What the opening version looks like
Most are top-hung. The sash lifts out from the bottom edge and pivots on hinges at the top, like a small hatch. Stays hold it at the angle you set. Other layouts exist, but on a slope the top-hung type is the one you will meet.
The catch is where the moving parts live. A fixed unit is one sealed pane in a frame with nothing to move, so the maker can thin the metal as far as it will go. A sash needs space for hinges, seals and the stay arm, which makes the frame deeper and lifts the glass a touch higher above the tiles. It is still a dark, slim, quiet thing. It is just not quite as flat as the fixed one beside it.
Judge by the drawings. Ask the maker for a cross-section of the fixed and opening versions and compare the height over the covering. A few millimetres count on a front slope and hardly at all at the back. Our conservation rooflights page sets out the range.
If you are not yet sure the style suits your roof, read what makes a rooflight a conservation rooflight first.
When opening earns its keep
Air is the reason, nearly every time. A bedroom under the slope bakes through the afternoon, because hot air climbs and gathers under the rafters. A sealed window gives it no exit. A loft bathroom needs steam to leave. A stairwell that rises to the roof behaves like a flue, and an opening light at the top lets it draw.
One opening unit helps. Two, one low and one high on the slope, can set up a gentle flow without any fan, as cool air slips in beneath and warm air slides out above.
Summer sun is the other side of it. A south-facing slope fills a loft with heat as well as light, and a sash that swings wide lets the worst of it out in the evening. Choosing where the opening unit goes, and which way it faces, matters as much as choosing the unit.
Cleaning is a smaller gain. A sash that swings in reach can be wiped from inside, with no ladder on a slate roof.
If daylight is the only goal and the room has a window elsewhere or a working extractor, a fixed light is simpler. Nothing to hinge, nothing to drip, thinnest frame. For help sizing the air side, see our piece on opening rooflights for ventilation.
Stay, pole or motor
Rooflights are mounted high, which turns the opening method into a real decision.
A pole with a hook, or a hand stay, costs little and has nothing to wire. It works well when the unit is within reach.
A motor, driven from a wall switch or handset, makes sense over a staircase or high up a long slope. Some systems close on their own when a rain sensor trips, which is useful when the unit is left ajar and nobody is home. The cable has to be run through the roof build-up to a control point before the ceiling is finished, so it is planned at survey stage. Our pages on electric opening rooflights and their wiring and controls go through the choices.
The motor is another thing to house in the frame, so the depth penalty grows slightly. On a slope seen from the street, factor that in. At the back, pick whatever suits you.
Rules that come with a sash
Opening brings a few points that a fixed pane never raises.
First, escape from a bedroom. Approved Document B counts a window as an escape route only on storeys up to 4.5m from the ground. In a two-storey house, a loft bedroom has to be reached by a protected stair, so the rooflight is for air and light, not for climbing out. Building Control looks at that when the conversion is designed.
Second, planning. Most roof windows and rooflights on houses fall under permitted development, which keeps them close to the slope and below the highest point of the roof. A sash with a deeper frame still has to respect that. Listed buildings need consent, flats and maisonettes lack householder rights, and an Article 4 direction can remove them. Check the skylight planning guide before you commit.
Weather is a quieter rule of thumb. A sash left open in a storm lets rain onto the lining, and an open rooflight on a south slope in summer lets in sun as well as air. Closing it on the way out is a habit worth building, or a rain sensor does it for you.
Third, energy. Under Part L 2021, a rooflight in an existing home is held to 2.2 W/m²K, while a roof window is held to 1.4 or a Window Energy Rating of band B. A historic building may be treated more flexibly, if building control agrees.
Picking one, and what happens next
A quick way to decide. Does the room already get fresh air from somewhere else? Go fixed and enjoy the slim look. Is it a loft bedroom, a hot landing or a bathroom with no extractor? Go opening and live with the extra depth. Does the slope face the street inside a conservation area? Talk to the council about that slope, and think about putting the opening light on the back one.
On the survey we climb into the loft, check the rafters, note which way each slope faces and look for where the stale air is trapped. A fixed written quote then names the unit, the opener, any cable run and the finishing inside. The work carries a 10-year workmanship guarantee, and the Building Control notification is ours to make when the job needs one.
To begin, request a free survey, or message a photo of the roof on WhatsApp.