Conservation Rooflight Glazing: Safety, Self-Cleaning, Solar Control, Acoustic and Security Options

The frame of a conservation rooflight gets most of the attention, but the glass does most of the work. This chapter explains what sits inside a typical sealed unit, what each glazing option actually does, and which questions to put to the manufacturer before you order.

This chapter sits within our Conservation Rooflight Guide. Frame materials, sightlines and whole-unit U-values are covered in the rooflight comparison chapter; here we stay with the glass.

What is inside a conservation rooflight sealed unit?

A conservation rooflight sealed unit is two panes of glass bonded to a spacer around the edge, with a sealed, gas-filled cavity between them. Each layer has a job, and knowing them makes a manufacturer’s specification sheet much easier to read.

Working from the outside in, a typical build looks like this:

Layer	What it is	What it does
Outer pane	Toughened (tempered) glass, often with a self-cleaning coating on the weather face	Resists thermal stress, hail and handling; breaks into small blunt fragments if it fails
Cavity	A sealed gap between panes	Slows heat loss by limiting conduction and convection
Gas fill	Usually argon	Denser than air, so it conducts less heat across the cavity
Spacer	A “warm-edge” spacer bar with desiccant, sealed with primary and secondary sealants	Holds the panes apart, keeps moisture out, and reduces cold bridging at the edge of the glass
Inner pane	Laminated glass with a low-emissivity (low-e) coating facing the cavity	Low-e reflects heat back into the room; the laminate holds together if broken

The inner pane is the one to check first. A laminated inner pane is widely recommended for overhead glazing, because if the glass is broken the interlayer keeps the fragments bonded in place rather than letting them fall into the room. Check the exact build with the manufacturer, and note that the BS 6262 series covers glazing in buildings more generally.

The warm-edge spacer matters on a rooflight because the glass edge is the coldest part of the unit, and condensation there runs straight down to the cill. We cover condensation more broadly in our note on condensation.

How do conservation rooflights keep a single-glazed appearance?

They keep a single-glazed look by hiding the sealed unit’s edges behind a fronting that reads like traditional putty. Historic cast iron and wrought iron rooflights held a single sheet of glass bedded in linseed oil putty, and the visual test is whether the new unit looks like that from the ground.

Manufacturers approach this differently. Clement, for example, uses a silicone fronting shaped to resemble putty. Whatever the method, look for three things:

• A narrow, even bead line at the frame edge, with no visible spacer or sealant behind it
• No reflective strip where the spacer would otherwise catch the light
• A glazing bar that belongs to the design, not an applied strip that emphasises a modern unit behind

A structural bar divides two separate units; an applied bar sits on the face of one larger unit. Conservation officers often ask which is proposed. We explain how to specify it in the specification chapter, and we would suggest settling it before the glazing options below, because it changes the size and number of sealed units.

How does self-cleaning glass on a rooflight work?

Self-cleaning glass uses a thin coating on the outer face that works in two steps. First, daylight activates the coating so that it slowly breaks down organic dirt, such as leaf residue, pollen and bird droppings (a photocatalytic effect). Second, the same coating makes the surface hydrophilic, which means rain spreads into a thin sheet rather than forming beads, and that sheet rinses the loosened dirt away.

The coating does not repel water; it attracts it, so the glass wets evenly and dries with fewer spots. It depends on two inputs: daylight to break the dirt down, and rain to wash it off.

That is why self-cleaning glass is useful but not a cure-all:

• Shallow pitches drain slowly. Water and debris linger longer on a low slope, and the sheeting action is weaker. A rooflight near the bottom of its pitch range will need more manual cleaning.
• Sheltered slopes miss the rain. A rooflight under an overhanging parapet, a tall chimney stack or trees may get too little direct rain to rinse properly.
• North-facing and shaded slopes get less daylight, so the breakdown of organic dirt is slower.
• Inorganic dirt is not broken down. Mortar dust, soot, lichen spores, grit and paint overspray need washing off.
• Heavy deposits overwhelm it. Leaves, moss and tile chips still need clearing, particularly from the head of the frame.

