Lantern pitch, and why 25 degrees keeps coming up

What the pitch has to do

Shed water fast enough to carry dirt with it. Stay low enough that the ridge does not dominate the roofline. Stay inside the 150mm permitted development allowance where that applies. And produce a proportion that looks like a lantern rather than a conservatory.

Twenty five degrees satisfies all four on a typical domestic span, which is why it has become the default.

Below about 15 degrees

Self cleaning coatings stop working properly, because they rely on rainwater sheeting off rather than sitting in droplets. Dirt stays put, and on a lantern above reach that matters.

Drainage slows, and any debris that lands tends to stay. Below about 10 degrees you are no longer really building a lantern, and a flat rooflight on a kerb is the better product.

Above about 30 degrees

The lantern starts to read as a conservatory roof, which is usually not what anyone came for. It is taller, which can push it past the permitted development allowance. And the extra height buys nothing functionally: the drainage and self cleaning benefits have already been achieved by 25.

Where pitch is forced

On a very large lantern, keeping a 25 degree pitch produces real height. Where the 150mm rule applies and the span is wide, the pitch sometimes has to be reduced to stay compliant.

That is a legitimate reason to reduce it, and it is worth knowing that it comes with the self cleaning trade-off rather than being free.

Hipped or gabled

Related to pitch and worth deciding at the same time. A hipped lantern slopes on all four sides and is the usual domestic form. A gabled one has two vertical ends, which gives more internal volume and a stronger architectural statement, and it looks deliberate rather than default.

Where the number comes from

Most lantern ranges offer glazed slopes somewhere between 15 and 25 degrees, and 25 is the figure that keeps appearing because it sits at the top of the useful range without becoming awkward.

It is not a regulation. It is the point where four separate requirements happen to meet, and understanding them explains why the range is what it is.

1. Water has to run off

The lower limit, and the reason lanterns are pitched at all.

Below about 5 degrees water does not run off glass reliably. It holds as a film, evaporates, and leaves behind everything it was carrying. That is why flat rooflights get dirty in a way pitched ones do not.

By 15 degrees rain sheds properly and takes deposits with it. By 25 it does so briskly, which on a lantern over a space that is awkward to reach is worth having.

It is also why self cleaning glass genuinely works on a lantern and does very little on a flat rooflight. The coating breaks down organic material and relies on rain sheeting off to carry it away, and sheeting needs pitch.

2. The frame needs depth to span

A lantern is a structure rather than a panel. The pitch is what gives the frame its depth and therefore its stiffness.

A shallower lantern needs heavier members or intermediate supports to span the same opening, and heavier members mean thicker sightlines and less glass. A steeper one gets structural depth for free from its geometry.

That is why very shallow lanterns tend to look chunkier than their steeper equivalents: the frame is doing the work the pitch is not.

3. Proportion in the room and on the roof

The upper limit, and it is aesthetic rather than technical.

Beyond about 25 degrees a lantern starts to look like a small conservatory roof sitting on an extension. It stands tall, it is visible from further away, and on a modest extension it dominates.

From inside, a steeper lantern gives more volume and more sense of height, which on a broad room is exactly what it is for and on a small one is overbearing.

So the choice within the range is largely about the size of the extension and the ceiling height below.

4. Solar gain

Runs the other way, and it is the argument for a steeper pitch that people rarely consider.

A shallower glazed surface is closer to horizontal, which means it presents more directly to a high summer sun. A steeper one presents more obliquely.

So a 15 degree lantern collects more midday sun than a 25 degree one of the same area, on the same roof, facing the same way.

That is a real difference, and it does not remove the need for solar control glass on a south or west facing lantern. It just makes it slightly less critical.

How to choose within the range

Steeper suits: a broad room with a flat ceiling at normal head height, where the volume the lantern adds is the point. An extension where the roof is not overlooked from an upper window. A south or west aspect, where the reduced solar incidence helps. And any position where cleaning access is awkward, because it sheds better.

Shallower suits: a smaller extension where a tall lantern would look out of proportion. A situation where the extension is overlooked from a bedroom window above and the profile matters. A permitted development extension close to its height limit. And a room that is already fairly tall, where more volume is not needed.

The height limit that catches people

A single storey rear extension built under permitted development has a maximum height, and a lantern standing on it counts toward that.

A steeper lantern on a low extension can push the whole thing over the limit, which turns a permitted development extension into one needing an application. That is worth checking before choosing a pitch rather than after.

