What decides the size of a roof lantern

1. The structural opening

A lantern sits inside an opening formed in the flat roof, bounded by trimmers on all four sides. Those trimmers carry the roof and the lantern, and they transfer that load into whatever is holding them up.

On a modest opening in a new flat roof, timber trimmers are usually adequate. Widen the opening and the span increases, and at some point timber stops working and a steel is needed. That is not a problem, but it has consequences: a steel has to be designed by an engineer, it costs money, and its position sets the kerb position.

Which is why the structure is decided before the lantern is ordered rather than after. A made to size unit cannot be adjusted to fit a steel that turned out to sit 40mm from where the drawing assumed.

2. The room below

A lantern lights the area beneath it and roughly a metre past its edge in each direction. Past that the light falls away quickly.

In a square room, one central lantern is the right answer. In a long kitchen-diner, which is what most rear extensions produce, two smaller lanterns spaced down the length of the room deliver more usable light than one large one in the middle, and they usually need less structure.

It is worth resisting the instinct that bigger is better. A 3m lantern in the centre of a 7m room leaves both ends dark.

3. The roof you have to leave

This is the one that gets missed, and it is the one that causes leaks.

A lantern needs a working margin of flat roof around the kerb so the covering can be dressed up the face of the upstand and terminated properly under the lantern’s flange. Squeeze the lantern out towards the walls to gain glass and there is nowhere to detail that termination.

A roof with no margin is a roof that will leak at the perimeter, and no amount of sealant fixes a detail that was never possible. As a working figure, leave at least 300mm of flat roof between the kerb and any wall or upstand, and more on the upslope side where water arrives.

Height, and permitted development

Size is not only about plan dimensions. A larger lantern at the same pitch is a taller lantern, and under permitted development a rooflight generally must not project more than 150mm above the existing roof plane.

On a rear extension that is usually assessed against the extension’s own roof, and lanterns are routinely fitted without difficulty. In a conservation area, on a listed building, or where an Article 4 direction applies, it is a different conversation and it belongs on the conservation page.

What we do about it

The survey measures the opening, the roof build up, the fall and the structure, and the size comes out of that rather than going into it. Where what you want and what the roof allows are different, we say so before you order rather than after.

More on the product itself is on the roof lantern installation page.

Start from the opening, not the catalogue

People arrive with a size in mind, usually taken from a photograph. That is the wrong end of the problem.

A lantern is made to your dimensions, so there is no catalogue size to work back from. What constrains it is the opening the roof can give you, and that comes from the structure, the roof area available, and what the room below actually needs.

Get those three settled and the size follows. Pick a size first and you spend the project trying to make the building accommodate it.

The proportion that reads well

A lantern needs roof around it. It is a form that rises from a plane, and if it occupies most of that plane it stops reading as a lantern and starts reading as a glass roof with a border.

As a working guide, leaving a clear margin of roof on every side, generous enough to be obviously deliberate, is what makes the finished thing look considered. A lantern crammed to within a short distance of the parapet on one side looks like it was sized before anybody measured.

Rectangular proportions generally sit better than square ones on a rectangular room, because they follow the geometry of the space below. A square lantern in a long narrow kitchen fights the plan.

And centring matters. Centred on the room reads better than centred on the roof, where those differ, because the room is what people experience.

How much light you actually need

More glass is not automatically better, and this is the correction worth making early.

A horizontal opening admits considerably more daylight per square metre than a vertical window, because it faces the whole sky rather than a portion of it. A modest lantern over a kitchen island transforms that space. Doubling its area does not double the perceived brightness; the eye adapts, and the returns fall away quickly.

What extra area does deliver, reliably, is extra solar gain, extra heat loss, and extra cost.

So the honest question is not how large the lantern can be, but where the light needs to land. Over the island. Over the table. Into the deep part of the plan against the original house wall. Size follows from that.

Overheating sets a practical ceiling

On a south or west facing single storey extension, this is the constraint that should bite before any other.

Horizontal glazing takes the full summer sun. A large lantern over a room that also has bifold doors can make that room genuinely unpleasant on a hot afternoon, and it is the most common regret people report.

