What goes into a quiet rooflight specification, line by line

Sound reaches a room by two routes. It passes through the glass, and it slips round the glass through gaps. A good specification deals with both, which is why it runs from the weather face of the unit to the plasterboard around the opening.

The note that comes with every quote, described on the quiet spec page, is the summary. This page is the longer working behind it. We take the lines in the order the sound meets them because that is also the order a fitter builds them, from the kerb upwards and then the glazing in.

Two habits help when you read any specification. First, look for the thing that is named rather than described. "Laminated inner pane" is a named part. "High-performance glazing" is a mood. Second, look for the reason beside each part. A pane thickness with no reason beside it has usually been chosen from a price list.

The parts interact. A heavy inner pane set in a thin, hollow frame wastes the glass. A solid kerb under a single thin sheet wastes the kerb. So the specification is read as a chain, and it is only as quiet as its least considered link.

Nothing here needs a measuring instrument to check. Every line in a quiet rooflight specification can be confirmed with a tape, a delivery note and a torch, and that is deliberate. The figures that matter to you are the ones you can verify on your own roof.

The outer pane is the surface rain, hail and debris strike, so it is chosen for strength and for how it behaves under a downpour before it is chosen for anything else.

On most rooflight units the outer pane is toughened glass. Toughening makes it strong and makes it break into small, blunt pieces if it ever fails. The specification gives its thickness, because thickness decides how much a pane resonates when raindrops land on it. A thin sheet behaves like a drum skin. A thicker sheet is stiffer and heavier, and it answers a raindrop with a duller sound.

The outer pane is frequently made a different thickness from the inner pane. That is an acoustic choice as much as a structural one. Two sheets of identical thickness tend to ring at the same pitch, and the sealed gap between them can reinforce that pitch. Making the two unequal spreads the resonances apart, so there is no single note the unit passes easily.

The outer surface carries a coating on some products. Self-cleaning coatings shed dirt, and solar-control coatings cut heat. They do little for sound, but they are written into the specification so that the unit you are offered can be compared with the unit you are delivered. The full thermal picture is covered in energy efficient rooflights.

On a flat roof, the pitch of the glass matters as much as its thickness. A nearly level pane holds a film of water and takes the full impact of each drop. A steeper lantern roof sheds the water faster and softens the strike. The specification states the pitch of the glass for that reason, and the page on reducing rain noise under a flat rooflight follows the effect through in more detail.

The inner pane is where most of the acoustic choice is made. Swapping a plain toughened inner sheet for a laminated one changes how the whole unit behaves, at modest extra cost in depth and weight.

Laminated glass is two sheets bonded to a clear plastic interlayer under heat and pressure. A common build is two 3 mm sheets around a single interlayer, written as 6.4 mm laminate, though thicker builds exist and suit some rooms. The interlayer is soft compared with glass. When a sound wave shakes the pane, the soft layer absorbs a share of that energy as the two sheets try to move slightly against each other. A single sheet of the same total thickness has nothing to soak it up, so it rings.

The interlayer has a second job, and it is why laminated glass is the normal choice for an inner pane overhead. If the glass cracks, the fragments stay bonded to the interlayer and hang together instead of falling into the room. The specification records that as a safety reason, and the quiet reason sits beside it.

Some interlayers are made specifically for sound, with a softer core than the ordinary clear film. The specification names which one is used. If an installer proposes an acoustic interlayer, ask for the product to be written down. The page on acoustic laminated glass for rooflights explains how the layers work, and is laminated glass quieter than toughened glass answers the plain comparison.

A laminated pane is also thicker and heavier than a standard one, so it asks more of the frame, the glazing bars and the fixings. That is why the specification is written as a whole rather than glass first and everything else afterwards. The frame has to be chosen to carry the glass that the room needs.

The gap between outer and inner pane is a spacing decision with two jobs: slowing the flow of heat and breaking the path of sound. The specification gives its width, what sits at its edge, and what fills it.

Width comes first. A wider cavity generally helps the acoustic side, because the two panes are further apart and move less in step. It has to be balanced against the depth of the frame, the weight of the unit and the thermal rating the unit must reach. A cavity that is generous for sound but leaves the unit short of current Building Regulations is not an option, so we set the two together and write down the compromise. The rules that govern the thermal side are explained in Building Regulations for rooflights.

The spacer bar runs round the edge of the cavity and holds the panes apart. Modern warm-edge spacers are made of low-conductivity material so the edge of the unit does not become a cold line where condensation forms. The specification names the spacer because it is also the first place a cheap unit loses quality, and because a misted sealed unit is almost always a failed edge.

The fill is usually argon. Argon slows heat transfer slightly better than air, and it is invisible in use. It does not matter to sound in any way you would notice, but it belongs in the specification because it is part of the unit you are buying.

A third pane is the question people ask next, and it depends on the build. A third pane adds a second cavity and more weight. If it is one more identical sheet it may add little for sound and a lot for the thermal rating. Does triple glazing make a rooflight quieter goes into it, and our habit is to put the thickness and the lamination where the noise is, rather than adding sheets.

A sealed glass unit can be excellent and still be bypassed. Air carries sound, so any path for air is a path for noise, and the frame and seals are what close those paths.

The specification names the gasket at each place it is needed: around a fixed pane, at the closing edge of an opening sash, and where the frame meets the kerb. A continuous gasket that compresses evenly all round is a different thing from a strip that has been squeezed shut over a worn surface. On a replacement job, the old gasket is part of what we heard at the survey, because a hardened seal explains a lot about why the old unit whistles.

Frames come in aluminium, timber, PVC and, on many roof windows, a coated timber or polyurethane core. The specification gives the material and the section. A thin hollow section can hum when the wind is up, and a thicker section with a thermal break sits still. For an opening unit, a two-point or multi-point closing action pulls the sash evenly onto the gasket, which keeps the seal tight along its full length.

