How much the extra pane actually adds
Glass weighs about 2.5 kg per square metre for every millimetre of thickness, so the arithmetic is simple once you know the build-up of the unit.
A typical double glazed flat rooflight uses a toughened outer pane of around 6 mm and a laminated inner pane of around 6.8 mm, because overhead glazing needs an inner leaf that holds together if it breaks. That is roughly 13 mm of glass, or about 32 kg per square metre. A triple unit keeps both of those panes and adds a middle pane, often 4 to 6 mm, taking the glass to somewhere near 45 to 47 kg per square metre. Spacer bars, gas and sealant add a little more, and the frame adds its own weight on top.
| Clear glass size | Double (glass only) | Triple (glass only) |
|---|---|---|
| 600 x 900 mm | about 17 kg | about 25 kg |
| 1000 x 1000 mm | about 32 kg | about 46 kg |
| 1000 x 2000 mm | about 64 kg | about 92 kg |
| 1500 x 3000 mm | about 145 kg | about 210 kg |
These are worked figures for the build-ups above, not a manufacturer's data sheet. Thicker laminates, walk-on glass or acoustic interlayers push the numbers up, and the real weight of any unit you are quoted for is on its specification.
What a fixed rooflight asks of the roof
For a fixed skylight on a flat roof, triple glazing adds a load the roof structure is designed to cope with, as long as the opening is framed properly.
Put the figures in context. Structural design for a flat roof already allows for snow and for someone walking on it to maintain it, and those allowances are larger per square metre than the difference between two and three panes. On a pitched roof the comparison is even kinder: plain clay tiles, common on Cambridge terraces and village cottages, weigh in the region of 60 to 80 kg per square metre once laid, so a triple glazed roof window often weighs about the same as, or less than, the tiles it replaces.
The weight is not spread evenly, though. When joists or rafters are cut to form the opening, the load they carried moves sideways into trimmers, and the rooflight's own weight sits on the kerb or the frame around the hole. That is why the question is less "can the roof take triple glazing" and more "is the opening trimmed to take it". Our guide to double vs triple glazed rooflights compares the two on heat, light and noise as well as weight.
Opening mechanisms are where weight really matters
A heavier sash has to be lifted, held open and closed again safely, so every hinge, spring and motor on an opening rooflight has a rated limit.
A centre-pivot roof window in a loft uses a balance or friction system tuned to the sash it carries. A top-hung window relies on springs or gas struts to hold it open. An electric hinged rooflight on a flat roof pushes the glass up on a chain or spindle actuator, and each actuator is rated for a maximum load and stroke. A sliding rooflight rolls the glass along runners that have their own weight rating.
Specify triple glazing on a sash designed for double and several things can go wrong: the window will not stay open at the angle you set, the motor labours or trips, or the hinge and fixings wear early. The fix is not to avoid triple glazing, it is to choose a unit where the triple sash is the one the manufacturer designed and tested with that mechanism, and to match the actuator to the glass. On a large opening rooflight that may mean two actuators rather than one.
Older Cambridge roofs and the structure around the opening
The roofs that need checking are old ones with slender timbers, and long spans where a big pane sits in the middle of the roof.
Victorian and Edwardian terraces in Romsey, Petersfield and around Mill Road often have rafters that are small by modern standards and have carried a slate or tile roof for over a century. A 1930s semi in Cherry Hinton may have a similar story. A roof window in either is well within reach, but the survey looks at rafter size, spacing and condition before recommending a triple unit, and the trimmers either side are sized for the load they will now take. On a flat roof over a side return, a long skylight over a kitchen puts its weight onto new joists or a steel, and the engineer's calculation for that beam needs the real glass weight rather than an assumption.
New homes at Eddington or Trumpington Meadows are usually built to modern loadings, though timber I-joists and trussed rafters must never be cut without a design. Where structure is altered, the work is notifiable and we handle the Building Control notification. The Part L requirements for rooflights set the thermal side of that sign-off.
Handling, fitting and large panes
On the day, weight is mostly a handling question: the size of the crew, or the lifting gear, it takes to get the unit onto the roof and set it down cleanly.
A small triple roof window can be carried up a scaffold by two fitters. A large flat rooflight above 100 kg or so is lifted with a glass vacuum lifter or a small crane, which affects access, parking and scaffold design on a tight terraced street. None of that stops the job, but it belongs in the plan and the quote, and it is one reason the cost of a rooflight depends on size and access as well as glass.
When the extra weight earns its place
Triple glazing is worth its weight where the room below loses the most heat or feels the cold most: north slopes, rooms used in the evening and exposed roofs.
It lowers the U-value and keeps the inner pane warmer, which helps with the downdraught described in why a rooflight feels cold underneath. On a north facing slope with no winter sun to offset heat loss, the case is strong, as covered in do north facing rooflights lose more heat. The third pane does cost a little daylight, explained in do energy efficient rooflights let in less light. On a south slope over a room that needs every bit of light, a good double unit with the right coating is often the better fit. Our light planning weighs all of this for the actual room, and the energy efficient rooflights service covers the full range.
Questions people also ask
Is a triple glazed skylight too heavy to open by hand?
Not if it is designed as a triple unit. Roof windows made with triple glazing have balance springs or friction hinges set for the heavier sash, so they open with about the same effort as a double version. Problems come from fitting heavier glass into a sash built for lighter glass.
Do I need a structural engineer for a triple glazed rooflight?
For a small roof window between existing rafters, usually not. Where joists or rafters are cut and trimmed, where a long rooflight sits on a new beam, or where the roof is old and the timbers are slender, a calculation is sensible and Building Control may ask for one.
Can I put triple glazing into my existing rooflight frame?
Rarely. A triple unit is thicker and heavier, so it usually will not fit the rebate of a frame designed for double glazing, and the hinges or motor may not be rated for it. Replacing the whole rooflight with a unit designed for triple glass is the dependable route.
Does triple glazing make a rooflight more likely to break?
No. The outer and inner panes are the same toughened and laminated glass used in double units. The extra pane sits in the middle, adds insulation and weight, and does not weaken the unit.
