What pitch does a roof lantern need?
Around 25 degrees by default. Below 15 you lose self cleaning; above 30 it starts to look like a conservatory.
Part of our Roof Lantern Installation guide
Most lanterns are pitched around 25 degrees, and it is a well chosen default rather than a rule.
What the pitch achieves
Enough slope that rain runs off carrying dirt with it, low enough that the ridge does not dominate the roofline, and short enough overall to stay inside the 150mm permitted development allowance on a typical span.
The lower limit
Below about 15 degrees, self cleaning coatings stop working. They rely on water sheeting across the surface rather than beading, and on a shallow pitch it beads and sits. Dirt stays, and on a lantern above reach that becomes a cleaning problem rather than a cosmetic one.
Below about 10 degrees you are not really building a lantern. A flat rooflight on a kerb is the right product and it will cost less.
The upper limit
Above about 30 degrees the proportions shift towards a conservatory roof. It is taller, which can breach permitted development, and it gains nothing functionally over 25.
When pitch is forced down
On a very wide lantern, holding 25 degrees produces real height. Where the 150mm rule applies, the pitch sometimes has to reduce to stay compliant, and that is a legitimate reason.
Worth knowing it comes with the self cleaning trade-off rather than being free. Detail on the lantern page.
The short answer
The lantern itself carries its own pitch. What matters is the flat roof it sits on, and that needs a fall rather than a pitch: a minimum finished fall of 1 in 80 is the usual design target, which in practice means designing to 1 in 40 so that deflection and workmanship tolerances do not leave you with a flat spot.
The glazed slopes of the lantern are typically between 15 and 25 degrees. That is a manufacturer’s specification rather than something you choose, and it exists for two reasons.
Why lanterns are pitched at all
Water has to leave. Below about 5 degrees, water does not run off glass reliably, it sits and evaporates, and what it leaves behind is dirt. A lantern at 15 degrees or more sheds rain, and the rain does most of the cleaning.
Self cleaning coatings need run off to work. The coating breaks down organic deposits and then relies on rain sheeting off rather than beading to carry them away. On a near flat pane it does very little.
Structural depth. The pitch gives the frame its rigidity, which is what allows a lantern to span a large opening without heavy intermediate members.
The trade off
A steeper lantern sheds water better, gives more internal height and reads as more of an architectural feature. It also stands taller above the roof, which matters if the extension is overlooked from an upper window or if planning is a consideration.
A shallower lantern is more discreet and looks better in proportion on a smaller extension, but it collects more dirt and gains more heat through the middle of the day because the glass is closer to horizontal relative to a high summer sun.
Most manufacturers offer a choice within their range, and it is worth deciding deliberately rather than accepting the default.
The kerb, which is the actual answer
A lantern sits on a kerb, and the kerb is what makes the installation work.
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 of the kerb itself, because an uninsulated timber kerb punches straight through the roof insulation around the whole perimeter. And a continuous vapour control layer sealed to the frame.
The kerb also has to carry the lantern, which is heavy. On a new extension that is designed in. Retrofitting a lantern into an existing flat roof means checking the structure, and that is a cost worth paying rather than avoiding.
What we check first
On any lantern enquiry we look at the fall on the existing roof before anything else. A roof with no fall, or with a fall running toward the opening rather than away from it, is the single most common reason a lantern installation goes wrong, and correcting it locally around the opening is part of the job rather than an optional extra.
What to check on your existing roof
Stand on the roof after rain, or look at it, and see where the water goes. Standing water, or a fall running toward the proposed opening rather than away from it, is the single most common reason a lantern installation causes trouble.
Correcting the fall locally around the kerb is part of the job rather than an optional extra, and it should appear in the quotation.
Ask us
Send a photograph showing how the flat roof meets the house. We will not price a lantern without seeing the fall on the existing roof, because that is where the trouble starts.
Choosing within the range
Most manufacturers offer a choice of pitch within their range and it is worth deciding deliberately. A steeper lantern sheds water better, gives more internal height and reads as more of a feature. A shallower one is more discreet and better proportioned on a small extension, but collects more dirt and gains more heat through the middle of the day.
One last point
The pitch of the lantern is a manufacturer’s specification. The fall on the flat roof it sits on is your builder’s responsibility, and it is the one that causes trouble when it is wrong.
More on Roof Lantern Installation
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