Specifying glazing by which way the slope faces
Two identical rooflights, the same size, the same frame, ordered on the same day, should often not carry the same glass. One is going into a south-west facing slope above Painswick and the other into a north-facing rear addition in Gloucester, and the sun they will see over the next thirty years has almost nothing in common. Orientation is not a refinement applied at the end of a specification. It is one of the first three facts a survey records, alongside the covering and the pitch, and it decides the solar factor, the coating, whether shading is worth fitting and sometimes whether the third pane earns its place. This page treats the compass as a specification input and works through what each aspect actually asks for. The theory behind the numbers themselves is on U-values and glazing specification, and the wider picture is on the energy efficient rooflights hub.
Sun angles over Gloucester, in June and in December
Gloucester sits at roughly 51.9 degrees north. At solar noon on the longest day the sun is about 61 degrees above the horizon. At solar noon on the shortest day it is about 15 degrees. That single pair of numbers explains most of what follows.
A 61 degree sun is nearly overhead, so it strikes a shallow rooflight almost square on and a steep pitched slope at an angle. A 15 degree sun is low and raking, so it barely touches a horizontal rooflight and shines almost straight into a steep south-facing slope. The summer problem and the winter benefit therefore land on different parts of the roof, and a slope that overheats in July may contribute very little in January.
Pitch and aspect together, because neither works alone
The quantity that matters is the angle between the incoming sunlight and a line perpendicular to the glass, known as the angle of incidence. At zero degrees the sun is square to the glass and transmission is at its maximum. As the angle opens up, transmission falls slowly at first, then sharply beyond about 60 degrees, and above 75 degrees most of the light is reflected off the surface rather than admitted.
The consequence is that a 45 degree Cotswold slope facing due south receives its strongest sun in spring and autumn, not midsummer, because in June the high sun is well off perpendicular to a steep slope. Meanwhile a five degree flat roof rooflight receives its strongest sun at midsummer noon and almost nothing in December. Two units on the same house, facing the same way, behave like different products because of the pitch alone.
A south-facing slope: two seasons pulling opposite ways
South is the aspect with the largest annual solar total and the most genuine conflict. A steep south-facing roof window is the best winter asset on the whole house, taking a low sun deep into the room in the months when daylight is short and the heating is on. The same window in July is manageable, because the high sun is at a poor angle to a steep slope.
That makes a permanently applied solar control coating a poor trade on a steep southern slope. It rejects the winter sun that was worth having to moderate a summer condition that was already the less severe one. The better specification on a steep south face is a clear, high-transmittance low-emissivity unit and a movable shade that can be deployed on the handful of afternoons that need it. The choice between putting that shade outside or inside is set out on external shading against internal blinds.
West, and why it is the hard case on any roof in this county
West is the aspect that causes the complaints. The sun arriving on a west-facing slope in late July is at 20 to 35 degrees above the horizon, which is a steep angle of incidence on a horizontal roof but nearly perpendicular to a pitched western slope. It arrives between four and seven in the evening, which is exactly when the outside air temperature peaks and when the building fabric has already absorbed a full day of heat.
So a west-facing slope combines the highest intensity of the day with the least favourable time for the room to shed it, and it does it into a loft conversion which is the lightest, least thermally massive part of the house. This is the one aspect where a permanent solar control coating is close to free, because the winter sun a west-facing slope gives up is low, brief and already largely gone by four in the afternoon. Specify the low solar factor, accept the winter loss, and add shading on top.
East, and the morning that has finished by eleven
An east-facing slope sees the sun from first light until late morning. The intensity can be high, but the air temperature is at its lowest of the day and the building has cooled overnight, so the same energy produces far less discomfort than it does at six in the evening.
For most rooms, east is comfortable and needs nothing special: a clear low-emissivity unit and a standard specification. The exception is a bedroom. A June sunrise here is around a quarter to five, and a bright east-facing roof window over a bed is an alarm clock. That is a blackout problem, not a thermal one, so it is solved with a side-channel blackout blind rather than with the glass.
When the slope faces north, the priority list changes completely
A north-facing slope in Britain receives essentially no direct sun at all for most of the year, and only a brief oblique touch around midsummer morning and evening. Everything arriving is diffuse skylight, and diffuse light has three properties worth designing for: it is steady, it is close to shadowless, and it produces almost no glare.
