Why there is a fire zone over a party wall
A party wall between two homes is built to stop a fire in one house reaching the other, and the 1500mm band on the roof exists so the fire cannot simply step over the top of it.
Approved Document B, the Building Regulations guidance on fire safety for dwellings, treats the wall between two houses as a separating wall. That wall has to run right up to the underside of the roof covering and be fire stopped at the junction. Stopping the wall there is only half the job, though. If the roof surface either side could burn, or could let flames and hot gases through, a fire in one loft or extension would travel across the top and drop into next door.
So the guidance also controls the roof itself. For 1500mm either side of the wall, the covering should achieve the BROOF(t4) external fire rating, the highest classification for a roof surface, and it should sit on a deck made of material of limited combustibility. A skylight in that band is a hole in the very surface the rule is trying to protect. Glass can crack in heat, frames and seals can fail, and the opening puts the fire straight into the neighbour's roof void. That is why the practical advice from rooflight makers and Building Control alike is simple: keep the rooflight out of the zone.
How the 1500mm is measured
Measure on plan from the face of the party wall to the nearest outside edge of the rooflight's upstand, not to the edge of the visible glass.
The glass on a flat rooflight sits inside a frame, and the frame sits on a kerb that is wider again. On a typical unit the outside of the upstand can be 100mm or more beyond the glass on every side, so a rooflight that looks clear of the zone on a sketch can have its kerb inside it. We set the structural opening from the upstand dimensions for that reason. The upstand guide explains how the kerb is built and why it is wider than the pane.
| Distance from party wall | What can go there |
|---|---|
| 0 to 1500mm | BROOF(t4) covering on a limited combustibility deck. No thermoplastic rooflight; other glazing only if Building Control accepts it |
| Over 1500mm | A normal flat rooflight, sized and placed for the room |
A wall shared between two buildings is what triggers the band. A garden wall standing on its own is a boundary, not a separating wall, and falls under separate Part B guidance on distance to the boundary instead. If you are unsure which your wall is, the survey settles it before anything is drawn.
What the zone does on a Cambridge terrace
On a narrow house the two 1500mm bands can take up much of the roof, so the size and shape of the rooflight follow from the width that is left.
A terraced house in Romsey or off Mill Road can be under 4.5m wide between its party walls. Put a full-width rear extension on it, with a party wall on both sides, and 3m of the roof width is already spoken for. That leaves roughly 1.5m of usable width down the middle, minus the kerb, which suits a long, narrow unit or a run of smaller ones along the centre line. The guide to multiple rooflights covers spacing when a single unit cannot be wide enough.
A side-return infill is tighter still. Where the new roof meets a wall shared with the neighbour's own extension, only the part of the roof more than 1500mm from that wall is available. On a 2m side return that can mean a slim strip against the house wall, which is often the right place for the light anyway. Our page on skylights for side-return extensions works through those narrow layouts. A long strip rooflight can still run along the rear wall or the house wall; the linear rooflight guide shows how, as long as the strip itself stays out of the party wall band.
Where the light lands once the rooflight moves off the wall
Pulling a rooflight 1500mm away from the party wall moves the patch of sunlight towards the middle of the room, which can suit the room better than you expect.
A skylight tight to a wall washes that wall with light and makes a narrow room feel wider. Held back from the party wall, the rooflight loses some of that wash on the neighbour's side but lights the floor, the table or the worktop more directly. In a Cambridge summer the sun is high, around 61 degrees at midday in June, so the sun patch falls almost straight down under the glass. In winter the sun is only about 14 degrees up at noon, so the light arrives at a low angle and lands well to the north of the opening, sometimes on a wall. The guide to distance from the wall and positioning a flat roof skylight go further into that choice.
This is part of every light plan we write. The fire zone is fixed, so we plan the rest of the room around it rather than trying to squeeze glass into it.
The Party Wall Act is a separate question
The Party Wall etc. Act 1996 deals with work to the wall itself and your neighbour's rights, not with fire spread, so a rooflight that respects the 1500mm band may still need a notice if the job touches the wall.
Setting a rooflight into your own flat roof, well clear of the wall, normally involves no work to the party wall at all. It is different if the extension roof bears on the party wall, if flashings are cut into it or if a parapet is raised. That kind of work can call for a party wall notice to the neighbour, and you may want advice from a party wall surveyor. If your roof sits behind a parapet on the shared wall, the page on rooflights on a parapet roof covers the drainage and detailing side. Planning permission is another separate matter again, set out in does a flat rooflight need planning permission.
Questions people also ask
Can a fire-rated rooflight go inside the 1500mm zone?
Some rooflights are tested for fire, but the guidance only rules out thermoplastic units outright; anything else in the zone would need Building Control to accept that the roof still meets Part B. That is a case-by-case decision, usually needing test evidence, so we plan the rooflight outside the band from the start and handle the Building Control notification for the work.
Does the same rule apply to a pitched roof window near a party wall?
Yes. The band applies to the roof covering either side of a separating wall whatever the pitch, so a roof window on a terrace slope or a semi-detached loft should also sit at least 1500mm from the wall.
Does it apply to a semi-detached house?
It does, on the side that shares a wall with the other half. On the detached side, the distance to the boundary guidance applies instead.
Who checks the distance?
Building Control, as part of the notification for the work. We mark the zone on the survey drawing so the position is agreed before the roof is opened. To see how it works out on your own roof, book a light survey or read more on flat roof rooflight installation in Cambridge.