Two controls. The g value of the glass, which sets how much solar energy gets through, and can be brought down with solar control glazing at a modest cost in light. And ventilation, particularly an opening vent at the top of the lantern that lets hot air out at the highest point in the room.

Where the orientation is difficult, the right answer is frequently a smaller lantern with better glass rather than a larger one with standard glass. The room ends up brighter in practice, because it stays usable.

Heat loss sets another

Even a good lantern loses heat faster than the insulated roof it replaces. That is unavoidable; glass is not insulation.

Ask for the whole unit U value rather than the centre pane figure. On a lantern with many frame members, rafters, hips and a ridge, the frame is a substantial proportion of the total area, and the centre pane number flatters the reality considerably.

A lantern with more, smaller panes has more frame and performs worse thermally than one with fewer, larger panes of the same overall size. That is a real trade against the more traditional appearance of a finely divided lantern.

What the structure allows

The opening needs trimming, and on anything but a small lantern that means a structural engineer and usually steel.

The engineer works out what the roof needs to carry the lantern and to span around the hole, and the answer sets the maximum clear opening achievable without disproportionate cost. There is usually a size beyond which the steel becomes a different order of expense, and it is worth asking where that step is before committing to a dimension.

On an extension being built new, this is easy: the structure is designed around the lantern from the start. On an existing roof it is harder, and the existing joist arrangement frequently makes one dimension much cheaper to achieve than another.

Height, and what it does to consent

A lantern rises above the roof plane, and on a single storey rear extension under permitted development there are height limits that the lantern has to sit within.

A larger footprint generally means a taller lantern, because the pitch is maintained across a wider span. So footprint and height are linked, and a lantern sized without checking the resulting ridge height can push the extension over a limit that was comfortable before.

In a conservation area, with an Article 4 direction, or on a listed building, the rules differ and the check comes first.

Weight, glass size and access

A practical limit people forget until delivery day.

Large glass panes are heavy, and a lantern is assembled at height. Beyond a certain pane size the installation needs mechanical lifting, which needs access for the equipment, which on a rear extension behind a terraced house may simply not exist.

Access should be assessed before the size is fixed. A lantern that has to be craned over a house is a different job from one that can be carried through it, and the difference is substantial.

The sequence that produces the right size

Decide where the light needs to land in the room. Establish what the structure will give you and what it costs at each step. Check the height against the consent position. Confirm access for the panes. Then draw the lantern into the roof plan and look at the margins.

That process usually produces a lantern somewhat smaller than the one people first imagined, better glazed, and considerably better to live with.

Working within the height limit

The constraint people meet last and should meet first.

A lantern rises above the roof plane, and on a single storey rear extension under permitted development the total height is limited. Footprint and height are linked, because the pitch is maintained across the span, so a wider lantern is a taller one.

Which means a lantern sized on appearance alone can push an extension over a limit that was comfortable before, and turn a permitted development project into a planning application.

Establish the height limit, then work the footprint and pitch within it. That is the order.

The structural cost steps

Cost does not rise smoothly with size. There are steps.

Beyond a certain clear span the steel section changes, and beyond another the glass build up changes and the panes need lifting mechanically.

Ask the engineer and the supplier where those steps fall before fixing a dimension. A lantern 200mm narrower may cost substantially less, and nobody in the room will ever know the difference.

Access, which decides more jobs than people expect

Large panes are heavy. Beyond a certain size the installation needs mechanical lifting, which needs a standing position for the equipment and a route to the roof.

On a rear extension behind a terraced house with no side access, that may simply not exist, in which case the size is set by what can be carried through the house.

Establish this before fixing dimensions rather than on delivery day.

Margins, and what looks considered

A lantern needs roof around it. Leaving a clear margin on every side, generous enough to be obviously deliberate, is what makes the result look designed rather than fitted.

A lantern crammed to within a short distance of the parapet on one side looks like it was sized before anybody measured.

And rectangular proportions generally sit better on a rectangular room than square ones, because they follow the geometry of the space below.

The honest conclusion

Working through the height limit, the structural steps, the access constraint and the margins usually produces a lantern somewhat smaller than the one people first imagined.

Better glazed, better ventilated, considerably cheaper, and better to live with. More glass does not deliver proportionally more perceived brightness, because the eye adapts, and it does deliver proportionally more solar gain.