The coating can also be damaged. Abrasive pads, scrapers and some household glass cleaners can scratch or strip it, and silicone-based products can leave a film that stops the glass wetting evenly. Follow the manufacturer’s cleaning advice exactly. The Rooflight Co.’s guidance, for example, recommends warm soapy water, a soft cloth, no abrasives, and a rinse; de-ionised water reduces streaking. Our maintenance chapter sets out a full routine.

On a well-exposed slope at a sensible pitch, a self-cleaning outer pane noticeably stretches the interval between cleans. On a sheltered, shallow or shaded slope, expect it to help rather than to solve the problem.

Do you need solar control glass in a rooflight?

You need solar control glass when a rooflight faces the sun over a room that is likely to overheat, particularly a small room, a bedroom, or a top-floor space in a new dwelling. A rooflight admits far more solar heat than a vertical window of the same size, because it faces the sky and catches the sun for much of the day, including in summer when the sun is high.

The figure to look at is the g-value, or total solar energy transmittance. It is expressed as a fraction between 0 and 1; the lower the number, the less solar heat gets through. For reference, The Rooflight Co. publishes a solar energy transmittance of 0.51 for its standard Conservation Rooflight glazing (EN 410). Solar control glass lowers that figure, usually at some cost to light transmittance and sometimes with a slight tint or reflectance.

In England, Approved Document O addresses overheating in new residential buildings, and rooflights are one of the high solar gain elements that the assessment picks up. That includes new dwellings created by conversion, so a barn with several south-facing rooflights over first-floor bedrooms can face real scrutiny. The barn conversions chapter deals with layout; the glazing is the other half of the answer.

Specify it by slope and room, not across the board. North-facing rooflights rarely need it; south and west slopes over bedrooms are the usual candidates. Some solar control coatings look darker or more reflective from outside, so on a listed building ask for a sample and discuss it with the conservation officer. An internal blind helps with glare but does less against heat than glass that rejects it before it enters.

How much light does a conservation rooflight let through?

A standard double-glazed conservation rooflight typically lets through around 70% of visible daylight. The Rooflight Co. publishes a light transmittance of 0.71 for its standard glazing (EN 410), which is a reasonable benchmark for a low-e, argon-filled double unit.

Solar control coatings, extra laminates, a third pane or a tinted interlayer all reduce that figure. Because a rooflight faces the sky, it delivers far more daylight per square metre than a vertical window, which is why a small, well-placed unit is often enough. Ask for the light transmittance of the exact build you are ordering, not the standard one.

Can glazing reduce rain noise on a rooflight?

Yes. Rain drumming on glass is the most common acoustic complaint about rooflights, and an acoustic laminated interlayer is the main glazing answer. Glass is hard and resonant, so heavy rain or hail on an overhead pane can be noticeably louder than on the surrounding slates or tiles.

An acoustic laminate uses a special interlayer that damps vibration in the glass, so less of the impact noise passes into the room. It can be used in the inner laminated pane, which is already there for overhead safety, so the upgrade is often a change of interlayer rather than an added layer.

Ask the manufacturer which pane carries the interlayer and how it affects weight. If the concern is traffic or aircraft rather than rain, say so, as the best build may differ. For bedrooms under rooflights, particularly in timber-lined attic rooms with little mass, acoustic glazing is worth pricing at the outset. It is much harder to add later.

What security options are there for conservation rooflights?

Security for a conservation rooflight comes from three things working together: the glass, the locking, and whether the unit needs to open at all. Focus on units that can be reached from a flat roof, lean-to, balcony or low single-storey eaves. For those, the options typically include:

Measure	What it does	Trade-off
Laminated outer pane or enhanced laminate	Resists attempts to break through the glass; holds together when struck	Extra weight and some cost
Lockable winders or handles	Stops the sash being prised or wound open from outside	Keys to manage; check escape requirements
Restricted opening	Limits how far the sash can open	Reduces ventilation
Fixed unit	Removes the opening sash altogether	No ventilation or escape from that opening

Where a project is being designed to Secured by Design principles, for example on some new-build or conversion schemes, ask the manufacturer directly what their rooflights can achieve and what evidence they can provide. We do not list product certifications here, because they change and depend on the exact configuration.

Two cautions. First, do not lock or fix a rooflight that is needed as an escape window. The escape requirements in Approved Document B (paragraph 2.10) cover the clear opening size, sill height, and the need for the window to stay open unheld; a key-locked escape rooflight defeats the purpose. Second, a motorised rooflight left open is not a secure position; The Rooflight Co. also notes that an open motorised unit is not locked and can be closed by wind.

