Adding a roof lantern to an existing flat roof

The answer sits in the roof structure, which you cannot see from the garden. A flat roof is a deck laid over timber joists and finished with a waterproof covering such as felt, EPDM rubber or GRP. The joists run one way between the walls, and each one carries a share of the load. A lantern adds weight in one spot, and cutting a joist removes part of the frame that was carrying it. So the first question is whether the timbers, once trimmed, can still do their job.

Three facts decide it: how deep the joists are and how far apart they sit, how far they span between their supports, and the condition of the timber and deck where the opening will go. Sound timber at ordinary spacing is the straightforward case. Joists that are slight for their span, or a deck that has been wet, change the plan. Extra timbers are added alongside, the lantern moves to a better spot, or a smaller one is chosen.

Weight counts as well. A triple-glazed unit across a wide opening weighs more than a double-glazed one across a modest opening, and the maker's stated weight for the lantern you pick goes into the check.

The room under the roof gets a say as well. A deep kitchen and dining space with a low ceiling suits a different position and size from a narrow utility room, and the place where you stand, sit and eat decides where the glass should be. The structure sets what is possible; the room decides what is worth doing.

None of this can be judged from a picture. The survey costs nothing and comes ahead of any price, and for a head start you can send a picture of the roof and the room to WhatsApp 07880 284133.

Once the position is agreed, the opening is marked out from above and from the room below so the two line up. The covering and deck inside the line are cut away. A joist that crosses the gap is cut back, and its loose ends are carried on a new timber across each end of the opening, called a trimmer, which is fixed to the joists either side. Those neighbouring joists take extra load, so they are doubled up where the structure needs it.

A narrow lantern that drops between two joists needs less cutting. A wide one that crosses several needs more, and heavier timbers to carry the cut ends. Anything already coming through the roof near the spot settles where the opening can go: a soil pipe, a flue or an extractor outlet stays where it is, and the lantern is placed around it.

The structure and the trimming are among the reasons Building Regulations come into a new opening at all. That is also why the cutting is planned before anyone is on the roof. The timbers, their sizes and the order of work are settled first, and the lantern is not fitted into a hole that was cut and then thought about.

The kerb is the raised box the lantern stands on. On a new extension it goes up with the roof. On an existing roof it has to be added to a surface that is already waterproof, which is where care pays off. There are two routes. The maker can supply an insulated kerb with the lantern, made to the agreed opening and fixed to the trimmed structure. Or a timber kerb is built on the roof, insulated, then covered. Either way the kerb sets how high the glass stands and how deep the lining looks from inside.

For energy purposes the lantern is a rooflight, going by the way Approved Document L defines one. In England, a new or replacement rooflight in an existing home should have a U-value no worse than 2.2 W/m²K. Judged across the unit's whole outer skin, that figure can cover more area than the opening it sits over, and a kerb supplied with the unit as one assembly falls inside it. A kerb formed on site should reach 0.35 W/m²K or better.

The join between kerb and covering is where a lantern job holds or fails, so it gets the most thought. The covering is cut back round the opening, taken up the outside face of the kerb and finished so that water running off the roof cannot get behind it. How that is done depends on what the covering is. Felt is lapped and bonded up the upstand. EPDM rubber is cut and stuck to the kerb with the maker's flashing details. GRP is laminated up the side in place. Each system has its own method and parts, and the sealing follows the system already on the roof, so the old and the new read as one waterproof layer.

The lantern frame then sits over the top edge of the kerb, which shelters the covering on its sides from the weather. If the covering near the position is in poor condition, that is raised at the survey, so the join is planned on sound ground and not found out halfway through.

Two separate things need checking. The first is planning. A roof lantern added to an existing flat roof usually stands well over 150 mm proud of the roof, so it normally falls outside Class C and needs planning permission unless the council confirms otherwise. We check this at survey. Putting a rooflight into a house in England usually falls under permitted development, within limits that our planning permission page sets out, but a lantern's height is what changes the answer, as the lantern planning answer explains. On a new extension, the lantern is assessed with the extension; we confirm the position with the council before work starts. Where the house is listed, a new lantern will almost always need listed building consent too, and that consent settles the work.

The second is the Building Regulations. A new rooflight will generally need approval, because the roof is cut, the opening is trimmed and supported, the structure takes extra weight, the unit is held to energy limits, and a boundary nearby brings fire performance into play. Where the work is notifiable, we handle the Building Regulations notification with the council's building control team or a registered building control approver.

Fitting is a sequence, and the order matters more than the pace. The room below is protected before the roof is opened, and the roof is closed against the weather before the inside is finished. Opening a roof suits settled weather, with the lantern and kerb already delivered. Access to the roof, whether by scaffold tower or by ladder at the edge, is agreed at the survey, and the household can expect dust and the room under the opening to be out of use while the roof is open.

Once the lantern is fixed, look at it from the room as well as from the garden: the glass should sit level on its kerb, the seals should be in place and the lining should be square to the ceiling. Anything that looks wrong from the room is easier to raise before the scaffold tower or ladder has gone.

At handover, the controls, the blind choices and how to clean the glass outside are talked through. Most of that depends on choices made earlier. A fixed lantern has nothing to open. A vent panel is an option on some makers' lanterns, raised by pole or driven by a motor. Which suits you comes down to the ceiling height and who will reach for it. Blinds for a lantern can be made to the shape of its glass panels by specialist blind makers, and they are bought for the maker and size. If you want a motor on either, tell us at the survey, because the cable has to reach it before the ceiling lining closes.

Cleaning stays an outside job: a water-fed pole in the hands of someone standing in the garden, or a ladder set at the edge of the roof. A lantern is not a walk-on unit, so nobody stands on it; the fitting itself sits inside our 10-year workmanship guarantee.