Can a roof lantern go on a pitched roof?

A roof lantern is a raised, multi-faced glazed structure. It has its own pitch built into it: the glass bars climb from a rectangular perimeter up to a ridge, usually at somewhere between 15 and 25 degrees, which is what gives the unit its shape and lets rain run off it. That geometry only works if the frame it sits on is level, or close to it. The perimeter has to be square and flat so every rafter bar starts from the same height and meets the ridge cleanly.

Set the same frame on a slope and the geometry falls apart. One side of the lantern would sit much higher than the other, the ridge would run downhill, and the weathering detail at the base would have water pooling against the high edge and sheeting off the low one. Manufacturers size and cut a lantern for a flat opening on a level kerb. There is no standard lantern made to be dropped onto a 30 degree tile slope, because the product is not meant to go there.

Flat is a relative term here. A well-built flat roof is never truly flat. It carries a shallow fall, often around 1 in 40 or 1 in 80, to move water to the outlet. A lantern copes with that easily, because the kerb is built up to a level top even where the deck below slopes. The limit is roughly a couple of degrees under the unit itself. Beyond that you are no longer fitting a lantern to a flat roof, you are trying to force it onto a pitch.

Most houses with a lantern have a pitched main roof. The lantern is not in that slope. It is in a flat-roofed element attached to the house, and that is nearly always the honest answer to the question. If you want a lantern, you build or use a flat base for it.

• A flat-roofed rear extension. The single most common home for a lantern. A kitchen or dining extension off the back of a house takes a flat or shallow warm-deck roof, and the lantern sits centrally in it over the island or table. The pitched roof of the house is untouched.
• A flat section within a pitched roof. On a larger project a roof can be designed with a flat central deck between pitched slopes, and the lantern lands on that deck. This is a design decision made at drawing stage, not something retrofitted into finished tiles.
• A raised kerb or box dormer. A structural upstand can be built up from the roof to present a level opening for the lantern. This is more involved, because it adds height and needs its own weathering and structural support, but it is how a lantern reaches a roof that has no flat area of its own.

Whichever route suits your house, the work sits within roof lantern installation as a whole, from the timber kerb up to the flashing, and the pitched surrounding roof is left as it is.

Sometimes the roof you want daylight in genuinely is the pitched slope: a loft conversion, a room in the roof, a vaulted ceiling that follows the rafters. A lantern is the wrong tool for that, but you are not short of options that are made for a pitch.

A pitched rooflight or roof window is designed to sit in the slope, flush or slightly proud, weathered into the tiles or slates with a flashing kit matched to the covering. These come in fixed and opening versions and in sizes from a single bedroom window up to banks of several units side by side. For anything in a sloping roof, this is the category to look at, and our pitched roof skylight installation page covers how they are set into the covering. If you want the drama of glass overhead but the roof is pitched, several roof windows grouped together give you a large glazed area without pretending a lantern can do a job it cannot.

There is also a middle option. Some manufacturers make low-pitch and pitched glazed units that read a little like a lantern from inside but are engineered for a slope. If the lantern look matters to you, ask about those rather than assuming a standard flat-roof lantern can be adapted.

Here is where the decision earns its keep, because in Southend a lantern or a bank of rooflights on the back of the house very often faces south. The 1930s semi belt was largely built with the garden to the rear, so a great many rear extensions and back slopes look south across the estuary, straight at the sun for most of the day.

That matters more than it would inland. The estuary gives an open southern horizon with nothing to shade the glass, and reflected light off the water adds to the total on a bright day. A lantern is a lot of glazed area, most of it tilted up towards the summer sun, over the one room that already makes its own heat from cooking. Get the glass specification wrong and you have built a greenhouse over the kitchen.

The number to control is the G-value, the share of the sun’s energy that comes through the glass as heat. Clear double glazing sits around 0.6; solar-control double glazing sits nearer 0.3 and blocks roughly half as much heat for a modest hit to daylight. On a south-facing lantern or a south-facing bank of pitched rooflights, ask for the low figure. On a north-facing one you can leave it clear and keep the free winter warmth. The physics is the same whether the glass ends up in a lantern on a flat deck or in a rooflight in a pitch, and it is worth deciding before anyone quotes you. Our energy-efficient skylight installation work is built around exactly this question.

Start with two facts about your roof: is there a flat area for a lantern to sit on, or is the whole roof pitched, and which way does the glass face. If there is a flat extension or deck, a lantern is on the table and belongs to roof lantern installation. If the roof is all pitch, look at roof windows in the slope instead. Either way, if it faces south, settle the glass specification first.

Tell us the roof type, the rough opening size and the orientation, and we will come back with the right unit and a glass specification to match. Ring 01702 898232 or request a quote, and we will look at the whole roof rather than just the hole you want to put glass in.