Roof answer
Can a roof lantern go on an existing flat roof?
A lantern can usually go into a flat roof that is already there, without a rebuild. What decides it is the structure under the covering, the weight of the lantern and the way the new kerb ties into the old roof.
The answer
Yes, in many cases. A lantern is fitted by cutting an opening in the existing deck, trimming the joists around it and building an insulated kerb that the new roof covering is dressed up to. It works when the joists can carry the extra load and the roof has a sound fall. A weak or sagging roof must be strengthened first, which we assess at the free survey.
What happens to the roof when a lantern goes in?
A hole is cut through covering, deck and insulation, the joists are framed round it with trimmers, a kerb goes up and the covering is carried up its sides and sealed.
The existing roof is not thrown away. Its covering is cut back around the marked opening, and the deck underneath is opened up. Joists that cross the hole are cut and their loads are picked up by new, doubled timbers, called trimmers, on each side of the opening.
The kerb and the covering
A kerb is then built on the deck around the opening. It is insulated, so the junction does not become a cold edge, and it is wrapped in the roof covering, which is carried up the outside face and tucked beneath the lantern's base. This is where a lantern leaks if it is done carelessly, so the junction detail matters more than almost anything else on the job.
Where the old roof stays
Away from the lantern the original covering can stay as it is, provided it is in good condition and compatible with the new work. The lantern then sits as a feature in an otherwise untouched roof, and a rebuild is not part of the plan unless the survey says the roof is worn out.
Cutting a new opening is building work, so we handle the notification to Building Control, either the local authority or a registered building control approver.
Can the joists carry a lantern?
Often, but a lantern with a kerb and glass adds weight over a small area, so the joists, their spacing and their bearings are assessed before an opening is marked.
A flat roof is a deck laid on timber joists, and the joists are fixed to the walls at each end. The roof already carries its own weight, the covering, snow and anyone who steps on it. A lantern changes where that load goes, because the opening removes some support and the glass and frame add weight at the edges.
What is looked at
The survey takes in the joist size and spacing, the span from wall to wall and the condition of the timber. Old roofs can have joists that are lightly sized, notched for pipes or darkened with past damp. If the timber is sound and the spans are reasonable, the trimmers can carry the load across. If the joists are marginal, the options are to strengthen them, to move the opening or to choose a lighter frame.
Walls at the ends
The loads end up in the walls, so a weak or cracked wall plate is also a consideration. This is more likely in an old rear offshoot of a Tyneside flat or terrace than in a recent extension. A structural engineer may be involved if the opening is big or the roof unusual, and a lighter frame material, discussed in our guide to lantern frame materials, can ease the demand.
Does the old covering need replacing first?
Only if it is worn out or the roof ponds water. A sound covering with a gentle fall can stay; a tired or sagging one is better renewed while the roof is open.
No flat roof is perfectly level. It is laid to a slight fall so rain runs to a gutter or outlet. If the deck has dipped, water can sit in a pool, and a pool near a new kerb is the last thing you want.
Looking at the covering
The covering is checked for blistering, splits and soft areas. If it is old, the sensible time to replace it is when the roof is already being opened for the lantern, since the new kerb detail can then be integrated with a fresh covering across the whole area. That is a decision to make with the numbers in front of you, not a default. Our page on flat roof rooflights explains how openings, upstands and falls work together.
Why a lantern wants a fall around it
Water running off the roof meets the kerb and has to go round it. A kerb across the direction of flow acts like a small dam, so the roof around it should be shaped to carry water past. Sometimes this means a small tapered insulation detail behind the kerb.
Does being in a flat change whether I can do this?
The building work is the same, but a new lantern in a flat must go through a planning application, and the title or lease may involve the other owner in any work on the roof.
The technical answer above is the same whether yours is a freehold house or a converted or purpose-built flat. The permission answer is different, and it comes first.
On a house
A lantern on a house can be permitted development under Class C if it projects no more than 150mm beyond the plane of the roof slope and sits no higher than the highest part of the roof. Because a lantern sits on a kerb and then rises to a ridge, many exceed 150mm and need an application. Our answer on lantern planning permission sets this out.
In a flat
Flats and maisonettes have no householder permitted development, so a new lantern on a Tyneside flat needs planning permission from the council. Your title or lease should say whose the roof and loft void are, and whether a neighbour has to agree to work there. That talk with the neighbour upstairs or down should happen first. We cannot give legal advice on that point, but we can describe the work so the conversation is simple.
Gather these before the survey visit
- Photos of the roof from any upstairs window that overlooks it
- Any drawings or certificates from when the extension was built
- A note of past leaks, damp patches or ponding after rain
- Your title or lease if the property is a flat
- A rough idea of the room's ceiling height beneath the roof
Before your roof is opened
Will fitting a lantern disturb the rest of the roof?
Only around the opening and kerb. The covering is cut back locally and made good against the new upstand, while the rest of a sound roof remains in place. A survey tells us whether the wider covering is a reason to renew it.
What if the existing flat roof already leaks?
Fix the cause before a lantern is added, because a new kerb on a wet deck hides the problem. When the roof is opened for the lantern, the deck and joists are visible and any rot is dealt with as part of the quoted work.
Can a lantern replace an old flat rooflight?
Often, since an opening and a kerb already exist. The old kerb height and size may not suit a lantern, though, so it may be rebuilt. See our roof lantern page for how replacements are handled.
Must someone be in during the survey?
Someone needs to open the door and walk us through the room under the roof, because the ceiling height and the inside finish form part of the design. Access to the roof itself is arranged at the visit. The visit costs nothing and happens before pricing.