Do you install rooflights on thatched roofs?

The short answer

The honest answer is about the covering rather than the house. A rooflight goes into a thatched slope only where a thatcher forms the opening, because thatch is not a lapped waterproof surface that a manufactured frame can be flashed into, and no flashing kit is made for it. On a thatched property the unit almost always belongs somewhere else on the building: the flat roofed rear extension, the tiled outrigger, the slated wing added in the nineteenth century, the garage or the garden room. That is where Severn works on these houses, and it is usually where the dark room is anyway. What follows is why the thatch itself behaves the way it does, and how the rest of the building is assessed. The general installation sequence is on the skylight and rooflight installation page.

How a thatched roof actually keeps water out

Every other covering on a Gloucestershire roof works by overlap. Slate, clay tile, concrete tile and stone slate all shed water across an impervious surface, and each course laps the one below so water travels downhill over a series of joints. A rooflight is flashed into that system by interrupting it and re-forming the laps around a frame.

Thatch works on a completely different principle. Water reed, combed wheat reed or long straw is laid in a coat commonly 300mm deep or more, at a pitch of 45 degrees or steeper, and the water never crosses a surface at all. It runs down the outside of the stems, from one bundle to the next, and leaves the roof at the eaves having penetrated only the top few tens of millimetres. There is no underlay behind it in a traditional coat, and there is no lap to interrupt. There is only depth and angle.

Why a manufactured frame and a coat of thatch cannot meet

Four things follow from that, and each of them defeats a standard unit.

Depth. A roof window frame is designed to sit in a covering a fraction of the thickness of a thatch coat. Set into 300 or 400mm of reed it is buried, and the glass sees a deep trench of straw rather than the sky.
Movement. A new coat settles and compresses over its first years and continues to weather down over its life. A rigid frame fixed to the rafters stays exactly where it is while the material around it moves away from it.
No lap to work with. Soakers, aprons and head flashings all depend on interleaving with courses. There are no courses to interleave with.
The re-coating cycle. A coat is stripped or overlaid and the ridge renewed on a cycle measured in decades, and everything set into it has to come out and go back with the thatch.

Saying that a conventional rooflight is not achievable in a thatched slope is a statement about how the roof is built, and it holds regardless of who is doing the work.

Fire, which is the other half of the answer

Thatch is a combustible roof covering and the regulations treat it as one. Approved Document B sets out separation distances from a boundary for a thatched roof, and the guidance for new and renewed thatch covers the treatment beneath the coat, the separation from a chimney and the fire barrier laid over the rafters.

Anything set into the coat is a discontinuity in that arrangement, and it has to be designed rather than improvised. Insurers of thatched properties in this county take the same view, and the policy conditions on a thatched cottage are often more specific about alterations to the roof than the planning position is. That is a conversation to have with the insurer before anything is drawn.

Ventilation of the coat itself

A traditional thatched roof breathes. The coat dries from both faces, and the space beneath it is deliberately open rather than sealed, which is why thatched lofts feel draughty and are meant to. Moisture that reaches the underside evaporates away instead of sitting in the base of the stems.

An impermeable frame set into that arrangement interrupts the airflow and puts a cold surface where warm moist air can reach it, and the material that stays damp is straw. Rot in the fixings and the base of a coat is expensive and it shortens the life of the whole roof, not just the area around the opening. This is the argument that most often decides the question, ahead of the weathering one.

Glazing that does exist in thatch, and who forms it

Thatched buildings do have windows in their roofs, and they are almost always eyebrow dormers: a small casement in a framed structure over which the thatcher sweeps the coat in a continuous curve, with no straight junction anywhere. The waterproofing is entirely a matter of how the thatch is laid over and around the dormer, and it is thatcher’s work from start to finish.

A small number of specialist thatchers do set glazed units into a coat, using a purpose formed timber curb with a lead apron and the reed dressed up and around it. It is a bespoke detail, it is designed for that roof, and it comes out and goes back at each re-coat. Where a homeowner wants glass in the thatch itself, the master thatcher is the person who designs it and the person who warrants it.

Where the thatch is in Gloucestershire

There is more of it in this county than people expect, and it concentrates in particular places. The Severn Vale villages north and west of Gloucester hold most of it: Ashleworth, Hasfield, Corse, Staunton, Maisemore and Down Hatherley, along with cottages around the green at Frampton on Severn. There is a scatter through the Forest fringe near Newent and Dymock, and a handful in the Cotswold villages where thatch survives alongside stone slate.

Almost all of it is long straw or combed wheat reed rather than water reed, because that is what the local farming produced, and long straw in particular has a shorter cycle and a softer profile. It is also, almost without exception, on buildings old enough to be protected.

The parts of a thatched property that take a unit easily

This is where the practical answer usually lands, and it is a longer list than people assume. A thatched cottage that has been extended in the last century almost always has a section of tile, slate or flat roof somewhere, and it is frequently over the kitchen.

Flat roofed rear extensions, in felt, single ply or GRP, which take a made to size flush glazed unit on an insulated kerb.
Tiled or slated outriggers and rear wings, added in the nineteenth or twentieth century, where a conventional roof window and its flashing kit work normally.
Later side extensions with a shallow pitch, where a kerb mounted unit suits better than an in plane one.
Garages, annexes, garden rooms and converted outbuildings, which carry none of the thatch constraints at all.

Where the dark room is under thatch and the extension is where the light is not needed, the honest answer is that the roof decides, and that gets said at the survey rather than after an order.

Listed status, which nearly always applies

Most thatched buildings in Gloucestershire are listed, and listing protects the whole building rather than the thatch alone. A rooflight in a later tiled wing of a listed cottage still needs listed building consent, because consent follows the building and not the visible part of it, and the internal reveal counts as much as the external appearance.

The workable route is pre-application advice from the conservation officer with photographs of the roof and a clear note of what is proposed, before anybody orders anything. Officers in this county have seen a great many of these and will usually indicate early what would be acceptable, which is far less expensive than a refusal after a scaffold is standing. The position is set out on rooflights in listed buildings and in the guide to conservation areas and listed buildings in Gloucestershire.

What to send so the answer comes back quickly

Four things make a first reply useful on a property like this. A photograph of the whole building from the garden, so the thatched and non thatched sections are both visible. A photograph of the room that is dark. The address, because listing and conservation status are usually clear from it. And a note of what the covering is on the extension, if it is known.

From that it is normally possible to say which part of the building will take a unit, what type it would be, and whether consent is likely to be needed, before anybody drives out. Severn fits new rooflights and replaces units that have reached the end of their life across Gloucestershire and thirty miles by road around the city, and the coverage is listed on the areas pages.