Can a roof lantern be fitted to an existing extension?

The short answer

Yes, and it is the most common lantern job in this county by a distance. A lantern goes into an existing flat roof the same way it goes into a new one: joists trimmed, kerb built and weathered, frame set, glass in, ceiling lined and plastered. What decides the job is never whether it can be done but what the existing roof turns out to be once a small area is opened up. Deck condition, joist depth and centres, the falls, whether there is any usable insulation, where the outlet sits, and how the covering was detailed thirty years ago all decide whether the lantern is a three day job or a re-roof with a lantern in it. The sequencing behind all of it is on the roof lantern installation page.

Six things about the existing roof that decide the answer

Every survey on an existing extension answers the same list. Joist size, depth and centres, which set what can be trimmed and whether a calculation is needed. Deck material and its moisture content, because a 1980s chipboard deck that has been damp is not a substrate for anything. The designed fall and the actual fall, which are different numbers once thirty years of deflection are in. The insulation, its position in the build-up and its thickness. The position of the rainwater outlet and the abutment against the house. And the age and type of the waterproofing, because tying new work into an old covering has its own rules.

Nothing on that list can be established from inside the room, which is why a lantern on an existing extension is not priced from photographs alone.

Opening up a small area rather than guessing from below

A ceiling tells you very little. A 300mm square lifted in the covering near the proposed kerb line tells you everything: the layers in order, whether there is insulation and where it sits, the deck material and its condition, the joist depth and direction, and whether the deck is dry. It is made good the same day.

The alternative is pricing on assumption and discovering the truth with the scaffold up and the covering off, which is where fixed prices come apart. Anything found in that opening goes into the survey notes in writing with its cost consequence, so the figure that follows holds. Where the roof turns out to be sound, that is written down too.

Deck condition, joist depth and what a moisture meter finds

Flat roofs built across the back of Gloucester and Tewkesbury terraces in the 1970s and 1980s share a specification: 100 by 50 joists at 400 centres, a chipboard or softwood boarded deck, three layers of bitumen felt, and either no insulation or 25mm of something between the joists with no vapour control layer under it. Ventilation to the void was frequently a token pair of vents at one end.

The consequence is interstitial condensation over decades: the deck goes soft near the abutment and at the low corners, joist ends bearing into the wall darken, and a moisture meter reads high across a strip you cannot see from below. Adding a lantern to a deck in that state is putting good work on top of a problem. Where that is what the survey finds, the sensible sequence is a new warm deck across the whole extension with the lantern kerb formed as part of it, which costs less as one operation than as two.

Adding insulation while the roof is already open

An existing cold deck, insulation between the joists with a ventilated void above, is the build-up that fails. The upgrade is a warm deck: the deck boarded, a vapour control layer over it, rigid PIR above that, and the waterproofing on top. Typical thicknesses run from 100 to 150mm of PIR depending on the system, and the whole roof surface rises by that amount.

That rise is not a detail. It changes the height of the extension for planning purposes, it changes where any door threshold or window cill sits relative to the roof, and it changes the kerb height, because the 150mm upstand is measured from the new finished surface rather than the old one. All three get checked before the build-up is agreed rather than after the insulation is on site.

Trimming the existing joists, and the ceiling below them

Joists inside the lantern footprint are cut, doubled trimmers go across the head and foot of the opening, and the joists either side are doubled up as trimming joists. Over a single bay that is routine carpentry. Over a 1200mm or wider opening the trimming joists carry considerably more than they were sized for, and the answer is a deeper trimmer, a flitch plate or a small steel, with the calculation going in with the Building Control application. The threshold at which that becomes structural is set out on do roof lanterns need a steel beam.

The awkward version is a Gloucester terrace where the kitchen and the rear addition were knocked into one and the roof now bears on a steel of unknown size with no drawings. A lantern kerb landing on or beside that beam changes its loading, so it gets exposed and checked rather than assumed.

The outlet, the abutment and everything already coming through the deck

A lantern cannot sit wherever the room would like it. The kerb has to leave the falls to the existing outlet intact, so it cannot stand across the drainage path or crowd the outlet itself. It has to clear the abutment against the house wall by enough flat roof to form a proper upstand and flashing, usually 150mm at the very least and comfortably more in practice.

Then everything already penetrating the deck: soil vent pipe, boiler flue, extract terminal from a kitchen hood, sometimes an old aerial mast base. Each is measured onto the survey drawing before a position is agreed, because moving a soil vent pipe is a plumbing job with its own consequences and moving a flue is a gas job.

What the existing roof shows, against what the work then includes

What the opened area shows	What it means	What the job then includes
Sound deck, insulation above it, membrane under ten years old	The roof is doing its job	Trim, kerb, weather into the existing covering, lantern. The short version
Sound deck, insulation between joists, no vapour layer	Cold deck, working for now, will not improve	Usually a warm deck overlay across the whole roof while the scaffold is up
Damp chipboard deck, soft near the abutment	Long term condensation or a failed detail at the wall	Deck renewed, joist ends checked, falls re-formed, new covering with the kerb built in
Ponding water sitting 20mm deep in one corner	Insufficient or reversed fall after deflection	Falls corrected with tapered insulation or firrings before any kerb goes on
Asbestos cement sheet on an older lean-to	Not a substrate for a lantern at all	Licensed removal first, then a new insulated deck, then the lantern
Joists at 600 centres, 150mm deep, sound	Good news for a wide opening	Trimming across two bays, often without a steel

Tying a new kerb into a covering that is already twenty years old

New waterproofing has to bond to old, and the systems are fussy about it. EPDM will not weld; it bonds with contact adhesive and a proprietary cover strip, and the existing sheet has to be cleaned back to sound rubber. Single ply welds to itself but only to the same polymer family, and a PVC membrane will not weld to TPO. Built up felt takes a new cap sheet lapped a generous distance and torched, provided the layers underneath are sound. Liquid systems need the old surface primed and the reinforcement taken continuously round the base of the kerb with no joint at the change of plane.

Where the existing covering is near the end of its life, tying into it is buying a junction that will need attention before the lantern does, and that is said at survey rather than discovered later.

A conservatory roof swapped for a lantern, and why that is a different job

Plenty of enquiries are really about a conservatory: glass or polycarbonate on lightweight bars, running down to a low eaves on three sides, no insulated deck anywhere. Replacing that with a lantern means building a flat roof first, and a flat roof needs a structure to sit on. Conservatory dwarf walls and window frames were not designed to carry a warm deck plus a glazed lantern plus snow load, so the perimeter has to be assessed properly and frequently rebuilt.

Done well, the result is an insulated room with a lantern over it that is usable in February, which is exactly what the conservatory was not. Done as an overlay on an existing conservatory frame, it is a structure carrying a load nobody calculated.

Gloucester rear additions, Quedgeley garage roofs and Cotswold garden rooms

Three patterns account for most of this work locally. The Severn Vale rear extension, red brick, flat felt roof, dark kitchen underneath, where the deck is the question and flood history makes homeowners rightly sceptical of anyone cheerful about cutting into a roof. The modern estates at Quedgeley, Abbeydale, Bishops Cleeve and Ashchurch, where a garage or a utility room with a shallow flat roof became part of the kitchen and the roof is newer and generally sound. And the wolds villages, where a stone garden room or an orangery behind a coped parapet takes a lantern easily, provided the parapet gutter keeps its full width and its outlet.

To get an answer for your own roof, email info@severnskylightsgloucester.co.uk with a photograph of the flat roof looking down from an upstairs window, a photograph of the room, the rough dimensions and the address. Coverage is on the areas pages.