Opening units are a balance. A rooflight that opens gives ventilation and a route for smoke to clear, but it also has a seal line that a fixed unit lacks. If the room is noisy and never needs airing through the roof, a fixed pane is the quieter choice. If it needs to open, the specification says which seal and which closing action are used. Electric and opening rooflight installation describes the options.

Roof windows on pitched roofs follow a related logic, with a sash set in a frame that is flashed into the roof covering. The flashing kit, the insulation collar and the lining all belong in the specification. See roof window installation for that side of the work.

On a flat roof the glass sits on an upstand, and that upstand is the piece most often left out of the conversation. A hollow or thin kerb behaves like a sounding board, taking rain and traffic noise and delivering it to the room.

A solid upstand built from insulated timber or a factory-made kerb, with the waterproof membrane dressed up its outer face and over its top edge, is quieter and warmer. The specification gives its height above the finished roof, the insulation inside it, and the way the membrane is turned up. A height of about 150 mm above the finished roof surface is the working minimum for the waterproof turn-up, and a taller upstand also lifts the frame clear of standing water and settled snow.

The lining sits on the inside of the opening. It is the plasterboard or plaster that makes a splay or reveal from the ceiling up to the frame. The specification says how thick the insulation behind it is and how the joint between plasterboard and frame is sealed. An unsealed hairline there is a path for air and sound, and it is also a spot where warm, damp room air meets a cold surface and makes condensation. The same detail helps both problems.

The kerb also has to be fixed to the structure properly, and the fixings are listed. A loose kerb hums. One that is screwed down through the insulation to a solid timber upstand, or bedded on a continuous sealant strip, stays put. On a large lantern the kerb may sit on a built-up structure, and the specification records what carries the load. Flat roof rooflight installation and roof lantern installation show the build-up step by step.

The same parts are combined differently depending on what the room hears. The table sets out where we put the weight for each kind of noise, as a starting point for discussion rather than a fixed recipe.

The last row matters. A specification that recommends the same heavy build for every roof is a sales script. If the survey finds a quiet room, the written advice says a good standard unit is enough.

Low, steady sound from roads and rails responds best to mass and to a solid kerb, since it is carried through the structure as well as the glass. The sky is different. Aircraft overhead arrive from above onto the largest area of glass in the room, so the inner pane and the gaskets carry most of the work. Our page on rooflights under the Gatwick flight paths covers what that means for homes around Crawley, and road and rail noise through rooflights does the same for the corridors beside the M23, the A23 and the Brighton main line.

Room finishes are part of the choice as well. A bare loft bedroom with a hard floor and plastered slopes sounds louder under glass than a carpeted one with curtains. The specification notes the room as it is, not as the brochure shows it, and suggests soft finishes where they are the cheaper gain. Does a blind reduce rain noise under a skylight is worth reading for that reason.

The specification is only useful if it is checked. There are three moments to do it: when the unit arrives, when it is fixed, and when the lining is finished.

At delivery, the glass carries an etched or printed mark in one corner that records the maker and the make-up of the pane. A laminated pane shows a visible edge, a thin clear line between two sheets, where a toughened pane shows none. Compare the thickness on the delivery note with the thickness in the specification, and ask for any difference to be explained before the old unit is out.

At fixing, look at the kerb before the glass goes on. You are looking for a solid upstand, insulation filling the cavity, the membrane turned up its full height and a continuous bed for the frame. Once the frame is on, those details are hidden for good, which is why we photograph them and file the pictures with the job.

At lining, run a finger round the joint between plasterboard and frame. It should be continuous and firm, with no gap and no crumbling sealant. Stand in the room on a wet evening if you can, and listen. The specification described what we expected the room to do, and your own ears are the final check.

The job is notifiable to Building Control in some cases and not in others. Where it is, we handle the notification. The pages on whether a new rooflight needs Building Control sign-off and whether replacing a rooflight is notifiable explain the difference. The 10-year workmanship guarantee described on our guarantee page covers the installation of the parts the specification lists.

Ask for a quote and the specification arrives with it. You do not need to know the vocabulary first, because the survey is partly a listening visit and the note is written from what we hear.

Tell us which room, what bothers you, and whether you are replacing a unit or opening a new one. We survey across Crawley and the 30 miles around it. If you already have a quote from another installer, ask them to write the glass build, the seals and the upstand beside their price. What a quote for a new rooflight should include lists the other lines worth looking for.

The quote form on this page starts the process. You can also message us on WhatsApp or use the contact page. If you are still deciding whether the extra glass is worth it, the answer page on whether acoustic glass is worth it for a rooflight weighs it plainly. The wider decision sits under our main skylight and rooflight installation in Crawley page.

Which line of the specification makes the most difference?

For sound through the glass, the laminated inner pane. For sound that bypasses the glass, the seals and the upstand. A specification that names only one of those is leaving half the route open.

Can a quiet skylight be added to an existing roof?

Yes. A replacement unit can carry a better build than the one it replaces, provided the opening, the kerb and the frame can take the heavier glass. The survey checks that, and the specification records what has to change around the glass as well as the glass itself.

Will a quieter unit be darker?

Laminated glass has a very slight tint and cuts a little light, but the difference is small beside the effect of the size and position of the opening. Daylight is still the point, and the specification gives the glass type so you can see it in a sample before you agree.

What if my room is quiet already?

Then the specification says so and proposes a good standard unit with sound seals. Not every roof needs an acoustic build, and a short note saying that is a perfectly good result.

Does the specification change the price?

It can, because thicker and laminated glass costs more than standard glass. The costs page explains how the glazing build feeds into a quote, and the written specification lets you see exactly what each part is adding.