Fixed flat rooflights for extensions, and why sealed glass is enough

A fixed unit is glazed into its frame at the factory and stays shut for life. The frame sits on an upstand, the membrane dresses up to it, and nothing on the roof is designed to be opened, adjusted or wound.

The same product is sold as a fixed flat rooflight, a fixed flat roof skylight or simply a non-opening rooflight. The names differ by brochure, not by construction. What matters is the list of things that are absent: no sash that lifts away from the frame, no hinge to carry the weight of a double-glazed pane, no stay, no motor and no cable run back to a switch.

That absence is easiest to see in a cross-section. An opening rooflight has a moving sash inside a fixed outer frame, and the gap between the two is closed by a compression seal. A fixed unit has a single glazing bead and a single line of sealant where the glass meets the frame. There is one junction to get right instead of three.

Fixed units come in two forms. The first is a framed unit with a visible aluminium or PVC-u border around the glass. The second is a frameless unit, where the glass steps over the upstand and the frame is hidden below it. Our page on frameless flat rooflights covers that second form. Both are fixed in the sense used here, and both belong under the wider subject of flat roof rooflight installation.

A room that already has openable glazing on a wall does not need a second route for air through the roof. The wall window handles ventilation, and the rooflight handles light.

Think of a typical single storey rear extension: a kitchen and dining room with a set of sliding doors onto the garden, perhaps a side window, and a flat roof over the middle. The doors can be opened on a warm afternoon. The kitchen has an extractor. The space does not need a roof opening to breathe, and it gains nothing from paying for one.

The same reasoning holds for a home office built at the bottom of a garden, a utility room with a side door, or a hallway extension with a window at each end. Wherever a person can already reach a window, a fixed rooflight over the same space duplicates nothing and adds no moving part to look after.

It is worth separating two ideas that get blurred. Daylight and ventilation are different requirements. A rooflight is usually bought for the first. The second is met by other means and is governed by its own part of the regulations, which we return to below. Choosing fixed glass does not mean choosing a stuffy room. It means assigning each job to the part of the building best placed to do it.

Every moving part on a roof is a part that can wear, stick or let in weather. A fixed unit takes most of that list away before the building is even finished.

Consider what an opening rooflight asks of its components over years of use:

• A gasket that is compressed and released every time the sash is closed.
• Hinges and stays that carry the weight of the glazing through a full swing.
• An operating arm or a motor that needs power, a control and a rain sensor.
• A drip channel or vent slot that can clog with leaf litter and pollen.

None of those exists on a fixed unit. The seal is made once, under controlled conditions, and is then left alone. That is why a fixed unit is the tightest choice over a room that does not need the extra function: there is simply less that can go out of true.

The weak point on any flat rooflight is seldom the glass. It is the junction between frame, upstand and membrane, and the fewer interruptions there are in that line, the more reliable it stays. A fixed unit keeps the perimeter uninterrupted. There is no cable gland piercing the kerb, no control box to fix on the outside, no lug for a rain sensor.

For an older unit that once opened and no longer closes properly, this is also a reason to renew with fixed glass where a wall window now covers the air. We come back to that in the section on replacing old units.

Noise gets into a room through the thinnest, most flexible path. On an opening rooflight that path includes the gap around a sash. On a fixed unit there is no such gap, so the glazing build decides the sound, not the seal.

This is where the quiet light idea applies to something as ordinary as a sealed pane. A flat rooflight over a kitchen is not a loud thing on a dry day. In a downpour, rain lands on a large flat surface and the glass rings like a drum skin. Over a home office it can drown out a call. Over a loft-style bedroom it is what wakes the sleeper first.

We deal with that through the specification, never through a number:

• A laminated inner pane, where a plastic interlayer between two sheets of glass damps the vibration that thin single glass passes straight through.
• Pane thickness chosen for the span, because a thin large pane flexes and a thicker one does not.
• A sealed cavity that is not interrupted by an opening sash.
• A well-formed upstand and a continuous bead, so the glass does not sit on a hard contact point.

In the Crawley area the noise worth thinking about depends on the address. Some homes sit under the Gatwick routes, some sit near the M23 or the Brighton main line, and others are simply under rain. Whichever it is, we note what we heard at the survey in the written quiet spec that comes with every quote. You can see how that document is laid out on the quiet spec page, and the glazing detail for bedrooms is in quieter glazing for a flat roof rooflight over a bedroom.

