Replacing Wired Glass Rooflights

Chapter 1

Where wired glass rooflights are still found

Wired glass was the default choice for roof glazing on small extensions, garages and lean-tos from the 1950s into the 1980s. Replacing it today swaps a single sheet of mesh-reinforced glass for a sealed, insulated unit with a safety pane on the inside, and it changes the room below as much as the roof above.

You can recognise it straight away: a square grid of fine steel wire, usually about 13mm across, set inside the glass. The glass itself is either clear and polished, so you can see the sky through the mesh, or rolled and textured, so the light comes through as a grey, even glow. It was sold as Georgian wired glass, and builders used it wherever they wanted glass overhead that would not fall into the room in pieces if it broke.

In Cambridge it turns up in predictable places. Behind the gault brick terraces of Romsey, Petersfield and the streets off Mill Road, a narrow scullery or side passage was often roofed over in the 1960s with a timber-framed wired glass light on a low kerb. On 1930s semis in Cherry Hinton, Arbury and Chesterton, a flat-roofed kitchen or garage extension added a generation later may carry a small wired glass rooflight or a short run of patent glazing with aluminium bars. Village bungalows and cottages around Histon, Cottenham, the Shelfords and Sawston have wired glass lean-tos over back doors, utility rooms and outside toilets that have since become part of the house.

Some of these units are sixty years old. The frames are timber, steel or early aluminium, the putty has hardened, and the glass is a single sheet between the room and the weather. That is the starting point for everything in this guide.

Chapter 2

What the wire was for, and what it does not do

The wire mesh was there to hold broken glass together, mainly in a fire. It was never designed to make the glass stronger, and in modern terms wired glass is not treated as a safety glass for places where people can be hurt by it.

When ordinary annealed glass breaks it falls as large, sharp shards. Wired glass also breaks into sharp pieces, but the embedded mesh keeps most of them hanging in place for a while. That behaviour made it useful as fire-resisting glazing in doors and screens, and it gave builders some comfort about glass over people's heads. The wire does nothing to resist the impact in the first place. Setting a steel mesh into molten glass actually leaves the finished sheet weaker than plain glass of the same thickness, because the wire creates internal stress and a line of weakness wherever it meets the edge.

Current practice for glazing overhead is built around a different idea. The inner pane of a modern rooflight is laminated: two or more layers of glass bonded to a clear interlayer that holds the whole pane together if it cracks. The outer pane is usually toughened, which is several times stronger than annealed glass and breaks into small, blunt fragments rather than blades. Guidance on sloping and overhead glazing expects glass above people to stay in place when it fails, and a laminated inner pane does that far more reliably than a wire grid.

So the case for replacement is not only about age. A wired glass rooflight over a kitchen, a bathroom or a hallway sits above people every day, and it was specified to a standard that has since moved on. If yours already has a crack running through it, our guide to replacing a cracked rooflight covers making the opening safe before the new unit goes in.

Chapter 3

Why old wired glass cracks on its own

Wired glass can crack without anyone touching it. The two usual causes are rust in the mesh at the edges and heat stress on a sunny slope, and both get worse with age.

Where the glass has been cut, the ends of the wire are exposed at the edge of the sheet. Over decades, water gets past the old putty or glazing tape and reaches those ends. Steel expands as it rusts, and the swelling wire pushes the glass apart from the inside. The result is a short crack starting at the edge, often at a corner, that slowly lengthens. You may also see brown staining in the mesh near the frame, which is the rust showing through.

The second cause is thermal stress. Glass cracks when one part of a sheet is much hotter than another. On a rooflight the centre of the pane heats up in direct sun while the edges stay cool inside the frame, and the dark wire absorbs more heat than the glass around it. A south or west facing wired pane that is partly shaded by a chimney, a gutter or an overhanging tree can crack on a bright spring morning after a cold night. The crack usually starts at the edge and runs across the pane at a right angle to it.

Neither of these can be stopped once the process has begun. A cracked or rusting wired pane is simply at the end of its life. The practical question is not whether to replace it, but what to replace it with, and whether the old frame has any part to play. Our guide to glass only or full unit replacement explains that choice in general. With wired glass the answer is almost always the full unit, because a single-glazed frame cannot take a thick sealed unit.

