Is a Roof Window Possible in a Thatched Cottage?

A thatched roof is not a difficult case that rules out roof window installation, only a different one. A thatched roof is not simply a tiled roof with a deeper covering laid on top. Reed or long straw thatch can be 300mm thick or more, laid over battens or sarking on rafters that were often cut and pitched long before anyone thought about putting glass through them. The pitch itself tends to be steep, frequently 45 degrees or beyond, which is part of what sheds water off a soft covering so quickly.

That thickness changes where the glass can sit. On a plain tile or slate roof the frame sits close to the rafters, and the covering butts up against a flashing kit only a few centimetres deep. On thatch, the frame has to clear a much deeper build-up, and the thatcher working alongside the window installer needs the reed or straw cut back cleanly rather than simply tucked around the frame.

Because there is no rigid tile or slate edge for a flashing kit to key against, thatch calls for its own detail rather than an off-the-shelf kit.

The usual method raises the window on a timber upstand kerb built into the roof structure, so the frame sits proud of the rafters and clear of the thatch base. The thatch is dressed back from the kerb, and lead flashing wraps the upstand much as it wraps a chimney stack lower down the same roof, with a small lead tray above the frame to divert water round the opening rather than let it run into the reed. Getting this flashing right matters more on thatch than on any other covering, and it is one reason a thatched install takes longer to plan than a straightforward tile job.

• How thick the thatch is at the point the window needs to go, and how far the kerb has to be raised to clear it
• Whether the roof is listed or sits inside a conservation area, and what consent that adds alongside Building Regulations
• Where the rafters actually run, since older thatched roofs are not always laid to a tidy modern spacing
• How the frame will be dressed with lead, and how the thatch is kept back from the glass once the job is finished

A window through thatch has to earn its place twice over: once for the view, and once for safety, because thatch and any opening cut through it are not a combination to leave to chance.

Many thatch specifications call for a fire-resistant layer under the reed close to any opening, in the same way a chimney is treated, and for the thatch to be trimmed back a set distance from the frame rather than laid tight against it. We work to whatever the thatcher's own specification sets for that roof, and to current Building Regulations for the window itself. Where the work is notifiable, we handle that Building Regulations notification, so the paperwork does not sit with the homeowner.

Most thatched cottages are either listed or sit inside a conservation area, so cutting a new opening through the roof often needs listed building consent from the local planning authority as well as meeting Building Regulations. The two are separate processes that run alongside one another.

A rule that suits one thatched cottage does not always suit its neighbour, even on the same street, because listing and conservation status are set address by address. Checking the position for that specific roof before any date is booked saves a job being started and then paused.

A kerb raised through thick thatch produces a deeper reveal than a roof window in a slated or tiled roof, and a deep reveal left square narrows the sky to a letterbox seen from the bed or the landing below it.

Splaying the lining, wider at the room than at the glass, keeps that reveal from swallowing the view, and setting the glass at the right height for the sightline matters as much on a deep thatched reveal as anywhere. This is exactly the kind of detail a sightline sketch settles before the roof is opened rather than after, so the depth the kerb adds is planned for rather than discovered.

The size of window that will fit still comes down to where the rafters sit, and on an older thatched roof those centres are checked on site rather than assumed from a modern spacing table. Because thatch breathes differently to a felted tile roof, the collar around the frame also needs proper thought; see how a vapour barrier collar works for what that detail does and why it matters more where the covering either side of the frame is not a manufactured membrane.

A recessed installation, which lowers a roof window into the roof for a slimmer look from outside, is the opposite of what thatch generally needs: its depth is exactly why the frame is raised rather than sunk. Where a thatched cottage's street-facing slope is involved, a job can also need to work through the planning position for a front-facing roof window, on top of the listed building point above. Weather matters too: a thatched roof stripped back for a new opening sits more exposed than an equivalent patch of tile, so the work is timed and covered accordingly, and it typically takes a little longer than the usual roof window timeline once the extra flashing and lining are counted in.

Anyone weighing this up for a cottage roof can compare a general covering against thatch using the flashing detail for slate, tile and plain tile roofs, and every quote against a thatched roof, like any other, comes with a sightline sketch showing the view the finished reveal will actually give.

Does the thatch need to be replaced to fit a roof window?

Only the section immediately around the new opening. The thatcher strips and re-dresses the reed or straw local to the kerb and flashing, rather than re-thatching the whole roof, unless the roof is due for that work in any case.

Can an existing thatched roof take a roof window, or only a new one?

An existing thatched roof can usually take a new opening, provided the rafters can carry the trimming a wider frame needs and the roof's listed or conservation status has been checked for that address. The same kerb and flashing method applies whether the thatch was laid last year or decades ago.

Is there a special roof window made for thatched roofs?

The window itself is a standard pitched roof window; what changes is the kerb, flashing and lining built around it. The choice between opening types is the same one covered in centre-pivot or top-hung roof windows, unaffected by the roof covering.

Does a roof window in thatch cost more than one in tile?

The window and glass cost the same. What adds cost is the extra lead flashing, the raised kerb and the thatcher's own time dressing the reed, which is covered generally on what affects the cost of a rooflight.