Upgrading the glass while the roof is open

The marginal cost of a better unit is a difference, not a total

Split a replacement into its parts and the arithmetic becomes obvious. There is the survey, the scaffold, the stripping and re-laying of the covering, the flashing kit, the underlay and insulation work at the perimeter, the Building Control notification, and the unit. Change the glazing specification and exactly one of those changes. The scaffold is the same scaffold, the day is the same day, the flashing kit is chosen by covering rather than by glass.

Ordered as a separate exercise five years later, the same upgrade needs the scaffold again, the covering off again and a second full day, and the difference in the glass becomes a rounding error against the cost of getting to it. That asymmetry is the whole argument, and it is why the specification conversation belongs at the quotation stage rather than after somebody has already decided to keep it simple.

The decision belongs before the order, not on the scaffold

Glass is not a site variable. A sealed unit is manufactured to the frame, the coating is applied at the float line and cannot be added afterwards, and a solar control unit and a clear one are different products with different lead times. By the time the covering is off, the specification was settled weeks ago.

That has a practical consequence for how the enquiry runs. The aspect of the slope, the use of the room, what is underneath the opening and whether anybody has ever complained about the heat in July all need to be discussed at survey, because they are the inputs to the specification. An installer who takes the size and the size only will order the standard glazing code, which is the manufacturer’s default and is chosen to be acceptable everywhere rather than right anywhere.

The rebate depth, and where the frame sets the limit

A frame will accept a sealed unit up to a certain overall thickness and no further, because the glazing rebate is a fixed dimension moulded or machined into the sash. A current manufactured roof window sash typically takes a unit somewhere between 24 and 44mm depending on range and generation. A made-to-size flat roof unit is built as a frame and glass together, so the constraint moves from the sash to the frame section.

What this rules out is retrofitting a thicker unit into a sash that was designed for a thinner one, which is why the answer to a request for triple glazing in an existing frame is usually no and the answer to the same request in a replacement unit is usually yes. It also affects the coating strategy: where the cavity is forced narrow by a shallow rebate, argon stops performing at its best and krypton becomes worth its higher cost, because it reaches its optimum at a narrower gap.

On a replacement this is a survey question rather than a limit, since the frame is being renewed along with the glass. It only bites where the frame is being kept and the sealed unit alone is being changed, which is a smaller and different piece of work.

Weight, and the point at which the structure gets a second look

Glass is heavy and more glass is heavier. A double glazed unit with a toughened outer and a laminated inner runs at roughly 30 to 35 kilograms per square metre of glass before the frame is counted. Add a third pane and it goes to 45 or more. Against a 1990s air-filled unit with two 4mm panes, which is around 20 kilograms per square metre, a modern triple in the same aperture is more than twice the mass.

On a roof window in a sound trimmed opening in a cut roof, that difference is absorbed without comment. It becomes a real question in three places: a large made-to-size flat roof unit over an old joisted deck, a wide unit where the original trimmer was undersized or absent, and any opening in a trussed roof where the original design assumed a particular load. Those are checked at survey with a tape and a level rather than calculated after the fact, and where a calculation is needed it goes in with the Building Control submission. Lifting is the other consequence: a heavy unit in a Stroud valley cottage with a hand-carried scaffold is an access decision as much as a specification one.

Solar control, and the upgrade that changes how a room gets used

Of everything on the list, this is the one that produces the largest change in how a room feels, and it is the one most often left off an order. A low emissivity coating keeps heat in. A solar control coating keeps heat out. They are different layers doing opposite jobs and a unit can carry both.

The relevant number is the g-value, the fraction of solar energy that gets through the glass. A clear double glazed unit sits around 0.5 to 0.6. A solar control unit brings it to roughly 0.25 to 0.3. Overhead glass on a south or west facing slope receives sun at close to normal incidence through the hottest part of the afternoon, and halving what gets through is the difference between a loft bedroom that is usable in July and one that is abandoned by two in the afternoon.