Chapter 4

The warmth gap: single glazing against current standards

A single sheet of wired glass loses heat roughly two and a half times as fast as the limit set for a new rooflight today. In a small room with a large rooflight, that one pane can be the coldest surface in the house.

Heat loss through glazing is measured as a U-value, in watts per square metre per degree of temperature difference. The lower the number, the better. Single glazing sits at around 5.5 to 5.8. Approved Document L, in its 2021 edition, sets a limiting U-value of 2.2 for a new or replacement rooflight in an existing home, now measured with the unit lying horizontal, which is how a flat rooflight actually sits. A good modern double glazed unit comes in well under that figure, and triple glazing lower still.

You notice the difference in three ways. The room holds its warmth on a winter evening instead of losing it straight up through the roof. The cold downdraught under the glass, which makes a small utility room or a back hallway feel chilly even with the heating on, goes. And condensation stops running down the inside of the pane and dripping off the frame, because the inner surface of a sealed unit stays much closer to room temperature. That last point matters in bathrooms and kitchens, where a single-glazed rooflight can leave the ceiling around it permanently damp.

Chapter 5

What replaces it: matching the new unit to the roof

The right replacement depends on what the wired glass was sitting in. A flat roof, a pitched roof and a run of patent glazing each lead to a different kind of unit, and the choice is also a chance to rethink the size and the glass.

On a flat roof, the old light usually sits on a low timber kerb, sometimes only 50mm to 100mm above the roof surface. A modern flat rooflight needs an upstand of at least 150mm above the finished roof so that water and snow cannot reach the frame. That usually means building a new, taller insulated upstand and dressing the roof covering up it. On a small opening a flat glass rooflight is the natural choice; on a larger one, a lantern may give the room more height and a better spread of light. Our guide to replacing an old flat rooflight goes into the upstand in detail.

On a pitched roof, a wired glass light is usually a small steel or timber frame let into the slates or tiles, often over a stair or a box room. The replacement is a modern roof window or, on an older cottage or in a conservation area, a conservation rooflight that sits flush with the roof covering and has a slim central bar. Both come with their own flashing kit, which is a far better defence against the weather than the lead dressing and putty on the old frame.

A run of patent glazing, with long wired sheets held between aluminium or lead-capped bars, is common on lean-tos and covered side passages. These can be reglazed with a modern glazing bar system and sealed units, or the whole roof can be rebuilt as a solid, insulated roof with one or two rooflights set into it. That second route suits a lean-to that is now used as a proper room, and it is the same thinking behind replacing a conservatory roof with a lantern.

The new unit does not have to be the same size or even in the same spot. A wired glass light was placed where it was easy to frame, not where the room needed light. If the size or position is worth changing, see repositioning a rooflight when replacing it.

Chapter 6

How the light in the room changes when the grid goes

Taking out wired glass changes the character of the light, not just the amount of it. The soft, grey, shadow-gridded glow of the old pane is replaced by clear daylight, and on a sunny slope that includes direct sun you may not have had before.

Textured wired glass scatters light. It spreads the brightness evenly and hides the sky, which is why many of these rooms feel dim but never glaring. Clear wired glass lets more through but throws the shadow of the mesh across the floor and the walls as a moving grid. Both lose light to dirt that has built up over decades in the texture and at the frame edges. When the old sheet comes out and clear, low-iron glass goes in, the room gets brighter and the colours in it look truer.

That is good news on a north or east facing slope, where the light stays soft and cool. It needs thought on a south or west slope. In Cambridge the land is flat and there is little to shade a roof, so a clear south facing rooflight will put a sharp sun patch on the floor for several hours around the middle of the day, and a west facing one will bring low, warm sun into the room in the late afternoon and evening. Over a kitchen worktop, a desk or a television, that can mean glare and summer heat that the old textured pane used to mask.

Before we quote, we work out which way the slope faces and where that sun patch will fall through the day and the year. The glass is then chosen to suit: a neutral solar control coating on a south or west slope, a clearer high light transmission glass on a north slope, and a satin or obscured inner pane where privacy or diffused light matters, such as in a bathroom. The written light plan that comes with the quote sets that out, and our page on how we plan the light explains the method in full.

