Roof lantern pitch and how steep it should be

The minimum pitch, and what happens below it

Fifteen degrees is the figure most manufacturers set as a floor, and a few will go to 12 with specific conditions. Below that, three things start to go wrong at once.

Water stops running and starts creeping. Surface tension holds a film on shallow glass, and that film evaporates in place leaving whatever was dissolved in it. Dirt stops being carried off, because run-off velocity drops with the sine of the angle and there is not enough energy in the flow to move a leaf or a deposit of moss. And the gaskets around the glass, which are designed to be drained by gravity, sit in water rather than shedding it.

The result is a lantern that looks dirty within two years and needs cleaning far more often than the same unit at 25 degrees. Self-cleaning coatings help, but they rely on rain sheeting across the glass, and at 10 degrees rain does not sheet.

Why 25 degrees is the working default

Twenty five degrees is where most of the competing pressures balance. It is comfortably above the shedding threshold, so water leaves and dirt goes with it. It produces a rise that is generous enough to be felt in the room but not so tall that the lantern becomes the subject of the roof.

It also produces a slope that reads correctly from the ground on a single storey extension viewed from a garden, which is the angle nearly every lantern in Essex is actually seen from. Steeper than 30 and it starts to look like a small conservatory roof has been dropped on the extension. Shallower than 20 and it loses the character that made someone want a lantern rather than a flat rooflight in the first place.

What the extra height does inside the room

The rise is not just a number on a drawing. It is volume added to a ceiling, and in a room with a standard 2.4 metre ceiling a 450mm rise is close to a twenty per cent increase in height at the centre of the lantern.

The effect is disproportionate to the figure because the eye reads the converging bars as perspective. Four slopes running up to a short ridge give the ceiling a direction and a focus that a flat panel of glass does not, and standing under it the room feels taller than the tape says. That is the entire argument for a lantern over a flush rooflight, and pitch is the variable that controls how much of it you get.

Pitch and how much summer sun lands on the glass

Solar energy arriving on a surface follows the cosine of the angle between the sun and the surface normal. In Essex the sun reaches about 61 degrees above the horizon at midday in June and about 15 degrees in December.

A south facing lantern slope at 25 degrees presents itself to the June midday sun at an incidence angle of about 4 degrees, which is very nearly square on and close to maximum energy. Steepen it to 35 and the incidence widens to about 14 degrees, which sheds a little. The bigger effect is on the north facing slope, which at a steeper pitch turns further away from the summer sun and collects less. Steepening a lantern is a modest solar control measure rather than a decisive one, which is why the coatings do the heavy lifting. That specification is set out on the glazing page.

Pitch and the low winter sun

The more valuable half of the solar story runs the other way. In December the sun is low, around 15 degrees at noon, and a shallow lantern presents almost its edge to it. A steeper south facing slope catches more of that low light, both as brightness and as useful warmth.

On a north facing rear extension, which describes a great many houses in Chelmsford where the road runs east to west, the steeper south facing slope of the lantern is the one that faces back over the roof of the house. It still collects sky, and on a clear winter day it collects reflected light off the main roof as well. This is one of the few situations where 30 degrees rather than 25 is worth the extra height.

Run-off, and how fast rain leaves the glass

The volume of water a lantern sheds is easy to underestimate. A 2000 by 1500 lantern presents about three square metres of collecting surface, and in heavy rainfall at 75mm per hour that is around 220 litres an hour arriving at the base of the slopes.

All of it lands on the roof immediately around the kerb, in a concentrated line. A steeper pitch delivers it faster and further out from the frame, which is generally good, but it makes the falls and the drainage of the surrounding roof more important, not less. Where the flat roof around a lantern has a poor fall or a distant outlet, that concentrated discharge is what finds the weakness. How the kerb and the covering handle it is covered on the kerb page.

Rain noise, and the angle of impact

A raindrop hitting glass at 90 degrees delivers all of its momentum into the pane. One hitting at a slant delivers a component of it and slides off with the rest. So a steeper lantern is quieter than a shallow one under the same rain, and both are quieter than a flat rooflight.

The difference between 20 and 30 degrees is audible but not dramatic. Glass specification does more, and the laminated inner pane that a lantern should have anyway is the single biggest factor. Pitch is a contributing decision here rather than a governing one, but on a lantern directly over a bedroom or a study it is worth a couple of extra degrees.

Snow, and the load a shallow lantern carries

Snow load in the south east of England is modest by national standards, with a characteristic ground snow load around 0.5 kN/m², but it is not nothing, and it accumulates differently with pitch.

Above about 30 degrees, snow tends to slide off glass once the surface warms slightly from the room below, so the design load is reduced. Below 15 degrees it sits until it melts, and the full load stays on the structure for the duration. Between the two it depends on the weather. This is a factor in the frame calculation rather than something the homeowner needs to work out, but it explains why a shallow wide lantern needs a heavier ridge than a steep narrow one of the same span.

Wind uplift, and why steeper is not always better

Wind does not push down on a roof, it pulls up on it. Air accelerating over a raised object generates suction on the leeward slope and around the edges, and the taller the object the more of the wind stream it interrupts.

