The short answer
Yes, in most cases, provided the roof structure and the covering are in fit state. Putting a roof lantern on an existing flat roof means cutting a new opening, trimming the joists around it, building a kerb, and tying the new upstand into the old covering, be it felt, EPDM or GRP. Where that covering is near the end of its life, the better plan is to renew it during the same job.
How a retrofit differs from building new
On a new extension the lantern opening is formed as the roof is framed. On a finished one the roof already exists, so the opening is cut through a deck that carries a covering, insulation and usually a plasterboard ceiling beneath. Each layer is taken back to a clean edge and rebuilt in order afterwards.
That is why a retrofit begins with the survey and not the order form. We look at what can be seen from the loft or ceiling void, note the direction and depth of the joists, and record the build-up of the roof. Those facts are entered into the written job plan before a lantern is ordered. Our lantern installation service page walks through the whole job in order.
Cutting the opening and trimming the joists
A lantern needs a clear rectangular opening, and any joist that crosses it has to be cut. The cut ends are carried on new trimmers, which are timbers fixed across the opening and bearing on the uncut joists either side. Their sizes depend on the opening and the span, and in some cases an engineer calls for steel instead. That decision is covered in timber trimmers versus steel.
Before the first cut, the joist ends are supported from below with props, so the ceiling does not move while the new framing goes in. The opening is then checked for square by measuring the diagonals, since a lantern frame will not forgive an opening that has drifted. Any wiring, pipework or ducting that runs through the roof is found at survey and moved at this stage, so nothing is cut by surprise.
The cutting is planned so the roof stays open for as short a time as possible. The opening is formed, trimmed and covered on the same day, and nothing is left exposed overnight.
- Mark the opening out from the drawing in the job plan
- Take back the covering and deck to the line
- Fix the trimmers before any joist is released
- Cut the joists and secure their ends to the trimmers
- Close the opening with a temporary cover until the kerb goes on
Building the kerb the lantern stands on
The kerb is the raised upstand that the lantern sits on. It is built in timber or, with some systems, supplied as a prefabricated section, then insulated and lined before the covering is turned up its face. Its height matters, because the covering needs a proper upstand against the lantern frame to shed water, and it is the point where the lantern's size meets the real opening.
A kerb has to be square and level, because the lantern is made in a factory to tight dimensions. A kerb that is out of true will not take the frame cleanly. This is the stage where the surveyed measurements are checked again against the delivered unit, before anything is fixed.
Tying the new upstand into the old covering
This is the step that decides whether the retrofit stays dry. The covering up the new kerb must join the existing roof covering, and how it joins depends on what is already there. Felt is lapped and bonded. EPDM is a single membrane, so the new sheet is joined to the old with the maker's tapes and adhesives. GRP takes a fresh laminate laid with a generous overlap onto prepared old surface.
Every one of these joins relies on the old surface being sound and clean. Where it is, the new upstand ties in neatly. Where it is not, the join is only as good as the weakest area it touches.
| Existing covering | How the tie-in is made |
|---|---|
| Built-up felt | Lapped and bonded with an overlap |
| EPDM membrane | Joined with the maker's tapes and adhesive |
| GRP | New laminate laid with an overlap onto prepared surface |
When a re-roof is the better plan
If the existing covering is old, blistered or lifting, tying a new lantern into it asks the new work to rely on tired material. The sensible plan then is to renew the covering across the whole roof as part of the same job, so the lantern upstand and the roof become one new surface. It adds to the day, yet it avoids opening a good new kerb to a failing covering a year later.
We judge the covering at the survey and recommend a route, with the reason, in the job plan. Nothing is assumed from the garden. What the council may need to see is explained in the lantern planning answer, and Building Control is notified wherever the work is notifiable.
A re-roof also gives the chance to bring the insulation up to the current standard, because the old covering is already off. That can be done once, under one set of drawings, instead of in two separate visits with two sets of scaffold. For a roof of any age, the covering and the lantern are best decided together.
What you receive in writing
The free survey comes first. After it you receive a quote on paper, with a plan attached that sets down the opening size, the trimmer and kerb approach, the method for joining the covering, the council position we found, the Building Control route and the date booked for the fit. Our workmanship carries a ten-year guarantee, given to you on paper.
If you want to know how many days the retrofit occupies, read the fitting time for a lantern, which sets out the day or days involved and what can lengthen them. Because the work is sequenced, a retrofit is planned backwards from the delivery of the lantern, so the opening is never cut before the unit is confirmed on its way.
The room below is protected throughout. Floors and furniture are covered, the ceiling edge is sealed against dust, and the opening is lined out neatly once the lantern is in. You are told in advance which morning the work starts, and the roof is weathertight again before the team leaves each day.