Fixed or opening flat rooflights

A fixed rooflight is a sealed glass unit in a frame on an upstand. It does not move, so there is no hinge, no seal that has to close and no mechanism to go wrong. That makes it the simplest, most weathertight option, and usually the lowest in profile, which matters when you are close to the permitted development limit on projection.

The glazing carries the whole performance. Because nothing opens, the manufacturer can build the frame slim and set the glass as large as the roof allows, and the thermal performance of a fixed unit is generally better than an opening one of the same size, since a frame that moves needs more material around it. Where the glass is large, that slim frame also means more of the opening is glass.

Fixed units suit rooms that are already ventilated some other way: a hallway or landing with doors and windows nearby, a utility room with an extractor, a dining area in an extension with large sliding doors that open fully. In a stairwell with a flat roof above, a fixed rooflight over the landing gives the house daylight in the middle without any user having to think about it.

What a fixed unit cannot do is let heat out. A south-facing flat roof in a glazed extension can collect heat all afternoon, and with no way to open the top of the room, the warm air sits there. Glazing choices help, and we cover them in what glass goes in a flat rooflight, but glass alone does not move air.

An opening flat rooflight is a hinged sash set in a kerb, raised by a mechanism, and closed again onto a seal. There are several ways of working it, and the differences are worth knowing before you choose.

Manual, by pole or crank

A hand-operated stay or a rod lets you lift the sash by hand. It is the simplest opening unit and needs no power. It suits a rooflight within reach, or one with a fixing for a pole, and it suits a room where you only want the window open now and then.

Electric, mains or low voltage

A motor, usually a chain or spindle actuator, lifts the sash at the touch of a wall switch or remote. It is the practical choice for a rooflight above reach, a larger sash or one in a lightwell. A power feed is needed, often a low-voltage transformer, so a cable has to run to the rooflight, and that cable is one of the things the finish schedule records.

Electric with a rain sensor

A sensor on the roof closes the sash when it detects rain. It is worth having where a rooflight is likely to be left open and nobody is home, such as over a kitchen or a bathroom.

Smart or remote-controlled

Some electric units connect to a home control system or a handset. They can be set to open and close on a schedule or in response to a thermostat. The more control you add, the more there is to plan at the wiring stage.

Walk-on or access hatch

A larger opening that lets a person out onto a flat roof, usually used for roof access or a terrace. It is a different product from a ventilating rooflight and is covered under walk-on rooflights, not here.

We cover the electric options in more depth on electric opening rooflights.

Warm air rises, and a rooflight is as high as an opening can get. Open it, and warm air leaves from the top of the room while cooler air is drawn in through windows or doors lower down. That upward flow, called the stack effect, can clear a hot kitchen or a bathroom full of steam faster than a window in the wall can.

Approved Document F covers ventilation in dwellings, and an opening rooflight can contribute to the purge ventilation a room needs, meaning the rapid ventilation used to clear pollutants or steam. It is not a substitute for background ventilation or for an extractor where the room needs one, and the rules depend on the room and the work, so we check them at survey instead of assuming.

A few cautions apply. An opening rooflight cools best when there is somewhere for fresh air to come from, so a closed room with a single rooflight over it does less than you might hope. Rain can come in if the sash is left open without a sensor. And an open sash is a draught path in winter, so the seals around it have to be good when it is shut.

In a loft room or a flat-roofed dormer, an opening roof window may also be part of the escape route, which brings Approved Document B into play and has its own size and position rules. A flat rooflight is a different product from a roof window, and that question is better taken up with us at survey than assumed from a catalogue.

An opening sash needs room to swing, and that changes the upstand. The kerb has to be tall enough, and stiff enough, to carry the sash and the mechanism and keep the seal true when it closes. On a fixed unit a kerb that is a few millimetres out will be hidden by the frame. On an opening sash, the same error stops the seal closing evenly.

