The short answer
Yes. A triple glazed rooflight carries a third pane of glass, so the sealed unit is heavier than a double one, and the frame is usually deeper and sturdier to hold it. As a rough guide, glass weighs about 2.5kg per square metre for every millimetre of thickness, so each extra 4mm pane adds around 10kg to every square metre of glass. On a small roof window that is easy to handle. On a large flat rooflight or a roof lantern it changes how the unit is lifted, supported and fitted.
Where the extra weight comes from
A sealed unit is a stack of panes held apart by a spacer bar. Going from double to triple adds one pane and one more cavity, and the cavities carry no real weight, only gas. So the extra mass is the glass, and how much depends on its thickness.
The pane thicknesses are not the same in every unit. Outer panes are often toughened, and inner panes laminated, which means two sheets bonded with an interlayer and therefore more weight than a single sheet of the same thickness. A laminated inner pane overhead is good practice and industry guidance, so a triple unit with a laminated inner pane carries noticeably more mass than a triple with three plain panes. Heavier glass for a very large span, chosen for strength, adds more again.
The frame adds its share. A triple glazed unit is thicker, so the frame or sash that holds it is deeper, and the structure beneath it has to carry that.
What it does to the fitting day
For a roof window, the weight is usually a handling matter. Two people can generally lift a single unit into position, and the frame is fixed to the rafters as for any other. The difference is that you do not want to fumble a heavier unit at the edge of a roof, so the plan for getting it up there matters more.
For flat rooflights, roof lanterns and large fixed glass, the weight is a lifting question. Large units may need a crane, a hoist or several people, and access has to be thought through: a narrow side passage, a terraced house with no rear access, a first-floor extension. We note these points at the survey so the lift is planned and the cost reflects it. Our page on flat roof rooflights covers how those units sit on their kerbs.
Scaffolding comes into it as well. Where a heavier unit has to be moved across a roof, a safe working platform is a necessity, and access equipment is one of the things that moves the price of a job.
Does the roof need to be stronger?
Not usually for an ordinary roof window. The rafters are already trimmed to take the opening, and the extra weight of one triple glazed unit is small compared with the weight of the roof covering it replaces. A slate or concrete tile roof is far heavier per square metre than glass.
The picture changes for a large flat rooflight or a roof lantern on a flat roof. Here the structure under the kerb has to carry the glass, the frame and the kerb itself, plus wind and any snow load. The upstand sits on a trimmed opening, and a structural engineer or the Building Control process may call for a stronger trimmer or a different support detail. Forming a fresh opening in a roof that already exists is building work regardless, since the rafters are altered. The Building Control notification is ours to make, and the weight of the chosen glazing is part of what gets recorded.
Replacing an existing rooflight with a heavier triple unit deserves a check. If the old frame was built for a lighter double unit, it may not take the new weight without alteration, which is one of the reasons a straight swap is not always possible. We look at this at the survey before specifying.
Hinges, motors and opening units
Weight matters most when the rooflight opens. A manual or electric opening unit has hinges, arms and a motor or winder rated for a particular weight. Fit heavier glass to an opening unit that was designed for a lighter one, and the mechanism works harder, wears faster and may not hold the unit securely open. This is one reason manufacturers publish combinations: a given sash and opener are designed to go together.
If you want an opening rooflight with triple glazing, choose the opener and the glass as a pair from the maker's own range, and do not try to mix them. Our page on electric opening rooflights explains how the motor and the unit are matched.
Is the extra weight worth it?
The weight is only one side of the choice. A third pane improves the U-value, makes the inner surface warmer and can cut condensation on the glass in a cold room. Against that it adds weight, thickness and cost, and can reduce the amount of daylight passing through slightly, because every added pane absorbs and reflects a little.
For a large rooflight over a kitchen or dining area, or a bedroom where you notice a cold pane, the case is often strong. For a small roof window on a landing, the gain may be too small to justify the added weight and cost. The comparison is set out in our guide to double or triple glazed rooflights, and the heat-loss background is in do rooflights lose a lot of heat?
Weight also affects the inside, because a deeper, heavier unit changes the depth of the lining and how the reveal meets the glass. We settle that before work starts, and the written finish schedule records the lining depth and shape alongside the glazing.
What to ask when you get a quote
Ask for the weight of the complete unit, or at least the glazing build-up and size, so you can see whether it will need lifting equipment. Ask whether the existing frame, kerb or roof structure will carry it, and whether any trimming or strengthening is needed. And ask how the unit will reach the roof and who is responsible for the access.
If you are unsure whether triple glazing suits your room, ask for a quote and we will survey the roof, the ceiling and the access, then price the glazing options side by side. The rest of the specification choices are covered under energy efficient rooflights.