The cost is daylight and, to a small degree, colour. A solar control unit passes less visible light, and some products give the daylight a faint tint that is noticeable when a treated and an untreated unit sit on the same elevation. That is an argument for treating a whole slope rather than one window on it. Aspect by aspect guidance is on specifying glazing by which way the slope faces, and shading alternatives on external shading against internal blinds.

Laminated inner panes, and where they stop being a preference

Safety glazing overhead is not really an upgrade, it is a correction. A 1980s unit may well have two panes of annealed float glass in it, which is a material that breaks into blades. Current practice is a toughened outer pane, which breaks into small blunt fragments, and a laminated inner pane, which holds together on its interlayer and stays in the frame.

The laminated inner becomes a requirement rather than a choice wherever there is habitable space, a stair, a bath or a shower beneath the glass, and it is worth specifying anywhere anybody walks. The interlayer thickness is the variable: a standard 0.38mm polyvinyl butyral layer satisfies the safety case, and a thicker 0.76mm or acoustic-grade interlayer does the same job while noticeably damping sound. Since the price gap between the two is small and the labour is identical, the thicker interlayer is usually the better order.

Rain noise, the change nobody asks for and everybody notices

Ask a homeowner what they want from a new rooflight and they will say light, warmth and a window that shuts properly. Ask them six months later what surprised them and a good number will say the rain. A single sheet of toughened glass on a shallow or near-horizontal unit behaves like a drum skin, and a heavy shower over a rear addition kitchen in Tewkesbury is loud enough to interrupt a conversation.

Mass and damping are what quieten it, and a laminated outer pane provides both. Moving from a 4mm toughened outer to a 6.8mm laminated one is a modest change on paper and a large one in the room. Making the two panes different thicknesses helps again, because two identical panes resonate together. None of this requires a different frame, a different kerb or a different day, which puts it firmly in the category of things worth doing while the roof is open. The detail is on acoustic glazing and rain noise.

Self-cleaning outer surfaces, and the units that justify one

A self-cleaning coating is a thin pyrolytic layer on the outer face that does two things: it breaks down organic dirt under ultraviolet light, and it makes the surface hydrophilic so rain sheets off rather than beading and drying into spots. It is not a substitute for cleaning and it does nothing about the moss and grit that collects at the head of a unit in a wooded valley.

Where it earns its place is on any unit nobody is realistically going to clean: high above a stairwell, on a steep Forest of Dean slope with no safe standing, on a Cheltenham parapet flat reached through a sash window, or on a fixed unit in a slope where the sash cannot be rotated. On a centre pivot at head height in a loft bedroom, where the sash turns through 180 degrees and locks for cleaning, it is a smaller benefit. Access decides it more than anything else.

Going to three panes in a replacement frame

A replacement is one of the few realistic opportunities to move a rooflight from two panes to three, because the frame is being renewed and the rebate question therefore disappears. Whether it is the right call depends on the slope and the room rather than on the arithmetic alone.

The case is strongest on shallow and near-horizontal glass, where a double glazed cavity loses more heat than the same cavity in a vertical window because convection works harder when the warm surface is underneath. It is strong on a north-facing slope where there is no solar gain to trade away. It weakens on a steep south-facing slope where a solar control double often delivers more comfort than a clear triple, and it weakens again where the sash mechanism, the hinges or an operating rod were sized for a lighter unit. The full argument is on the triple glazing page, and the honest summary here is that the third pane is a real option on a replacement and it is not automatically the best use of the same money.

Obscured, patterned and switchable glass over a bathroom

Overlooking is a real issue on Gloucester and Cheltenham terraces where rear additions face each other across narrow gardens, and on stepped Stroud valley terraces where one house looks down onto the roof of the next. A rooflight over a bathroom or a shower on that kind of elevation can carry an acid-etched or sandblasted inner surface, which passes light and blocks the view without any moving part to fail.