That removes solar control from the specification entirely. Fitting a solar control coating on a north-facing slope reduces daylight, dulls the room and returns nothing. The priorities become the opposite ones: the highest light transmittance available, the lowest U-value affordable because the unit is losing heat all year and gaining almost none, and the best edge detail because the coldest glass in the house is the glass that never sees the sun. This is the aspect on which triple glazing makes the clearest case, discussed on triple glazing in a rooflight and when it pays.
Shallow and horizontal glass sees every aspect at once
A rooflight on a flat rear extension at five degrees does not really have an orientation. It faces the sky, and over a day it takes the eastern sun, the southern sun and the western sun in turn, plus the largest share of diffuse skylight of any surface on the building.
In annual terms a horizontal surface at this latitude receives more total solar energy than any vertical one and rather more than a steep north or east slope. In summer terms it receives the midsummer sun almost square on, at 61 degrees altitude, for hours either side of noon. That is why flat roof rooflights and lanterns over kitchens and rear extensions overheat more reliably than pitched roof windows do, and why solar control on shallow glazing is a good default rather than a judgement call. The physics of what happens once that energy is inside is on solar gain, overheating and shading.
Specification by aspect, in one table
| Aspect | Solar factor to aim for | Light transmittance | Shading | Other priorities |
|---|---|---|---|---|
| North, pitched | High, 0.50 or above | As high as available | Rarely needed for heat | Lowest U-value, warm edge, self-cleaning coating |
| East, pitched | Moderate, 0.40 to 0.50 | High | Blackout in a bedroom only | Standard low-emissivity specification |
| South, steep, 40 degrees or more | Moderate, 0.40 to 0.50 | High | Movable, deployed in summer | Keep the winter gain, control the summer |
| South, shallow, under 25 degrees | Low, 0.25 to 0.35 | Accept the reduction | External if it can be fitted | Purge ventilation high in the room |
| West and south-west | Low, 0.25 to 0.30 | Accept the reduction | External awning, plus internal blackout | The hardest case, specify for July evenings |
| Flat or near flat, any wall it sits on | Low, 0.25 to 0.35 | Accept the reduction | Fixed or external where possible | Laminated outer for rain noise as well |
Transmittance against solar factor: the trade inside a single pane
The two headline optical numbers are light transmittance, the proportion of visible light admitted, and the solar factor, the proportion of total solar energy admitted. A coating that rejects heat inevitably rejects some light too, because a large part of the sun’s energy arrives as visible light rather than as infrared.
The ratio between them is sometimes called selectivity, and a good modern solar control coating is more selective than an old one, keeping a reasonably high light transmittance while dropping the solar factor sharply. That is worth asking about rather than assuming, because the difference between a selective coating and a cheap one is the difference between a room that is cooler and a room that is cooler and gloomy. On a north-facing slope none of this applies, which is the point of specifying by aspect in the first place.
What a solar control layer does to the colour of the daylight
Coatings are not perfectly neutral. Older and cheaper solar control products often carry a visible tint, commonly a green or blue cast, and on a rooflight that tint colours the light falling on everything in the room below rather than just the view through a window.
In a kitchen it changes how food looks. In a room where anything is being matched, painted or photographed it is a genuine nuisance. Current high-selectivity coatings are far more neutral than they used to be, and the property to ask for is a high colour rendering index for the transmitted light. It is one of the few glazing decisions that is entirely about how the room feels and not at all about numbers on a certificate, and it is worth seeing a sample against the sky rather than reading a specification.
Where the use of the room outranks the compass
Aspect sets the default and the room can override it. A home office under a south-facing slope needs glare control more than it needs heat rejection, because a screen with a bright sky behind it is unusable regardless of temperature, and glare is fixed with a diffusing shade rather than a coating.
A bathroom under any aspect wants obscured or textured glass and a high resistance to condensation, so the edge detail and the ventilation matter more than the solar factor. A bedroom wants blackout wherever the sun rises. A north-facing studio wants the clearest, most neutral glass available and nothing in front of it. Write down what the room is for before choosing the glass, because the compass answers only half the question.