Are fire-rated glazing options available for rooflights?

Fire-rated glazing is available for some conservation rooflights, but it is a specialist option and must be confirmed with the manufacturer. Clement lists fire glazing among its options, alongside security, solar control and acoustic glass.

Fire performance depends on the whole assembly, not the glass alone. Where it is needed, involve building control early and ask the manufacturer what tested configurations they can supply.

Is triple glazing worth it in a conservation rooflight?

Triple glazing is rarely the best choice for a conservation rooflight. It adds a third pane and a second cavity, which improves the centre-pane U-value, but it also adds weight, thickness and cost, and reduces light transmittance.

On a slim frame meant to sit flush with the roof, the extra depth tends to make the frame bulkier, and the weight loads winders and hinges. In most heritage roofs, a good argon-filled double unit with a warm-edge spacer and low-e inner pane, plus careful insulation around the lining, gives a better overall result.

Ug or Uw: which U-value matters for rooflight glazing?

Both matter, but for different things. Ug is the centre-pane value of the glass alone. Uw is the whole-window value, which includes the frame and the edge of the glass, and it is usually the higher (worse) figure.

For example, The Rooflight Co. publishes Ug 1.0 W/m²K (EN 673) and Uw 1.4 W/m²K (EN ISO 10077-2). Clement publishes a centre-pane value of 1.2 and Uw 1.5 W/m²K, with a BFRC Window Energy Rating of A+.

Glazing options can nudge both figures. Approved Document L limiting values, and the approach for listed buildings and change of use, are set out in the comparison chapter. If you change the glass build from the standard, ask for the revised Uw.

What should you ask the manufacturer about glazing?

Ask for the exact glass build in writing, with the performance figures for that build rather than the standard unit. This checklist covers the points that most often get missed.

Question	Why it matters
What is the full build, pane by pane, including thicknesses and interlayers?	Confirms the inner pane is laminated and shows what you are paying for
What gas fill and spacer type are used?	Affects edge temperature, condensation and U-value
What are Ug, Uw, light transmittance and g-value for this build?	Needed for Part L, Part O and daylight checks
How is the unit fronted to give a putty-line appearance?	Key to the visual result on a listed building
Are acoustic, solar control, security or fire options available in this size, and do they change the weight?	Options can limit size, add lead time and load the operating gear
Can I see a glass sample?	Tints and coatings can look different on the roof
What is the guarantee on the sealed unit, and what voids it?	Glazing unit guarantees are commonly around five years

Record the answers on the order and in the approval drawings. The ordering chapter explains how to handle sizes, bespoke units and approval drawings.

How does rooflight glazing affect the rest of the roof?

Glass sheds water quickly, and a rooflight passes everything that falls on it down the slope in a concentrated stream, so the eaves gutter becomes part of the same specification. If the gutters are undersized, cracked or poorly jointed, deal with them while the scaffold is up. Our roof works chapter covers gutter capacity and inspection in detail, and the installation chapter covers flashings.

Common mistakes

• Assuming self-cleaning means no cleaning. On shallow, sheltered or shaded slopes, the coating helps but does not remove the need for regular washing.
• Describing self-cleaning glass as water-repellent. It is the opposite. The coating attracts water so that rain sheets off; treating it with a water-repellent product can stop it working.
• Leaving the inner pane unspecified. Confirm that the inner pane is laminated, especially over stairs, beds and circulation areas.
• Adding solar control glass everywhere. On north slopes it simply costs light. Specify it by orientation and room use.
• Locking an escape rooflight. Security measures must not compromise the escape requirements.
• Changing the glass after approval. Tinted or reflective options can alter the appearance the conservation officer agreed to.
• Ignoring the gutters. Water shed from the glass concentrates at the eaves.

Illustrative scenarios

The scenarios below are illustrative composites drawn from typical projects. They are not specific buildings or Tuscan Foundry commissions.

Where to go next

Other rooflight specialists are listed in the Building Restoration Index rooflights category. Unfamiliar terms are explained in the glossary.

