Fixed and opening flat rooflights compared

A fixed flat rooflight is a sealed pane of glass in a frame, bonded down onto a kerb or upstand and never intended to move. It is the simpler object by a wide margin. There is no hinge, no motor, no actuator, no chain, no gasket that has to compress and release thousands of times. You get light and you get a sealed weather barrier, and that is the whole specification.

An opening flat rooflight has a hinged or lifting sash that breaks the seal on demand. The glass unit sits in an openable frame, and either a person or a motor pushes it away from the kerb to leave a gap around the perimeter or along one edge. When it is shut it should perform almost like a fixed unit. When it is open it lets air move through the ceiling.

Everything else follows from that one difference. Fixed units are cheaper to buy, quicker to fit, and have nothing that can wear out. Opening units cost more, take longer to commission, carry a mechanism that will need attention over the years, and give you something a fixed pane cannot: a way to shift the hot air that collects at the top of a room. The comparison is really a question about whether that ventilation is worth the extra money and the extra maintenance for your particular roof and your particular room.

Both types sit on a kerb, the raised timber or insulated builder’s upstand that lifts the glass above the flat roof surface so water drains away from the aperture rather than pooling against it. The kerb is where most flat rooflight trouble starts, on either type, because it is the joint between the roofing membrane and the frame. On a flat roof the standing water problem is real: a truly level roof holds puddles, so flat roofs are laid to a shallow fall, usually between one in forty and one in eighty, and the rooflight has to shed water off its glass and away down that fall.

A fixed unit gives you fewer places for that to go wrong. The glass is bonded to the frame in the factory and the frame is sealed to the kerb once, on site. There is one static line of defence and it does not move again. A well-fitted fixed flat rooflight is about as watertight as a domestic roof detail gets.

An opening unit has to be watertight in two states. Shut, it relies on a compression gasket, a soft profile that the sash squeezes against the frame to close the gap. That gasket has to seal against wind-driven rain coming up the fall of the roof, which is a harder task than a vertical window faces, because gravity is not helping the water run off. Good opening rooflights manage this with a stepped frame, a drainage channel that catches anything the gasket lets past, and weep points that send it back outside. The mechanism matters here too: an actuator that pulls the sash down firmly and evenly seals better than a hinge a person has to lean on. This is one reason electric opening units often weather better than manual ones over time, since the motor closes them to the same pressure every time rather than however hard the last person happened to push.

The case for an opening flat rooflight is not the same as the case for an opening wall window, and it is worth being clear about why, because it is where the money is justified or wasted.

Warm air rises and then sits. In a room with a flat rooflight, the hottest air in the space collects directly under the glass, at the highest point, which is exactly where an opening unit puts its gap. Open it, and that trapped layer escapes upwards and out, and cooler air is drawn in low from a door or a window elsewhere in the room. This is stack ventilation, and a high-level opening drives it far harder than a window at head height ever can, because the temperature difference between the hot ceiling layer and the outside air is doing the work. A single opening rooflight can clear a stuffy kitchen faster than throwing both patio doors wide, and it does it without letting the dog out or the noise in.

A fixed unit gives you none of that. Whatever heat comes through the glass, and whatever heat the cooking and the bodies in the room add, stays until it conducts away slowly through the fabric or someone opens a door. On a flat roof this matters more than on a pitched one, because a flat rooflight faces straight up. It sees the whole sky and takes the summer sun almost square-on in the middle of the day, so it collects more solar energy per square metre than any other orientation of glazing on the house. The heat arrives at the ceiling, which is where it wants to stay, and a fixed pane has no way to let it back out.

That is the real reason ventilation and glass specification are two halves of the same decision. The glass controls how much solar energy gets in, and on a south-facing flat roof that argues for solar-control glazing with a low G-value regardless of whether the unit opens. The opening controls how fast the heat that does get in, plus everything the room generates, can get back out. Get the glass right and a fixed unit may be perfectly comfortable. Get it wrong on a south-facing flat roof, and no amount of opening will keep pace with the gain on a July afternoon. On most rear extensions here the honest answer is both: solar-control glass to hold the peak down, and at least one opening unit to clear what builds up.

Opening flat rooflights are not one thing. The mechanism decides how usable the ventilation actually is, and an opening unit you cannot reach is barely better than a fixed one.

Manual, pole-operated

The cheapest opening option. A winder or a push pole lets you crack the sash open by hand. It works on lower ceilings where you can reach a wall winder, or where you genuinely will fetch a pole. The catch is honesty about height. On a knocked-through extension with a two-and-a-half or three metre ceiling, a manual rooflight tends to get opened in April and shut in October and touched at no other time, which wastes the ventilation you paid for.

Electric, switch or remote

A concealed chain or spindle actuator opens the sash at the push of a button. This is the sensible choice for any ceiling you cannot comfortably reach, which is most flat-roof extensions. It closes to a consistent pressure every time, which helps the weather seal, and it can be wired to a wall switch, a remote handset or a home system. Electric opening units are what we tend to specify on high ceilings for exactly this reason: an opening rooflight only earns its keep if it gets used, and a switch by the door gets used.

