

Solar control coatings cut light transmittance as well as heat. Fitting one to a north rooflight throws away daylight you cannot spare in exchange for a benefit the slope was never going to need. What you want instead is the highest light transmittance available in a unit with a good U-value, because the glass on a north slope is a heat loss element rather than a heat gain one.

Triple glazing makes more sense here than it does facing south for the same reason. It is not a universal upgrade, but on a north slope in an exposed roof it earns its cost more readily.

Dew on the outside of the glass, and what it means

Owners of north rooflights sometimes find the outer pane misted on a still autumn morning while the rest of the roof is clear, and assume something has gone wrong with the unit. Nothing has.

External condensation forms when the outer glass surface drops below the dew point of the air touching it. That happens when the glass is losing heat to the night sky and gaining very little back from inside, which is exactly what a well insulated triple glazed unit does. It is evidence that the glazing is working. The sun clears it, and on a north slope the sun clears it later, so the effect lasts longer than it would on the opposite pitch.

The condition worth knowing about is misting on the inner face between the panes, which indicates a failed sealed unit and is a replacement, not an adjustment.

Sizing up rather than settling for less

The straightforward answer to reduced sky light is more glass, and it is usually available. Where a south slope might be well served by a single unit around 780 by 1180mm, the equivalent room lit from the north often wants the next size up, or two units instead of one.

Between rafters the rafter spacing sets the practical width, and the length is the dimension with room to move. A taller unit gains area without any trimming, which keeps the structural work and the cost down. Where a wider unit is wanted, a rafter has to be trimmed and the load carried on trimmers, which we cover on trimming rafters for a pitched roof rooflight.

Putting units on both slopes of the same roof

In a loft conversion or a vaulted room open to both pitches, the strongest result is usually one unit on each slope rather than two on the better one. The north unit supplies the steady baseline. The south unit supplies the sun, the warmth in winter and the sense that the room is connected to the weather.

It also balances the light across the room. Two units on one slope light one half brightly and leave the other in relative gloom, and the contrast makes the dim half feel worse than it measures. Splitting them evens the room out. That arrangement is different from the side-by-side case covered on two rooflights side by side on one slope, which solves a width problem rather than an orientation one.

Overshadowing usually matters more than the compass

Before accepting that a north slope is the constraint, look at what is actually in front of the south one. A mature lime, a neighbouring gable, a two-storey extension or a stand of conifers can take more daylight off a south rooflight than the orientation ever takes off a north one, and unlike the compass those things change.

North slopes are frequently the clear ones. On a terrace running east to west, the rear slope often looks out over gardens with nothing above the fence line, while the front slope faces a street of houses the same height. We measure obstruction angles from the roof itself rather than assuming, because the assumption is wrong often enough to matter.

What we establish at the survey on a north slope

Orientation is checked with a compass on the slope, not from a map, because a roof described as north-facing is frequently north-east or north-west, and thirty degrees of rotation puts early or late sun onto it. From there it is obstruction angles, the rafter spacing, the covering, the internal ceiling arrangement and where the light needs to land in the room.

The glazing recommendation follows from those. So does the size, and often the argument for a second unit. You get a range by email once we know the roof, then a fixed price after the survey, and the workmanship carries a ten-year guarantee. Fifteen years of fitting these tells us that on a north slope the specification error is almost always caution about size, not the orientation itself.