Can a roof lantern go on an existing flat roof?

What an existing flat roof has to offer

A new extension is designed with the lantern in it. An existing flat roof was not, so the question is what the roof already gives you to work with. A lantern needs an opening, a structure to hold it and a kerb to stand on. Plenty of flat roofs can provide all three, and the survey is where we find out. A lantern is a bigger commitment than a flat rooflight, so it deserves a proper look at what is already there, from above and from the room below.

The lantern is a skylight in the everyday sense, and in trade terms a rooflight: a small glass roof on a frame, stood on a raised kerb above the flat roof. The raised glass is what makes it different from a flat rooflight. It picks up more light and lifts the ceiling in the middle of the room. It also has more glass, more frame and more weight, and the roof underneath has to cope with all three.

The checks before a lantern goes on

Four things are looked at before anyone cuts anything.

The joists are the beams under a flat roof, usually timber, spanning the room. A lantern needs an opening, and an opening interrupts joists. Where one is cut, the load is carried round the hole by trimming timbers, and the joists either side must be able to take it. Joists that are undersized, notched or showing signs of damage change the plan. A steel beam may be needed, and in that case the beam is designed, not guessed. The weight of the lantern itself, plus snow, plus the kerb and covering, goes into the picture.

The deck is the layer of boards or sheet material the covering sits on. If it is sound, it carries the kerb. If it is soft, it is dealt with before the lantern goes on, because a new lantern sitting on a failing deck gets its problems for free. The covering could be felt, single-ply membrane, rubber sheet or fibreglass, and each has its own way of being dressed up a kerb. The covering round the lantern has to be compatible with the old covering, or the whole roof is recovered at the same time. Sometimes that is the sensible time to do it.

Every flat roof should fall slightly to shed water. A kerb blocks that flow, so where the opening sits matters. The lantern goes in a spot where the roof sheds water around it, not into it. If the roof has lost its fall, the answer may involve tapered insulation to put the slope back. That is explained in our piece on a rooflight on a flat roof with no fall.

The ceiling, the insulation, any services in the roof void and the line of the room all affect the opening. A lantern is seen from inside as much as outside, so the size is chosen to suit the room as well as the roof.

Why the kerb matters more than the glass

On an existing roof, the kerb is where the work concentrates. It is the raised frame under the lantern. It lifts the glass off the roof surface, gives the waterproof layer something to climb, and holds the insulation that keeps heat in round the edge of the opening.

Kerb height is a judgement. Too low and the covering cannot be dressed up it properly. Too tall and it adds height to the profile of the lantern, and shifts how it looks from the garden. The roof finish, the fall, the thickness of the insulation and the lantern's own frame all feed into the figure. We measure the roof, work out the kerb and put it on the quote.

The kerb is also what lets a lantern go onto a roof built long before anyone thought of one. It turns the lantern's footprint into something the existing roof can carry and keep dry. On any quote, check the kerb height, the insulation and how the covering meets it, and ask to see all three written down. For the full account of how kerbs are built, see the page on rooflight kerbs and upstands.

Rules, paperwork and what happens next

A lantern is a larger piece of glazing than most rooflights, and it stands proud of the roof, so the planning and regulations side deserves a look. On a house, rooflights are usually permitted development, within limits on projection and height. Flats and maisonettes lack those householder rights, listed buildings need consent of their own, and a local Article 4 direction can take the rights away. Our planning and Building Regulations guide has the detail. The glazing has to meet the Part L thermal figures, and if the job needs notifying to Building Control, we send the notification.

The sequence for a lantern on an existing flat roof is a free survey, a fixed written quote, a made-to-measure lantern ordered to the opening, then the fitting: protect the room, open the roof, trim the structure, build the kerb, dress the covering, set and seal the lantern, and finish the inside. Our 10-year workmanship guarantee covers the installation.

If you are still deciding whether a lantern is the right kind of rooflight for your room, read are roof lanterns worth it? The full picture of what we install is on the roof lanterns page. To have your own flat roof checked for a lantern, book the free survey.