The upstand height is one of the first things we settle at survey, and our guide to flat rooflight upstand heights and falls sets out how it follows from the roof build. An opening unit tends to need a taller or deeper kerb than a fixed one of the same width, because the hinge and actuator sit within it and the sash rises above it when open.

There is a planning angle too. The projection limit under permitted development applies to the highest part of the rooflight, so a tall kerb plus an opening unit on a low roof is worth checking with the local planning authority before you commit. We note the position on the quote.

Think about use as well as specification. A rooflight you cannot reach, or that needs a pole you never find, tends to stay shut, and then it behaves like a fixed unit that cost more. Be honest about whether you will open it, and whether you would rather it opened itself on a hot afternoon. That answer often points straight to manual, electric or fixed.

A mixed solution is common in a larger room. One fixed rooflight carries most of the daylight, and a smaller opening one sits near the heat source, such as the hob or the sunniest wall, to clear it. Two units cost more than one, but each can be sized for its own job.

You see the inside of a rooflight every day, and an opening one adds things to look at. A fixed unit is only a frame and a lining. An opening one adds a handle or a switch, a visible cable run, and sometimes a motor housing at the side of the frame.

Those are finishing decisions, and they are best made before the plaster goes on. Cable for an electric unit is run through the lining or down a wall before the board covers it, and the switch is put where you will use it. A rain sensor lead is routed so it does not appear on the ceiling. A manual unit with a pole needs a fixing point and a place to hang the pole that does not spoil the wall.

The lining itself is formed square and true so the frame sits straight, with the inside face of the kerb boarded and finished like the rest. Where a lightwell runs down through a flat ceiling, the sides are splayed to spread the daylight, and the splay is planned around the actuator. We go into the options on how a rooflight is finished inside, and the finish schedule written for your room records where each cable and control goes.

Whether a unit opens is separate from what its glass does, but the two work together. A south or west-facing rooflight in a flat roof gets strong sun, and a solar control coating cuts the heat that comes through. An opening sash lets you shed the heat that is already inside, so a hot room often wants both.

Overhead glass carries extra considerations. A laminated inner pane holds together if it breaks, and is good practice over a room where people sit or work. It is guidance, not a Part K requirement for a rooflight. Triple glazing and warm-edge spacers improve the U-value, and the unit still has to meet Part L when it is replaced or newly fitted. Blinds, whether inside or outside, give you control on the few days of the year that need it.

The glass choice is one reason we ask what the room is used for before we recommend a unit. A kitchen extension with a lantern, a loft landing and a bathroom sun tunnel all call for different answers, and our page on choosing between a flat rooflight and a roof lantern covers the shape question that comes before this one.

Putting a new rooflight into a flat roof is normally building work needing Building Control approval, since the opening alters the roof structure and the window has to satisfy Part L. Taking out a complete rooflight, frame and glass, and fitting another is notifiable too, whereas a new sealed glass unit set into the frame you already have is not. The notification itself is something we deal with for you.

Planning is usually permitted development on a house, subject to the 150mm projection limit, the height of the roof and the usual limits for flats, conservation areas and listed buildings. Fixed and opening units are treated alike for planning, but the extra height of an opening kerb can matter if you are close to the line.

On the day, the order of work is the same for both: trim the roof structure, form the kerb, fix the unit, dress the waterproofing up the kerb, then finish inside. An opening unit adds a wiring visit before the lining goes on, plus a test of the mechanism and a run of the rain sensor before we leave. Dust is managed throughout and the room is left clean.

If you are still undecided, these questions usually settle it.

• Does the room already have a way for hot air or steam to leave at a high level?
• Can you reach the rooflight, or will it need to be electric?
• Will it be left open when nobody is home?
• Where would a cable and a switch go without spoiling the wall?
• Is the roof close to the 150mm projection line?

Your answers shape the quote, which names the unit, the kerb and the controls, and the finish schedule that comes with it records the lining, the finish and where each fitting sits. The wider job is described in our guide to flat roof rooflight installation in Exeter. When you are ready, ask for a quote and we will survey the roof and the room together.