Ventilation, energy performance and overheating are each covered by a separate Approved Document. A fixed rooflight touches all three, and none of them forbids it.

Approved Document F covers ventilation. Habitable rooms need background ventilation and a means of rapid or purge ventilation, and the usual answer is an openable window or door in an external wall. Where the room already has one of those of suitable size, a fixed rooflight adds light without changing the ventilation position. Where the room has nothing openable, the question changes, and we come to that below.

Approved Document L covers the conservation of fuel and power. New glazing in a new extension has to meet limits on heat loss, and rooflights are held to their own limits, looser than wall windows because glass in a roof loses more heat by nature. A fixed unit has a small advantage here, because a sealed frame with no moving joint is easier to make airtight and to insulate around.

Approved Document O covers overheating. Glass over a south-facing room collects heat in summer, and fixed glass cannot vent it on its own. That is a real consideration and it belongs in the choice of glass, not in a hinge. A solar control coating, a modest total size and the wall openings that exist already do much of the work.

The work is notifiable in many cases, and we handle the Building Control notification where it is, and install to current Building Regulations. For a wider view of the energy side, see energy efficient rooflights.

A sealed unit is not the answer everywhere. There are rooms where an opening rooflight earns its place, and saying so plainly is part of specifying properly.

The pattern is simple. If the room has somewhere else to take air from, fixed is the tidy choice. If it has nowhere else, a rooflight that opens, or a mechanical extract, belongs in the design. Our page on electric and opening rooflight installation sets out that side, and the answer on a flat rooflight in a bathroom deals with the wettest room in the house.

Heat is the other honest caveat. A large fixed pane over a south-facing kitchen will warm the room on a sunny day. The remedy is rarely an opening sash, which lets out a small amount of air through a small gap. It is a coating on the glass, a blind, or a smaller unit. We raise that at the survey rather than after the first hot week. If a blind is on your list, can you fit a blind to a flat rooflight explains what the kerb needs to allow for it.

With no sash to think about, the glass becomes the main decision, and it is worth deciding on purpose. The choice is between clarity, heat, sound and strength.

A fixed rooflight is glazed on the outside with toughened glass, so a dropped tool or a fall of hail does not break it into sharp pieces, and on the inside with laminated glass, so that if a pane ever did crack it would hold together above your head. That toughened over laminated pairing is the standard approach for roof glazing, and it is also the natural place to add acoustic benefit.

Coatings come next. A low-emissivity coating cuts heat loss in winter. A solar control coating cuts heat gain in summer, at the cost of a slight tint to the light. On a north-facing or well-shaded roof you would usually leave the solar control off and keep the light as bright as it can be. On a south-facing extension you may want it.

A self-cleaning coating is an option on the outer face. It helps rain wash off dust, though it does not remove the need to clear leaves and debris from the glass. Our answer on what glass a flat rooflight should have goes through each layer, and the page on obscure glass covers privacy.

A fixed unit can be made in large single panes, because it does not have to carry an opening mechanism. That flexibility is real, and it still has to be fitted into a roof that has its own structure and falls.

The upstand is the raised kerb that the unit sits on. It lifts the glass clear of the roof surface and gives the membrane something to turn up. Its height, its insulation and its finish all affect how the unit behaves, and they are the subject of our guide to flat roof rooflight upstands. If you are not sure whether your roof needs a separate kerb at all, this answer is the place to start.

Falls matter because a flat roof is designed to shed water. A roof laid to a fall of 1:80 is shallow but real, and a long rooflight across the direction of the fall can dam the flow. We look at where the water will go before we agree the position, and falls, drainage and why flat rooflights are never perfectly level explains the thinking.

For sizing in a single storey extension, the answer on what size suits a single storey extension gives a working method. Where a roof needs more than one unit, laying out several flat rooflights covers spacing and symmetry.

Adding a fixed rooflight to a finished flat roof is ordinary work, and it follows a set order. The roof decides how much of that order is easy.