Two cautions worth knowing. Obscure treatments reduce light transmission, and on a small unit in a north-facing bathroom that is a noticeable loss. And switchable glass, which turns from clear to opaque with an electrical current, is available in rooflight sizes but adds a wired component with its own control to a position that is hard to reach. In a bathroom the humidity and the access both argue for the simpler answer. Wet room considerations are covered at flat rooflights over a bathroom or wet room.

What the regulations require, and why it is a floor rather than a target

Replacing a rooflight is work on a controlled fitting, and the replacement has to meet the current limiting standard rather than the standard the old one met. Current guidance puts that at 1.4 W/m2K for a replacement roof window, with rooflights quoted in the vertical plane at 2.2. Any current manufactured unit clears it comfortably, which is exactly the problem: clearing a limiting standard is not the same as choosing well.

The limiting figure says nothing about solar control, nothing about the spacer, nothing about acoustic performance and nothing about safety glazing beyond the separate requirements that cover it. A unit that meets Part L and overheats a west-facing bedroom every summer has satisfied the regulations and failed the household. The detail of how the standard applies is on rooflights and Part L for a replacement.

The perimeter decides whether the better glass performs

A 1.0 W/m2K unit set into an opening with insulation stopping 100mm short of the frame will still grow mould in the head corners, because the coldest surface in the reveal is no longer the glass. Upgrading the glass without correcting the perimeter moves the problem rather than solving it, and on a replacement both are available on the same day.

So the specification conversation runs alongside the perimeter one: insulation continuous and tight to the frame with no void behind the plasterboard, the vapour control layer taped to the frame rather than trimmed short of it, and the reveal geometry set so warm air can wash up the face of the glass. Those are the details that decide whether a good unit behaves like one. They are covered at thermal bridging at the rooflight perimeter and vapour control layers sealed to the frame.

Matching across an elevation when only one unit is being renewed

Where a slope carries three or four units of the same age and one has failed, upgrading that one alone creates a visual mismatch that is more obvious than people expect. A solar control coating gives the glass a faint tint from outside, a modern low emissivity soft coat reads slightly differently in reflected sky, and a new white polyurethane sash next to three weathered pine ones is not subtle.

There are three honest routes. Match the new unit’s appearance to the existing and accept a lesser specification on one window. Upgrade the failed one properly and accept the mismatch, which on a rear slope nobody sees is usually the right call. Or replace the group, which puts one scaffold and one specification across the whole elevation and generally costs less than two visits. That last option is set out at replacing several rooflights at once.

Where the upgrade earns most on a Gloucestershire roof

West and south-west facing loft conversions across Longlevens, Hucclecote, Tuffley and the Cheltenham suburbs are the clearest case for solar control, because the afternoon sun arrives at a low angle straight onto overhead glass in a room with a low ceiling and no thermal mass. Near-horizontal units over Severn Vale rear additions in Gloucester and Tewkesbury are the clearest case for a laminated outer, because the glass is flat, the room below is a kitchen and rain lands hard on it.

North-facing Forest of Dean slopes around Coleford and Cinderford are where a low emissivity coating and a warm edge spacer matter most and solar control matters least, since there is no gain to control and the glass stays wet and cold. Cotswold stone slate roofs through Painswick, Northleach and the wolds are a case of their own, because the replacement is often a conservation unit with a slim steel frame, and the glazing options in that product are narrower. What is available there is set out at conservation rooflight installation.

Getting the specification onto the order

An order that says a size code and a brand has left the glass to the manufacturer’s default. An order that carries a specification says what is wanted and can be checked against what arrives. The lines worth writing down are the overall unit thickness, the pane build-up outer to inner with thicknesses, the coating type and the surface it sits on, the cavity gas and width, the spacer type, the declared whole unit U-value with the standard it was measured to, the g-value, and the light transmittance.

Those nine lines belong in the quotation as much as in the factory order, and what a replacement quotation should carry more generally is set out at what a replacement quotation should contain. Send photographs and an address to info@severnskylightsgloucester.co.uk, say which way the slope faces and what the room is used for in July, and the specification conversation starts before the survey rather than after it. Wider context is on the energy efficient rooflights page and in the guides.