Can a flat rooflight be opened?

Opening flat rooflights come in a few distinct types, and the right one depends on why you want it to open and how far off the floor it sits.

• Electric. A concealed actuator, either a chain or a spindle, lifts the glass on a wall switch, a remote or a wired keypad. This is the usual choice on a flat roof, because the glass is above head height and you are not going to reach it by hand.
• Manual. A winding handle, a twist catch or a telescopic pole hooks into the frame and cranks the pane open. It costs less than electric and has nothing to fail electrically, but only if you can actually reach the unit from a landing, a mezzanine or a step.
• Hinged access. A larger flat rooflight can be built as a hinged hatch that swings up far enough for a person to climb out onto the roof. These are specified for maintenance access and for terraces, and they carry a much heavier frame and a stronger stay than a ventilation unit.
• Sliding. On bigger openings the glass slides sideways on a track rather than tilting up. A sliding rooflight clears a wide gap without a pane hanging out into the weather, which suits a walk-out roof terrace.

Whichever mechanism you pick, an opening flat rooflight seals against a gasket when it is shut, the same way an opening window does. A well-made unit closes down tight and weathers no worse than a fixed one. The seal is doing real work, so it is the part worth asking about.

The main reason to open a flat rooflight is to move air, and specifically to let hot air out at the highest point in the room. Heat rises and collects at the ceiling, so an opening at the very top of the space pulls warm, stale air out and draws cooler air in through a door or a window lower down. That is far more effective than an opening in a wall, which sits below most of the heat it is meant to shift.

This matters more on a flat roof than almost anywhere else, and it matters most of all in south Essex. Southend faces south across the Thames Estuary, so a flat-roofed rear extension here often sits under sun from mid-morning until late afternoon with nothing on the southern horizon to shade it. A flat rooflight points at the sky rather than at a compass direction, so it collects the high summer sun almost square on. Put glazing over a kitchen, which already makes its own heat, and the room can run hot by early afternoon. An opening rooflight vents that build-up, and it does the job the moment the room warms rather than waiting for you to prop a door.

Ventilation is not the only reason, though. An opening flat rooflight can also give safe access onto a flat roof for cleaning gutters or reaching a terrace, and larger units can be specified to open automatically as a smoke vent where a stairwell or a commercial space needs one. Those are design decisions to settle early, because the frame, the stay and the wiring all change with the job.

An opening unit costs more than a fixed one of the same size, because it carries a hinge or a track, a stay or an actuator, and on the electric versions a motor and a control. It is also slightly more to go wrong over the years, since a fixed pane has no moving part at all. Against that, a fixed rooflight gives you daylight and nothing else. If the room below never overheats and never needs venting, fixed is the honest choice and the cheaper one.

There is a size question too. Opening flat rooflights have a maximum pane size, because the mechanism has to lift or slide the weight of the glass, and a very large single opening pane gets heavy fast. Where you want both a big expanse of glass and some ventilation, a common answer is to pair a large fixed unit with a smaller opening one next to it, rather than pushing one opening pane to its limit. That also lets you put the ventilation exactly where the heat gathers.

Opening and glass specification are separate decisions, and the strongest results come from treating them that way. Venting hot air out helps, but it works best alongside glass that keeps some of that heat from entering in the first place. On a south-facing flat roof we would specify a low G-value on the glass, around 0.3 or below, so less solar energy comes through, and then add an opening unit to shift whatever heat does build up. The two together beat either one alone. Our energy-efficient skylight installation work is built around exactly that pairing.

A few things decide whether an opening flat rooflight makes sense for your roof.

Reach. If the unit is above head height, which on a flat roof it usually is, a manual opener is no use unless you fit a pole or there is a raised floor level nearby. That points most flat-roof jobs towards electric operation, or towards leaving a hard-to-reach unit fixed and venting the room another way.

The upstand. A flat rooflight sits on a raised timber or insulated kerb, the upstand, which lifts the glass clear of standing water and lets rain run off. An opening unit needs that upstand built correctly, because the hinge and the seal both depend on the frame sitting square and the water shedding away from the opening edge. A good upstand is what keeps an opening rooflight weathertight for the long run.

Glazing rules. A new or enlarged rooflight is notifiable under Building Regulations whether it opens or not. Part L covers the thermal performance of the glass and Part K covers the safety glazing, and an opening unit within reach needs to meet the same standards as any other. As a general point a flat rooflight on a house is often permitted development, but flats, listed buildings, conservation areas and Article 4 zones are the exceptions, so it is worth checking with the local authority before you order.

If your existing rooflight has failed and you want the replacement to open, that is straightforward: the opening is already cut and the upstand is already there, so swapping a fixed unit for an opening one is the moment to change both the mechanism and the glass specification at once.

Decide first why you want it to open. If the answer is heat, an electric opening unit paired with solar-control glass will do more than either would alone, and it will do it on a south-facing estuary roof where the sun is relentless. If the answer is only daylight, a fixed unit is cheaper and there is less to maintain. If you want roof access or a smoke vent, say so early, because the frame and the fittings are chosen for that from the start.

Then get the roof looked at properly rather than ordering off a size chart. Tell us which way the roof faces, roughly how big the opening is and whether the room runs hot, and we will come back with a mechanism, a glass specification and a price. See our flat roof skylight installation page for how the whole job is put together, ring 01702 898232, or request a quote and we will look at the whole roof rather than just the hole in it.