How close to a party wall can a flat rooflight sit?

Near a party wall, the rooflight is answering to your roof, the wall and the neighbour, all at once. Each has its own reason for wanting distance.

The first is fire. A party wall between two homes is built to hold back a fire for a set time, and the roof has to respect that. The wall is either carried up to meet the underside of the roof covering with the gap fire-stopped, or it stands above the roof line. Current fire guidance allows the wall to stand 375 mm above the covering on some roofs. Whichever detail your house has, a rooflight must not cut into that line of separation or weaken it.

The second is the roof covering itself. Building Regulations Part B sets limits on rooflights by the material they are made of and how close they sit to a boundary. Plastic rooflights and domes are the ones with tighter limits on area and distance. Glass is treated more generously, which is one reason a flat glass unit is a sensible choice along a shared boundary. Your installer should tell you which classification the unit carries before any position is fixed.

The third is practical. Whatever sits beside a wall needs a margin, because the upstand has a thickness, the flashing has a turn-up and someone will want to clean the glass.

A party wall is often a step in height. The neighbour's roof may be taller than yours, and everything that falls on it ends up on your side.

Rain running down a higher pitched roof arrives in a sheet, hits your flat roof, and splashes. If the rooflight is tucked in tight, that splash lands on the glass and frame. You hear it, and the pane is dirty sooner than one in the middle of the roof. Leaves, moss and tile grit travel the same path and settle in the corner where your glass meets the wall.

In Crawley, the set-up arises in the terraces and semis of the New Town, where a 1950s to 1970s house has had a flat-roofed rear extension added against a neighbour's wall, or where both neighbours have extended side by side. A rooflight set well out from the shared wall gets less of the neighbour's run-off, less of the drumming, and a cleaner pane. Position is one of the simplest ways to keep a room quiet, and it costs nothing.

A laminated acoustic inner pane still does the main work. The quiet glazing for a flat rooflight page explains the build we specify when the noise source sits right beside the glass.

The junction between kerb and wall is where the work gets fussy. It is a small area with three layers that must each stay dry.

• The kerb stands clear of the wall. We leave a gap wide enough to get a hand in, so water can drain and the glass can be cleaned. A kerb jammed against brickwork traps damp and debris.
• The roof membrane turns up the wall. It is dressed up the brickwork and covered by a flashing, usually lead, chased into a mortar joint. That flashing sits at the wall, not at the rooflight.
• The upstand is insulated all round. The side nearest the wall is the coldest and most likely to sweat, so it gets the same insulation as the others. The height and thickness are set out on the upstands page.

Where there is a parapet, the roof already turns up against a wall of its own, and the rooflight sits inside that line. A parapet can change the fall of the roof, so we check where water is meant to go before fixing a kerb. The falls and drainage guide goes through how the roof drains.

The Party Wall etc. Act 1996 covers particular kinds of work to shared structures. A rooflight on your own flat roof is normally outside it, but the edge cases are at the wall.

Work that can fall inside the Act includes cutting into the party wall, raising it, and inserting a new flashing tray or damp-proof course along its length. In those cases you serve a written notice on the neighbour, and the usual notice period for work to a party structure is two months. The neighbour can consent, or a surveyor is appointed to settle the terms. The government publishes plain guidance on the Party Wall Act, and it is worth reading before the job is priced.

At the survey we look at how the wall and the roof meet, and we say whether the proposed rooflight needs any of that. It is also the point where we tell you which side of the wall the glass should sit. The decision goes into the written spec attached to your quote, together with the Building Control notification, which we handle where the work is notifiable. Where the roof is complicated, a survey quote is the place to start.

Can I have a rooflight right up against the wall?

Physically you can, but we would not recommend it. With no gap there is nowhere for water to drain, the flashing has no room to lap, and the glass cannot be cleaned. A margin at the wall is part of a good detail.

Does a neighbour have to agree before I fit one?

Not for work confined to your own roof. Consent is a question when the work touches the shared wall. Where it does, the notice goes to the owner of the neighbouring property, and we will tell you when that applies.

Does glass or plastic matter near a boundary?

It does. Part B has tighter limits on plastic rooflights near a boundary than on glass. A glass unit, such as the ones described in our fixed flat rooflight guide, gives you more freedom of position.

Will a rooflight near the wall be noisier?

It can be, mostly from the neighbour's roof shedding water onto the glass. Moving the unit away from the wall and choosing laminated acoustic glass reduces it. Our quiet spec records the position and the build for your room.

If your flat roof runs up against a shared wall, the rest of the subject is covered on the flat roof rooflight installation page.