A tall lantern on an exposed site therefore attracts more uplift than a shallow one, and the fixings holding the base ring to the kerb are what resist it. On the exposed sites in the county, the bungalows on Canvey, the properties along the Blackwater and Crouch, and the open ground around the Dengie, this is a live consideration and it argues gently against the steepest pitches. The fixing centres specified by the manufacturer are followed exactly rather than reduced to save time.

Hipped and gable lanterns, and how pitch reads on each

A hipped lantern has four sloping faces meeting at a ridge, with the two short ends sloping in. A gable ended lantern has two sloping faces and two vertical glazed ends.

Pitch reads very differently on the two. On a hipped lantern the eye sees the slopes from every direction and a steep pitch reads as a pyramid, which is emphatic. On a gable ended lantern the vertical ends anchor it and a steeper pitch reads more as a roof, which is calmer. Gable ends also give useful head height at the ridge for a vent and let light in from a second direction. As a rule of thumb, a hipped lantern wants a degree or two shallower than a gable ended one of the same width to read the same way.

Permitted development, and the height limit

For most houses a lantern falls under permitted development, but the conditions are specific and pitch is where they bite. A rooflight must not project more than 150mm above the plane of the existing roof slope, and it must not be higher than the highest part of the roof.

The 150mm rule is written for units in a pitched slope and is applied to lanterns on flat roofs with more latitude, since a lantern is a raised structure by nature. The condition that matters in practice is the second one, and on a single storey rear extension there is normally plenty of room below the main roof ridge. Where it bites is on a two storey rear addition, a flat roofed dormer, or a bungalow with a shallow main roof, and it is checked before anything is ordered rather than after.

Pitch on a narrow lantern, and the reason to go steeper

A narrow lantern at a conventional 25 degrees produces very little rise. On a 1000mm wide unit that is 233mm, which from inside barely registers as a shape at all, and from outside reads as a shallow box.

Where the lantern is narrow, steepening it is nearly free. Going to 35 degrees on that 1000mm unit takes the rise to 350mm, which is 117mm more height for no meaningful change in the wind or planning position and a much better shape from both sides. Narrow lanterns on Victorian side returns are the classic case, and they generally want the steeper end of the range. The specific constraints of those returns are covered on the rear addition page.

Pitch on a wide lantern, and the reason to go shallower

The inverse holds, and more strongly. On a 3000mm wide lantern every extra degree adds roughly 30mm of ridge height, so the difference between 22 and 30 degrees is 260mm of additional structure standing on the roof.

That has consequences beyond looks. The glass panes get longer, which pushes them past the sizes at which a single pane is practical and adds an intermediate bar. The wind loading rises. The scaffold and the lift get more involved. And on a wide lantern the rise is already substantial at a modest pitch, so the room gains the volume without the penalties. Wide lanterns generally want 22 to 25 degrees, and the ones that look best are rarely the steepest.

Shallow contemporary lanterns, and what to check

There is a current fashion for very shallow lanterns, sometimes as low as 5 to 10 degrees, marketed as a contemporary alternative to a flush rooflight. They are a genuine product and several good manufacturers make them.

They ask more of the detailing, and the questions to put to the supplier are specific. What is the warranted minimum pitch for this system. Is the glazing bar gasket designed for drainage at that angle or is it a standard profile being used outside its intended range. Is the glass supplied with a self-cleaning coating. And what is the expected cleaning interval. Where the answers are confident and in writing, a shallow lantern is a legitimate choice. Where they are vague, the flush flat rooflight is the honest version of the same idea.

Pitch and the internal reveal

The lining between the ceiling and the base of the lantern is the reveal, and pitch changes how it should be built. Under a steep lantern the eye is drawn upward along the bars, and a splayed reveal that opens outward continues that movement and makes the opening read much larger.

Under a shallow lantern, a deep splay competes with the slope and the result reads as two conflicting angles. A tighter, squarer reveal suits it better. The rule that holds either way is that the reveal geometry is decided at the same time as the pitch, not left to the plasterer on the last day. It costs nothing in materials and it is the cheapest improvement available on the whole job.

Fitting a ridge vent, and the height it wants

A vent set into a slope pane or into the ridge needs somewhere to go, and a very shallow lantern gives it very little. A top hung sash on a 12 degree slope opens into a shallow wedge of air, and the resulting free area is small relative to the size of the pane.

At 25 to 30 degrees the same actuator stroke produces a considerably larger opening, because the sash swings away from a slope that is already inclined. If a ventilating lantern is the intent, that argues for the middle to upper part of the pitch range. How those vents work and how they are driven is set out on the ridge vent page.

Choosing the pitch for your roof

The order of decisions is width first, then height limits, then pitch. Establish what the roof can take in plan, which is worked through on the sizing page. Check what height is available under any planning constraint and under any window above. Then set the pitch to produce a rise that suits the room.

For most single storey rear extensions in Essex that lands at 25 degrees. Narrow side returns go steeper, to 30 or 35. Wide full width lanterns go shallower, to 22. Anything under 15 wants a conversation about cleaning and about warranties before it is ordered. Sunspire settles pitch at survey alongside size and structure, because all three are one decision made three times, and the full process is set out on the roof lantern installation page.