Three numbers people mean by "thick"
When someone asks how thick a flat skylight is, they usually want one of three measurements, and the answer changes a great deal depending on which.
The first is the glass itself: the sealed unit of two or three panes with gas-filled cavities between them. The second is the rooflight as a product, meaning the glass plus whatever frame holds it, measured from the underside of the frame where it sits on the kerb to the highest point of the glass. The third is the whole assembly as it stands on the roof: kerb, frame and glass together, measured from the finished membrane to the top of the pane. Manufacturers' data sheets tend to quote the first two. Neighbours, planners and anyone looking out of a first-floor window see the third.
There is also a fourth depth that nobody sees from outside but everyone notices from the room: the internal reveal, the lined shaft between the ceiling and the underside of the glass. That is the one that decides how much of the sky actually reaches the floor, and it is covered further down this page.
Framed and frameless units carry their depth differently
A frameless rooflight is little more than the glass unit bonded to a kerb, while a framed unit sits a separate aluminium or uPVC frame on the kerb and holds the glass in that.
That difference shows up directly in height. On a frameless unit the glass stack is almost the whole product: a double-glazed unit with a toughened outer pane, a warm-edge spacer and a laminated inner pane typically comes to around 30 to 40mm. A framed unit wraps that same glass in a profile that contains the thermal break, the drainage channels and the fixings, and that profile usually stands somewhere between 50 and 100mm above the kerb, depending on the maker and whether the unit opens. Opening units sit taller again, because the sash needs its own frame inside the fixed outer frame, plus room for the actuator. The trade-offs of each type (weathering, sightlines, how water leaves the glass) are set out in our guide to framed vs frameless flat rooflights, and the choice between a fixed or venting unit in fixed vs opening flat rooflights.
| Part | Typical depth | Notes |
|---|---|---|
| Double-glazed unit | 28 to 40mm | Toughened outer, laminated inner |
| Triple-glazed unit | 40 to 60mm | Heavier; frame must suit |
| Framed unit (frame and glass) | 50 to 100mm | Opening sashes add more |
| Kerb above finished roof | 150mm minimum | Often 150 to 250mm |
These are general figures from across the market, given as a guide. The actual depth of any unit comes from the manufacturer's section drawing, and we check it against the roof before ordering.
What sits underneath: the kerb and the roof build-up
Below the frame, the kerb lifts the glass clear of standing water, and below the kerb, the roof itself has a depth that the opening has to pass through.
Industry guidance asks for at least 150mm between the finished roof surface and the top of the kerb, so the membrane can be turned up and dressed properly and a heavy downpour cannot overtop it. The kerb may be site-built in timber and insulated, or a factory-made insulated upstand supplied with the rooflight; our guide to flat rooflight upstands explains the options, and how high a flat rooflight upstand should be covers when a taller one makes sense.
Under the kerb is the roof: a membrane, a deck, perhaps 100 to 150mm of rigid insulation on a warm deck, joists of 150 to 225mm on a typical single-storey extension, then plasterboard. Add those up on a newer side return behind a Romsey or Petersfield terrace and the roof alone is often 350 to 450mm deep before the kerb is counted. On an older cold-deck kitchen roof it may be less, but the insulation usually has to be improved when the opening is formed, which brings the depth back up.
- Joists, with doubled trimmers either side of the opening to carry the loads of the cut joists around it.
- Deck (usually 18mm ply or OSB) with a vapour control layer laid on it.
- Insulation boards above the deck (a warm roof), often tapered to give the roof its falls.
- Waterproof membrane dressed up the upstand and over its top, so the joint is lapped, not sealed.
- Upstand, or kerb: insulated timber, finishing at least 150mm above the finished roof surface to keep standing water and splash off the joint.
- Rooflight unit fixed over the kerb, its frame skirt shedding water outside the membrane turn-up.
- Plastered reveal lining the opening from the ceiling up to the frame, finished white to bounce the light down.
- Roof falls to an outlet: designed at about 1:40 so the finished roof never falls less than 1:80.
Put together, a frameless rooflight on a 150mm kerb typically stands around 180 to 200mm proud of the roof. A framed unit on the same kerb is more like 220 to 260mm. From the ceiling to the top of the glass, the whole stack can easily exceed 600mm.
How the depth of the reveal shapes the light
The internal reveal acts like a short chimney for daylight: the deeper and narrower it is, the more it trims the low winter sun and the more it narrows the area of sky the room can see.
Cambridge sits at about 52 degrees north. At noon on the winter solstice the sun is only around 14 degrees above the horizon; at the equinoxes around 38 degrees; at midsummer about 61 degrees. Take a 1000mm wide opening with a 500mm deep straight-sided reveal. In June the shaft shades only the first 280mm or so of the opening, so most of the direct sun reaches the room. In March and September the shadow grows to about 640mm, and more than half the direct sun lands on the reveal lining instead of the floor. In December the reveal shades the whole opening, and the room gets only diffuse sky light.
That is not always bad news. Over a south-facing kitchen in a new Trumpington or Eddington home, a deep reveal is a free form of shading that takes the edge off summer glare while still passing bright sky light. Over a dark north-facing side return, where every bit of sky counts, the same reveal is working against you. There are three ways to reduce its effect: choose a lower frameless unit to shave the stack, splay the reveal so it widens towards the ceiling, and finish the lining in a pale, matt paint so the light it does catch is bounced down into the room. We work this through on every light plan, together with where the skylight is positioned in the room.
When the overall height becomes a planning or design point
For most rear extensions the height of a flat rooflight above the roof is a detail nobody notices, but in a few situations the number matters and should be checked early.
Permitted development rules for rooflights on an existing roof include a limit on how far they project beyond the roof plane, and a framed unit on a standard kerb can exceed that. Whether it applies depends on the house, the roof and whether the work is part of a new extension, so it is worth reading whether a flat rooflight needs planning permission before choosing the unit. Height also matters behind a parapet, where the glass may need to stay below the parapet line to keep it out of view from the street; on a flat roof seen from bedroom windows above, where a slim frameless profile looks far tidier; and on green roofs, where the substrate depth pushes the kerb up further still. Our main page on flat roof rooflight installation in Cambridge covers how we weigh these on the survey.
Questions people also ask
Is triple glazing much thicker than double on a flat rooflight?
The glass unit is typically 12 to 20mm deeper, and noticeably heavier. On a framed rooflight that extra depth is usually absorbed within the frame profile; on a frameless unit it adds directly to the height above the kerb. Our guide to flat rooflight glazing choices explains when triple glazing earns its weight on near-horizontal glass.
Does the thickness of the rooflight change the internal opening size?
No, the clear opening in the ceiling is set by the kerb's internal size, not by the glass depth. What the depth changes is how much of that opening the sun can reach at a given time of year. The relationship between unit size and the room below is covered in flat rooflight sizes.
Can a thicker roof make a flat skylight too deep?
A very deep warm deck or a green roof can push the reveal past 700mm, and at that point a small rooflight starts to feel like a tube. The usual answer is a larger unit, a splayed reveal, or both, which we plan before the roof is cut.
Do I need to know the thickness before booking a survey?
No. We measure the roof build-up at the survey and set the unit, kerb and reveal in the written light plan. If you are ready to go further, book a light survey or look at general rooflight costs first.