Overshading, and the neighbour’s gable that changes the answer
The theoretical aspect and the actual sun are frequently different things. A due-south slope with a mature lime tree twelve metres to the south of it is a north-facing slope from June to October and a south-facing one from November to April, because the tree is deciduous. A terrace in Chalford with the valley side rising steeply behind it loses the sun at three in the afternoon in winter and never sees it at all on the lower slope.
Terraced streets across Gloucester and Cheltenham overshade each other, and a two-storey rear addition frequently shades the flat roof behind it for the first half of the morning. This is why the survey looks at the actual sky view from the proposed position rather than reading the compass and stopping. Specifying an aggressive solar control unit for a rooflight that is in shade until noon spends money to make a dark room darker.
South-facing Cotswold slopes, and the conversions built into them
The wolds run in long ridges and the villages sit on the sides of them, so Painswick, Bibury, Northleach and Chipping Campden have a great many houses with one long slope facing broadly south or south-west and a shorter one facing back into the hill. The southern slope is the one people want to convert, because it is the one with the view and the daylight.
Two things follow. First, these are steep roofs, commonly 45 to 55 degrees, so the summer sun arrives at a poor angle and the winter sun arrives beautifully, which argues for keeping the solar factor up and controlling the summer with shading. Second, they are usually conservation rooflights in thick stone slate, which constrains the frame and sometimes the glass options available within it. Where the slope is closer to south-west than south, the calculation shifts towards the western answer and the solar factor comes down. That is a survey judgement made with a compass in hand, and the unit detail is on conservation rooflight installation.
North-facing slopes in the Forest of Dean, and the glass that stays wet
A north-facing slope in Coleford, Cinderford or Mitcheldean is a cold, damp surface for most of the year. It never gets the direct sun that dries and warms other elevations, the Forest is wetter than the vale, and the tree cover keeps humidity high and drops organic debris on the glass.
The practical outcome is algae, lichen and a green film on the outer pane that returns every autumn, plus frost that sits on the glass until mid-morning long after the south slope has cleared. A self-cleaning outer coating, which uses a photocatalytic layer that breaks down organic soiling and a hydrophilic surface that lets rain sheet off rather than bead, is worth more on a north-facing Forest roof than on any sunny slope, even though it works more slowly in low light. Where the unit is also hard to reach, which in a steep former mining terrace it usually is, it stops being a luxury and starts being the only realistic cleaning arrangement.
The prevailing wind comes from the same quarter as the sun
One further thing the compass decides is exposure. The prevailing wind in Gloucestershire is from the south west, and it brings driving rain with real force behind it, particularly across the Forest and up the Severn Vale. That is the same broad quarter as the difficult solar aspect, which means a south-west slope is asking for a low solar factor and generous weathering at the same time.
On those elevations the head laps go beyond the manufacturer’s minimum, the side flashings are taken wider, and the underlay is dressed rather than slit. North and east elevations are more sheltered from driving rain and correspondingly less demanding on the weathering, though they hold moss longer. Aspect therefore shows up twice on the same order: once in the glass specification and once in the perimeter detailing, and the second one is where the ten-year workmanship guarantee lives.
What the survey writes down about aspect
The record is short and it settles most of the specification. The bearing of the slope, taken with a compass rather than estimated from a street layout. The pitch off a digital level. What is standing to the south, south-west and west of the position, and how tall it is. Whether the slope is overshadowed in the morning, the afternoon or not at all. Whether the room below is a bedroom, a kitchen, a bathroom or a workspace. Whether the unit will be reachable for cleaning.
From those six lines the glass specification follows almost mechanically: the solar factor, the coating, whether a self-cleaning layer is worth the cost, whether shading should be ordered with the unit or later, and whether the third pane pays. It costs ten minutes on site and it is the difference between a room that works in July and one that gets an external blind bolted on in a hurry the following August. The order of the process, and how it turns into a figure, is on costs.
Next on this subject.
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- Guide Condensation, ventilation and rooflights Surface condensation on a rooflight is a whole-house ventilation problem showing up at the coldest point in the building. The…
- Guide Triple glazing in a rooflight and when it pays Triple glazing in a roof is worth more than triple glazing in a wall, and for a reason that has…
- Guide Warm-edge spacers and where condensation starts Watch any rooflight through a cold week and the water always arrives in the same place first: a band following…
- Guide Low-emissivity coatings and argon fills Roughly three quarters of the heat crossing an uncoated sealed unit travels as infrared radiation from the warm inner pane…