1. Survey the room and the roof together, and note what the room hears and where the water runs.
2. Open the ceiling and deck to see the joists, so the opening can be framed without guessing.
3. Trim the structure around the opening with proper timber headers, so the load is carried to the joists either side.
4. Build or fit the upstand, insulate it and wrap it in the roof membrane.
5. Set the fixed unit on the kerb, fix it through its flange and seal the perimeter.
6. Line the reveal inside and make good the ceiling.

The membrane detail depends on what the roof is made of. Felt, EPDM rubber and GRP fibreglass each take an upstand differently, and we have a page for each: felt, EPDM and GRP.

If you are wondering whether your roof can take an opening at all, can you put a rooflight in an existing flat roof covers the question, and why the joists need trimming explains the structure. The full sequence is in how a flat roof rooflight is installed.

Older flat roofs can carry a dome or an opening unit that has reached the end of its life. Where the room has other openable glazing, the renewal is a good moment to choose a fixed unit.

Signs that a unit is due for renewal are plain to see: a cloudy film between the panes, a frame that has bowed, sealant that has cracked away from the glass, or a sash that no longer sits square. These are conditions of age, not of one bad day of weather. The answer is to replace the unit, and to rebuild the upstand junction at the same time so the new glass sits on a sound base.

It is sensible to ask whether the old opening function is still needed. If it has not been used for years because the room has a window, the new unit can be fixed. If the room relies on it for air, it cannot be, and we say so at the survey. Our service for misted and failed rooflight replacement covers that route in full.

A replacement is also the moment to address noise. An old thin unit that rattled in rain can be renewed with a laminated inner pane, and the difference is one the household notices the first wet evening.

A fixed unit needs less from its owner than an opening one, but it is not maintenance-free. Glass outdoors collects dust, pollen, bird droppings and leaf litter, and a flat surface holds them.

A good upstand and a correct fall help water run off the glass rather than sit on it. Even so, a soft brush and clean water on a dry day, from a safe position, will keep a unit clear. Avoid abrasive pads and harsh chemicals on any coating. There is more detail in how to clean a flat rooflight, and in why water sits on a flat rooflight.

Snow and leaves behave differently on flat glass than on a pitched roof, and what happens in snow and falling leaves deals with both.

A fixed unit is usually the most economical rooflight of its size, because it leaves out the mechanism. The bigger costs are the roof work and the glass specification.

As a guide, the price of a flat rooflight job is driven by the size of the unit, the glazing build, the roof covering and how much structural work the opening needs. On a small unit, the opening, framing and making good tend to outweigh the price of the glass itself. Our page on what affects the cost of a flat roof rooflight explains each driver, and rooflight and skylight costs in Crawley gives general market ranges.

Every quote comes with the written quiet spec, so what you are paying for is on paper. Ask for a quote through the form on this page or by WhatsApp.

These are the points that a household weighs when choosing between a sealed unit and one that opens.

Will a room with a fixed rooflight feel stuffy?

Not if the room has a window or door that opens. The rooflight supplies light and the wall opening supplies air. Stuffiness comes from a room with no route for air at all, and that is the case where we would suggest an opening unit or an extract fan.

Is a fixed flat rooflight more watertight than an opening one?

It has fewer joints, so there are fewer places for water to find a way in. Watertightness still depends on the upstand, the membrane and the fall of the roof, so the unit cannot make up for a poor build around it.

Does a fixed rooflight help with noise?

It removes the gap around an opening sash, which is one route for sound. The glass itself matters as much, so a laminated inner pane and a sensible thickness are the real acoustic choice. We write it into the quiet spec.

Can a fixed unit be turned into an opening one later?

Not in place. The frame and glazing are a single sealed assembly. If you might want an opening unit later, say so at the survey and we can plan the kerb and the wiring route so that a swap is straightforward.

Should I choose a fixed rooflight or a dome?

A flat glass unit sits lower and gives a cleaner ceiling line, while a dome is lighter and can be simpler on a tight budget. Is a flat glass rooflight better than a dome weighs them against each other.

If your room already has a window and you want more daylight overhead, a sealed unit is likely the simplest choice. We visit, listen to the room, and write down the glazing build, the upstand and the seals in a quiet spec that comes with your quote.

We cover Crawley and 30 miles around, and every installation carries a 10-year workmanship guarantee. Start with the quote form or visit get a quote.