Chapter 7

Fire, boundaries and the regulations for the new unit

Wired glass was sometimes chosen for fire reasons, and that history matters when it is replaced. The new unit has to meet today's rules on heat loss and safety, and in some positions the rules on fire as well.

Replacing a whole rooflight is controlled work under the Building Regulations. The new unit has to meet the Part L limit on heat loss, and the work is notifiable, which means Building Control must be told. We handle that notification where the work needs it, so it is recorded and you receive the paperwork you will want when the house is sold. Replacing only the glass in an existing frame is generally treated differently, but with wired glass the old frame is rarely worth keeping. Our guide to Building Control for replacement rooflights covers the process step by step.

Part B, which deals with fire, is where wired glass has its own twist. A garage or a lean-to built close to a boundary, or a flat roof next to a party wall with the neighbouring terrace, may have had wired glass fitted because it was the fire-resisting option at the time. Roof glazing near a boundary or within about 1.5 metres of a party wall has to be looked at carefully, and in some cases a rooflight should not sit in that zone at all. At the survey we note where the old light is in relation to the boundary and the party wall, and if the position is a problem we recommend moving the new unit to a better place in the roof rather than repeating the old one.

Planning permission is not usually needed for a like-for-like replacement on a house. Things change if the new rooflight will project further, be larger, sit in a listed building, or be on a roof covered by an Article 4 direction in one of the city's conservation areas. Greater Cambridge Shared Planning handles those questions for both Cambridge City and South Cambridgeshire, and we check the constraints before anything is ordered.

Chapter 8

How the replacement is carried out

A wired glass rooflight replacement is usually a one or two day job, planned so the room is open to the sky for as short a time as possible. Most of the work happens on the roof, with a short spell inside to finish the reveal.

1. Survey and light plan. We measure the opening, check the kerb, the frame and the roof covering around it, confirm the slope and orientation, and look at how the room is used. The written quote and light plan set out the size, position, glass and upstand.
2. Checking old materials. Some glazing putties, mastics and roof sheets from the 1960s and 1970s can contain asbestos. If anything around the old light looks as though it might, a sample is tested before work starts, and any removal is done by the right people.
3. Removing the old light. Dust sheets go down inside. The wired sheet is taken out whole where possible, or taped and broken out under control if it is already cracked, so that no loose glass falls into the room.
4. Building the new upstand or trimming the opening. On a flat roof the new insulated upstand is built and the roof covering dressed up it. On a pitched roof the opening is trimmed to the new size and the flashing kit fitted.
5. Fitting the new unit. The rooflight, lantern or roof window is fixed, sealed and checked for fall and drainage.
6. Finishing inside. The reveal is insulated, lined and plastered, and the room is cleared and cleaned. A splayed reveal is worth considering here, as it spreads the light further across the ceiling.

The old wired glass and frame are taken away with the rest of the waste. Wired glass is harder to recycle than plain glass because of the steel, so it is separated and sent to a handler that can deal with it; our answer on what happens to the old rooflight goes into disposal in more detail. The finished work is covered by our 10-year workmanship guarantee, alongside the manufacturer's warranty on the unit itself.

Chapter 9

Questions people ask

Is wired glass the same as safety glass?

No. The wire holds broken pieces together for a time, especially in a fire, but it does not make the glass stronger, and standard wired glass is not treated as safety glass where people could be hurt by it. A modern rooflight uses a laminated inner pane, which holds together if it cracks, and a toughened outer pane.

Can I just put a sealed unit into the old wired glass frame?

Rarely. The old frames were made for a single sheet about 6mm to 7mm thick, and a double glazed sealed unit is three or four times that depth. The frame also has no thermal break, so it would stay cold and gather condensation. Replacing the whole unit gives a better result and meets the current rules on heat loss.

Will the room be brighter after the replacement?

In nearly every case, yes, because clear modern glass lets through more light than aged, textured wired glass and slimmer frames block less of the opening. On a south or west slope the extra light comes with more sun and heat, so the glass and shading are chosen to match. Our answer on whether a new rooflight will be brighter explains when coatings trade a little light for comfort.

Wired glass replacement is one part of our wider misted and failed rooflight replacement service across Cambridge and the villages around it. To see where the light will land in your room with a new unit, book a light survey or message us on WhatsApp.