Often, yes, but not because commercial glass is a separate product range. The same manufacturers and the same laminated and toughened build-ups appear in both. What changes is the duty the glazing is put to: bigger spans, higher above the floor, a roof people cross, larger solar gains in a deep open space, and in some buildings a fire requirement. Those duties push the specification, and the specification is usually thicker, heavier and more expensive than a domestic equivalent.
Where a commercial specification starts to diverge
A domestic rooflight in a kitchen extension is typically 1000mm by 1500mm, two metres above a floor, over a room used by four people who know it is there. Almost any competent overhead build-up serves it. Change any one of those variables substantially and the specification moves.
The four that move it most are span, height above the floor, whether anyone can get onto the roof, and how much sun the space receives against how much heat it can lose. A commercial building usually changes at least two of them at once, which is why the answers diverge quickly even though the starting products are the same.
Overhead glazing rules are the same, the spans are not
Any glazing above head height, domestic or commercial, has to be designed so that failure does not drop sharp pieces on people. In practice that means laminated glass in the inner pane, so a broken pane holds itself together on its interlayer rather than falling. That is not a commercial rule, it is a rule for anything overhead.
What differs commercially is that the panes are larger. Glass thickness climbs with span, and it climbs faster than most people expect, because deflection under load rises with the fourth power of the span while stiffness rises with the cube of thickness. A domestic unit may be fine at 6mm outer and a 6.8mm laminated inner. Double the span and the same build-up will not do, and you find yourself at 10mm toughened outer over an 11.5mm laminated inner with a weight that needs mechanical lifting.
Non-fragility drives the build-up thicker again
If anyone can reach the roof, the glazing has to be judged against the possibility of somebody falling onto it. A rated non-fragile assembly needs a laminated outer pane as well as a laminated inner, with a thicker interlayer than the domestic norm, and it needs to be fixed in the tested configuration for the rating to mean anything.
That single requirement often doubles the cost of the glass and adds enough weight to change the lifting method. It is also non-negotiable on a roof with plant on it, because someone will be up there servicing the plant. Our page on what a non-fragile rooflight is explains what the rating covers and, just as importantly, what it does not.
Solar gain in a large open volume
A domestic rooflight overheats one room and the occupant opens a door. A large area of roof glazing over an open plan office or a retail floor puts a serious heat load into a space that may have no openable windows at all, and the mechanical cooling to remove it runs for years.
The lever is the g-value, the fraction of solar energy that gets through. A standard clear double glazed unit passes a high proportion of it. A solar control coating on the outer pane cuts that substantially while keeping most of the visible light, which is the trade worth making on a south-facing commercial roof. It costs more per square metre and it saves plant capacity and running cost, and on any reasonably sized scheme that arithmetic is easy.
The counter-case is a north-lit workshop or a building that is heated more than it is cooled, where the coating throws away useful winter gain for no benefit. It is a calculation, not a default.
Height above the floor and what a failure means below
The consequence of a glass failure scales with the drop and with who is underneath. Three metres above a private office is a different risk from 10 metres above a supermarket aisle or a school hall. At greater heights the inner laminated pane does more of the work and the interlayer thickness matters more, because the pane has to stay in its frame long enough for the area to be cleared.
Some specifications go further and call for the inner pane to be heat soaked, a process that provokes toughened glass containing nickel sulphide inclusions into failing in the factory rather than years later on the roof. Spontaneous failure in toughened glass is rare, but over a public space the cost of heat soaking is small against the consequence.
Where plastics still beat glass
Glass is not automatically the right answer on a commercial roof. On a large sheeted warehouse roof, in-plane GRP or multiwall polycarbonate is lighter, cheaper per square metre, diffuses light more evenly along an aisle, and matches the profile of the metal so the weathering detail is the same as the rest of the roof. Glass in that position would need a kerb, a trimmed opening and a lifting operation for every unit.
The trade is service life and appearance. Plastic sheets weather, yellow and lose their non-fragile rating with time, and glass does not to anything like the same degree. Where a client wants views out and a crisp finish, glass wins. Where they want cheap even daylight over 2,000 square metres of shed, it usually does not.
Acoustic build-ups near plant, roads or flight paths
A rooflight is a hole in the acoustic envelope of a building, and on a commercial roof there is often something loud on the roof already: condensers, extract fans, an air handling unit. In a meeting room, a treatment room or a hotel bedroom below that plant, the standard unit will be the weak point.
The fix is an asymmetric build-up, different thicknesses in the two panes so they do not resonate together, with a wider cavity and an acoustic interlayer in the laminated pane. It adds weight and cost and it works. Where a client tells us the space below is noise-sensitive, we raise this at survey rather than after the unit is in and someone is complaining about the fan.
What we need before anyone specifies glass
Six things settle it: the finished size of each opening, the height from finished floor to the underside of the glass, whether the roof is ever accessed and how, the orientation and shading, what the space below is used for, and whether any fire, acoustic or thermal figure has been set by a designer or by the building’s approval. If a fire rating has been called for, that comes from your designer rather than from us, and our page on fire ratings for commercial rooflights covers where that requirement originates.
Give us those six and we can put a real specification against a real number rather than pricing a guess. Call 01865 704245 or email info@heritageskylightsoxford.co.uk. We install VELUX, Fakro, Keylite and Roto units and fabricated units where the sizes demand it, installations and replacements only, with a ten-year workmanship guarantee behind the perimeter detailing.
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