It is also worth knowing that a lantern stands proud of the roof by considerably more than the 150mm limit that applies to a rooflight, so it does not qualify under those rules at all. It is dealt with as part of the extension.

What the pitch does not change

The kerb, which decides whether the installation lasts.

At least 150mm of upstand above the finished roof surface, the deck falling away from it so water cannot pond against the base, rigid insulation on the outer face, and a continuous vapour control layer sealed to the frame.

On a lantern the kerb carries the structure as well as weathering the opening, so it has to be square and level as well as tall enough. A lantern is manufactured to your dimensions and a kerb out of square will not take it evenly.

None of that varies with pitch, and all of it matters more than the pitch does.

Hipped against ridged

A related decision, and it interacts with pitch.

A ridged lantern has two sloping faces meeting at a ridge, with hipped ends. A pyramid lantern has four faces meeting at a point, which suits a square opening.

On a long rectangular opening a ridge is the natural form. On a square one a pyramid reads better, and at a given pitch a pyramid stands taller than a ridge because it comes to a single apex.

Where an opening vent is being included, it goes at the highest point, which on a ridged lantern is at the ridge and on a pyramid is at the apex.

What to ask

What pitch options the range offers, and what the overall height above the kerb is for each.

That second figure is the one to check against any permitted development height limit and against what is visible from an upper window.

And whether the pitch chosen still allows an opening vent at the ridge, because on a very shallow lantern the vent geometry can become awkward.

What the number is balancing

A lantern pitch is a compromise between four things that pull in different directions, and the middle twenties is roughly where they meet on a domestic roof.

Shedding water and dirt. Steeper is better. Rain runs off rather than sitting, deposits go with it, and self cleaning coatings actually work because they need run off. Below about fifteen degrees the glass starts to hold water and dull.

Overall height. Shallower is better. Pitch and span together set the ridge height, and on a permitted development extension the total height is constrained. A steep pitch across a wide lantern produces a ridge that can push the extension over a limit.

Appearance from the garden. A very shallow lantern looks apologetic, like a compromise. A very steep one dominates a modest roof. The middle reads as deliberate.

Glass area and cost. A steeper lantern has more glass and more frame for the same footprint, so it costs more and loses more heat.

Why span changes the answer

Height rises with both pitch and width, so a pitch that is comfortable on a small lantern produces a tall one on a wide lantern.

Which means that as the footprint grows, the pitch usually has to come down to keep the height sensible. A large lantern at a steep pitch becomes a conservatory roof rather than a lantern.

That is why the same pitch figure does not suit every job, and why it should be drawn rather than specified from a number somebody quoted.

The traditional argument

Historic lanterns and roof lights on Victorian buildings tend to be steeper than modern ones, because they were made in smaller panes and steeper pitch was needed for the coverings and putties of the period.

On a period property, a steeper pitch reads as correct. On a contemporary flat roofed extension, a shallower one reads as intentional. Matching the pitch to the architecture matters more than matching it to a rule.

The practical minimum

Manufacturers state a minimum pitch for their systems, below which the glazing is not warranted, and it exists because the gaskets and drainage channels in the rafter sections are designed around water moving at a certain rate.

Going below it produces water sitting in the rafter drainage, which over time works the seals. It is not a limit to be negotiated.

The one thing worth deciding early

Whether the ridge height matters for consent. Establish the permitted development height limit and work the lantern size and pitch within it, rather than designing a lantern and discovering the extension no longer complies.

Hipped against gabled lanterns

The pitch discussion changes with the form.

A hipped lantern slopes on all four sides to a ridge. The hips are additional frame members, so a hipped lantern has more frame and loses more heat than a gabled one of the same footprint, and it reads as more traditional.

A gabled lantern has vertical or steeply raked ends, which gives more headroom at the ends and a more contemporary appearance.

At the same pitch, the hipped version is lower overall, which can matter where permitted development height is tight.

Snow, at the shallow end

A shallow lantern retains snow rather than shedding it, which increases the load and keeps the glass cold and wet for longer.

Not usually a structural concern on a properly specified unit, and worth knowing where a very shallow pitch is being proposed for appearance.

Seeing it before deciding

Pitch is difficult to judge from a number. Ask the supplier for a section drawing showing the finished height above the roof, and mark that height on a wall or a pole on site.

People are consistently surprised by how tall a lantern is once it is built, and a ten minute mock up prevents a decision made from a plan view.