Why thatch and roof windows do not suit each other
A roof window depends on a flashing that laps under and over a thin roof covering, and thatch has no thin covering to lap onto.
On a tiled roof, the covering is perhaps 15 to 30 millimetres deep. A roof window sits almost level with it, and the flashing kit is shaped so that the tiles above lap over its top collar while the apron at the bottom dresses down over the tiles below. Water runs across the surface, meets the frame, is diverted round the sides and carried away. Our guide to roof window flashings shows how each kit is shaped for tiles, slate and pantiles, and none of them is shaped for thatch.
Thatch works in a completely different way. A coat of water reed or long straw is often 300 millimetres or more thick, laid in courses and fixed to the rafters and battens with spars, sways or screws. Rain does not run over a smooth skin; it runs down the outer stems and drips from stem to stem, soaking only the top few centimetres before it drains off at the eaves. The coat stays dry inside because the pitch is steep and the stems are continuous from ridge to eaves.
Put a rigid frame into that and three things go wrong. The stems above the window are cut short, so water that should run on down the roof is stopped at the head of the frame and concentrated there. The sides of the opening expose cut stem ends, which take up water rather than shedding it. And the window itself sits at the bottom of a deep, sloping pit in the thatch, where leaves, moss and damp collect. The pane is shaded by the thickness of the coat around it, so even the light you gain is less than the size of the window suggests.
Planning, listing and insurance come first
Before anyone thinks about flashings, a thatched house in the Cambridge area is quite likely to be listed or in a conservation area, and that changes what is allowed.
Thatched cottages survive in villages across South Cambridgeshire, Grantchester among them, and a number of them are listed buildings. Any listed building needs listed building consent for a new roof window, whatever its size, and the conservation officer will weigh how an opening would change a roof whose whole character is its unbroken, soft line. Permitted development for rooflights still applies in a conservation area unless an Article 4 direction removes it, but it never applies to listed buildings, and a window that projects from the thatch surface is hard to square with the rules anyway.
Insurance is the second check. Thatched homes are insured by specialist policies, and insurers take a close interest in any change to the roof. Ask your insurer before the design gets far, because their view may settle the question.
Building Control is the third. A new opening in a roof is notifiable work, as our answer on whether roof windows need building regulations explains, and on a thatched roof the inspector will also look at fire separation and how the lining under the thatch is treated.
Better ways to bring light under thatch
The traditional answer to a dark room under thatch is a small window that the thatch flows around, and several modern routes sit alongside it.
The options below are compared by what they give the room and who normally builds them. The right one depends on the house, the consent and where the light is needed.
| Option | Light it gives | Who builds it |
|---|---|---|
| Eyebrow dormer | A vertical window, a view out, soft daylight | Thatcher and carpenter |
| Gable window | Light into the end of a loft room | Builder, with consent |
| Roof window in a tiled section | Direct sky light into the room below | Roof window installer |
| Rooflight in a flat-roofed extension | Overhead light into the ground floor | Rooflight installer |
An eyebrow dormer is the form a thatcher knows best. The thatch is swept up and over a small vertical window in a smooth curve, so the stems stay continuous and water runs round the window instead of into it. It gives less sky than a roof window of the same size, but a vertical pane also means a view out, less summer heat and no rain on the glass. Our comparison of whether roof windows give more light than a dormer sets out that trade in more detail.
Where the loft room runs to a gable end, a window in the gable wall is often the simplest way to light it. It needs the same listed building or planning checks, but it leaves the thatch alone.
Planning the light when the slope is off limits
If the thatched slope cannot take glass, the question becomes which other surface can, and how far the light will reach from it.
A loft room under thatch tends to be long and low, with deep walls where the coat meets the eaves. A gable window lights the end of the room strongly and the far end weakly. A small eyebrow on the south slope brings a patch of sunlight in for a few hours in the middle of the day; on a north slope it gives steady, cooler light. A roof window in an adjoining tiled roof can light a landing or stair that then borrows light into the rooms off it, which is the idea behind our answer on whether a roof window can light a stairwell landing.
That is the kind of reasoning a light survey is for. We look at where the room is used, which surfaces can take glass, how each one faces, and what glass and shading each needs. The written light plan then says what each opening will do, so you and the conservation officer can judge it on paper. Our page on how we plan the light shows the method.
If the house has both thatch and tiles
Older village houses have often been extended over the centuries, and the later roofs are usually where a roof window belongs.
A tiled catslide, a slated back range or a pantiled outshot can take a roof window in the normal way, subject to the same consents. The pitch needs checking against the manufacturer's minimum, which our answer on the minimum roof pitch for a roof window covers, and the flashing is matched to the covering. Where a thatched roof meets a tiled one, the window should sit well clear of the junction, so the thatcher's work at the abutment is left undisturbed.
For a full picture of sizes, opening types and glass, see our main page on roof window installation in Cambridge, or read about the types of roof window available. For budgeting across the different routes, our skylight and rooflight costs page gives general guide ranges.
Questions people also ask
Can a sun tunnel go through a thatched roof?
It meets the same problem as a roof window: a rigid tube and flashing interrupting a deep, water-shedding coat. Where a tiled or flat roof is nearby, the tunnel can often be taken through that instead and run across the loft to the room that needs it.
Are there roof windows made specially for thatch?
Standard roof window ranges and their flashing kits are designed for tiles, slate and similar thin coverings. Any proposal for glazing within thatch should be discussed with an experienced thatcher and the conservation officer before a product is chosen.
Will re-thatching affect an existing window in the roof?
Yes. Each time the coat is renewed, the thatcher has to work around the opening again, so any window in thatch has to be designed with future re-thatching in mind. An eyebrow dormer is built for exactly that.
Does a listed thatched cottage always need consent for a new window?
Yes. Listed building consent is needed for any alteration that affects the character of a listed building, and a new opening in the roof does. Speak to the local planning authority before you commit to a design.