Tuscan Foundry supplies cast iron gutters, downpipes and hopper heads to BS 460 for listed and traditional buildings, with an in-house linseed oil paint finish. Rooflight glass sheds water quickly and in concentrated streams, so if the roof is scaffolded for rooflight work, it is the cheapest moment to inspect, repair or replace the rainwater goods below. Stock items dispatch promptly; bespoke and copy-cast items take around 12–14 weeks, so order at the start of the programme.

Frequently asked questions

What glass is used in a conservation rooflight?

Most conservation rooflights use an insulated double-glazed sealed unit. The outer pane is toughened glass, often with a self-cleaning coating, and the inner pane is laminated glass with a low-emissivity coating. The cavity is usually argon-filled and the panes are held apart by a warm-edge spacer. The unit is fronted so that it looks like a single sheet of glass bedded in putty.

Does a rooflight need laminated glass?

A laminated inner pane is widely recommended for overhead glazing, because the interlayer holds broken glass in place rather than letting it fall into the room. Many conservation rooflights include one, but you should confirm the exact glass build with the manufacturer before ordering.

How does self-cleaning glass work on a rooflight?

The outer pane has a coating that uses daylight to slowly break down organic dirt, and it also makes the glass attract water so that rain spreads into a thin sheet and rinses the loosened dirt away. It needs both daylight and rain to work, so it is most effective on well-exposed slopes at a reasonable pitch.

Is self-cleaning glass water-repellent?

No. Self-cleaning rooflight glass is hydrophilic, which means it attracts water rather than repelling it. Rain spreads evenly across the surface and runs off in a sheet. Applying a water-repellent product to it can stop the coating from working properly.

Will self-cleaning glass work on a shallow or sheltered roof?

It will help, but it will not remove the need for cleaning. Shallow pitches drain slowly, sheltered slopes get less rain, and shaded slopes get less daylight. Inorganic dirt such as dust, soot and grit is not broken down by the coating and still needs washing off.

What is a good light transmittance for a rooflight?

A standard double-glazed conservation rooflight typically lets through around 70% of visible light. The Rooflight Co. publishes 0.71 for its standard glazing. Solar control coatings, extra laminates or a third pane will reduce this, so ask for the figure for the exact build you are ordering.

What is the g-value of a rooflight?

The g-value is the proportion of solar energy that passes through the glass, expressed between 0 and 1. A lower figure means less heat gain. The Rooflight Co. publishes 0.51 for its standard Conservation Rooflight glazing. Solar control glass lowers the g-value to help prevent overheating.

Do I need solar control glass in my rooflight?

You may need it if the rooflight faces south or west over a room that could overheat, such as a bedroom. In England, Approved Document O covers overheating in new residential buildings, and rooflights are a high solar gain element. North-facing rooflights rarely need solar control glass.

How can I reduce rain noise from a rooflight?

The main glazing option is an acoustic laminated interlayer, which damps vibration in the glass and reduces the drumming of rain and hail. It can often be used in the inner laminated pane. Adding mass to the ceiling lining around the rooflight can also help.

How do I make a rooflight more secure?

Focus on rooflights that can be reached from a flat roof, lean-to, balcony or low eaves. Options include a laminated or enhanced laminated outer pane, lockable winders, restricted opening, or a fixed unit. Never lock a rooflight that is needed as an escape window.

Can you get fire-rated conservation rooflights?

Some manufacturers offer fire-rated glazing options. Clement lists fire glazing among its options. Fire performance depends on the whole assembly, not just the glass, so confirm tested configurations with the manufacturer and involve building control early.

Is triple glazing better for a conservation rooflight?

Usually not. Triple glazing improves the centre-pane U-value but adds weight, depth and cost, and reduces light. On a slim, flush conservation frame, a good double-glazed unit with argon, a warm-edge spacer and a low-e inner pane generally gives a better overall result.

What is the difference between Ug and Uw?

Ug is the U-value at the centre of the glass alone. Uw is the whole-window value, including the frame and the glass edge, and it is usually higher. Building regulations compliance is assessed on whole-unit values, so make sure you are quoting Uw.

Can I change the glass in an existing conservation rooflight?

In many cases a failed or misted sealed unit can be replaced, but it depends on the frame design and how the unit is fronted. Contact the original manufacturer with the serial number or order details if you have them, and check whether the replacement affects any guarantee.