Rain and wind sensors

Most electric units can take a rain sensor that shuts the rooflight automatically when it detects the first drops, and a wind or temperature input that can open it on a timer or close it in a gale. If you want to leave the house with the rooflight cracked for background ventilation, a rain sensor is the feature that makes that safe. It is a small addition at the time of installation and an awkward retrofit later, so it is worth deciding up front.

Comfort and smoke ventilation

One distinction to keep straight. A comfort-ventilation rooflight is for everyday fresh air. An automatic smoke-ventilation rooflight, sometimes specified over a stairwell to clear smoke in a fire, is a different product built to a different standard, and the two are not interchangeable. For a domestic kitchen or living space you want comfort ventilation. If a design calls for smoke ventilation, that is a specific requirement to raise with whoever is designing the building.

These are industry-typical figures for domestic flat rooflights. Exact numbers vary by manufacturer, size and glass build-up, so read them as the shape of the market rather than a fixed quote. The pattern is what matters.

Two things stand out. The U-values overlap almost entirely, so opening does not cost you much winter warmth if the unit is well made: the difference between the columns is one or two tenths, not a category change. And the G-value column is identical, because the glass you choose is a separate decision from whether the unit opens. You can have a fixed rooflight with poor solar control or an opening one with excellent solar control, and it is the glass that decides how hot the room gets, not the hinge. Size is where the real limit sits. Fixed units go much larger, because there is no mechanism straining to lift a heavy sash, so the biggest single panes of glass over an extension are almost always fixed.

The choice is a room-by-room judgement, not a whole-house rule, and a big rear extension often wants both types in the same roof.

Fixed makes sense over

Rooms where the job is daylight and nothing else. A dining area, a hallway, a landing, a home office lit from above, a large living space where the ventilation is handled by the doors and windows you already have. Fixed is also the answer wherever you want the largest possible pane of glass, because the size ceiling is higher and the price per square metre is lower. If a room already breathes well at low level and you simply want light pouring in from above, paying for a mechanism you will not use is money spent on nothing.

Opening makes sense over

Kitchens, above all. Cooking dumps heat and moisture into a room faster than almost anything else a house does, and an opening rooflight directly over the hob area clears both straight up and out. Bathrooms and utility rooms, where damp air needs somewhere to go before it condenses on cold surfaces. Any south-facing flat roof over a room that is used in the afternoon, because that is the aspect and the time of day when solar gain peaks and a fixed pane traps it. Bedrooms in a single-storey flat-roof extension, where a room has to shed the day’s heat overnight to be sleepable.

The mixed roof

On a wide extension with two or three rooflights, the sensible layout is often one opening unit positioned where the heat and steam collect, usually over the kitchen end, and fixed units elsewhere purely for light. You get the ventilation exactly where it does the work and you are not paying for three mechanisms when one will clear the whole open-plan space. This is the kind of split we work out at survey, plane by plane and room by room, alongside the glass specification for each unit. It is also the point at which the flat roof skylight installation stops being a catalogue order and starts being a specification for your actual roof.

An opening unit costs more to buy and more to keep. The purchase premium is in the table above. The running cost is maintenance: a gasket that should be wiped and checked so grit does not stop it sealing, a drainage channel that wants clearing of leaves and moss, an actuator and its wiring that should keep working for many years but is a mechanism and not a lump of glass. None of this is onerous, and none of it is a sign of anything going wrong. It is the ordinary upkeep any moving building component asks for. A fixed unit asks for almost nothing beyond an occasional clean.

A few points to settle before you commit, whichever way you lean:

1. Which way does this roof plane face, and when is the room used? A south-facing flat roof over an afternoon room is the strongest case for opening and for solar-control glass. A north-lit office may be perfectly happy fixed.
2. Can you actually reach a manual unit? Measure the ceiling. If the answer is no, an electric unit is not a luxury, it is the difference between ventilation you use and ventilation you paid for and ignore.
3. Do you want a rain sensor? Decide now. It is a straightforward addition during installation and an awkward one afterwards.
4. What is the G-value of the glass? This is separate from fixed versus opening and it matters more on a flat roof than on any other orientation. A number should come back. If it does not, the specification is not finished.
5. Is the U-value quoted whole-unit or centre-pane? Whole-unit is the honest figure and the one that has to satisfy Building Regulations. New and enlarged rooflights are notifiable, and we make that notification to Southend-on-Sea City Council or the relevant local authority for your address as part of the job.

The short version. A fixed flat rooflight is the right buy when you want light, a large pane and the fewest things that can ever need attention. An opening one is the right buy when the room generates heat or moisture, when the roof faces the sun, or when you simply want to be able to let the top of a room breathe. On a great many south Essex extensions, particularly the flat-roofed rear kitchens on the 1930s semi belt with a south-facing garden behind them, the best roof carries both: fixed glass for light, an opening unit where the heat gathers, and solar-control glazing across the south-facing planes to keep the peak in check. We survey the roof, work out the aspect of every plane and the use of every room, and specify the units to match. If you want that worked out for your own roof, ask us for a specification before you order anything.