A flat glass rooflight is a sealed double or triple glazed unit bonded into a slim aluminium frame and laid to a shallow fall so water runs off, giving a room a clean rectangle of sky with almost no visible frame from inside. It has taken over from the polycarbonate dome on new work because it is clearer, much warmer and does not yellow. The outer pane is toughened, the inner is laminated so nothing can fall into the room, and better units carry a self-cleaning coating and a bonded outer edge that leaves no frame lip for water to sit against. The catch is that a flush glass unit is made to order with no adjustment in it, so the kerb it sits on has to be built square and to a tight tolerance. Get the kerb right and the glass is the easy part.
This page covers what a modern flush unit actually is, the glass specification that matters, the sizes available and the tolerances the kerb has to hit. How it compares with a dome is on dome rooflight or flat glass, and the wider job is on flat roof skylight installation.
What makes a flush unit flush
The word doing the work is bonded. On a cheaper glass rooflight the sealed unit sits in an aluminium frame with a visible bead or lip of metal around the glass. That lip is an upstand in miniature. Water sits against it, dirt collects in the line around the perimeter, and in Suffolk that means a ring of pollen in May and, near the coast, a film of salt.
On a proper flush unit the outer pane is bonded so it extends past the frame and overhangs it, so the top surface is uninterrupted glass with no metal lip for anything to sit against. Rain runs off the edge and takes the dirt with it. That single detail is what you are paying the extra for, and it is worth it, because it is the difference between a rooflight that stays clear and one that grows a grubby frame you cannot easily reach to clean.
The internal fall
A flat glass rooflight is not laid dead flat. It is set to a shallow fall, typically two to five degrees, so water sheds to one edge rather than pooling on the glass. Some units build that fall into the frame itself, so the glass sits at an angle while the frame base stays level on the kerb. Others need the kerb head laid to the fall. Either way, water must run off the glass to a low edge, and that low edge must sit over roof that drains away from the unit, not into a pocket beside it. A flush unit that sheds its water into a dished area next to the kerb has simply moved the pond a metre sideways.
The glass specification that matters
A flat unit faces straight up, so it takes more solar gain per square metre than any window in the house and radiates heat to a cold sky at night. That makes the glass specification more important here than on any pitched rooflight, and it is the part cheap quotes leave vague.
| Element | Specification | Why it matters on a flat unit |
|---|---|---|
| Outer pane | Toughened glass | Resists hail, thermal stress and foot pressure during cleaning |
| Inner pane | Laminated glass | If it ever breaks, the interlayer holds it so nothing falls into the room |
| Cavity fill | Argon, warm-edge spacer | Lifts the U-value and cuts edge condensation |
| Low-emissivity coating | Soft-coat, on an inner surface | Reflects heat back into the room, brings U-value to 1.1 to 1.4 W/m²K |
| Solar control coating | g-value 0.28 to 0.38 | Cuts summer overheating on a unit facing straight at the sun |
| Self-cleaning coating | On the outer face | Rain sheets off and lifts dirt, useful on a surface you cannot easily reach |
If a quote does not state a U-value and a g-value, the glass has not been specified. Two quotes that both say energy-efficient glazing are not comparable. Two that say 1.2 and 1.9 are.
Self-cleaning, and what it does and does not do
A self-cleaning coating is a titanium dioxide layer on the outer glass. Daylight triggers it to break down organic dirt, and rain then sheets off the surface in a film rather than beading, carrying the loosened dirt away. On a flat rooflight you cannot easily reach, that is genuinely useful. It is not magic. It handles organic grime, leaf tannins and general film. It does not shift a salt deposit on a coastal roof, and near the sea a rinse with a hose twice a year is still needed. It also needs rain and daylight to work, so a unit shaded by a taller building keeps less of its benefit.
Sizes and the point where a unit becomes a special
Standard flush units run from small ventilation-sized panes up to roughly 2.5m by 1.5m as a single sheet of glass, in a wide range of stock sizes and made-to-measure within that envelope. Above that, and for any single pane over about 3m in a dimension, you move into oversized glass that is heavier, needs different handling and often craneage, and is priced as a special. Non-standard shapes, multi-panel arrangements and very large single spans go through our custom skylight design service.
Two practical points on sizing. First, on a narrow side return, two smaller units with a clear drainage channel between them usually light the room more evenly and drain better than one large unit spanning nearly the full width. Second, aim for the glazed area to cover roughly 15 to 20 per cent of the floor area of the room below, since a flat unit takes light from directly overhead and spreads it less widely than a pitched lantern. A 12m² kitchen wants somewhere around 2m² of glass.
The tolerances the kerb has to hit
Here is the part that decides whether a flush unit goes in cleanly. A flush glass rooflight is a bonded assembly made to a measured size, and there is no adjustment in it. It cannot be trimmed on site, packed out or eased into an opening that is out of square. The kerb below it has to be built to the unit, not the other way round.
- Square. The two diagonals across the kerb head must match to within 3mm on a domestic opening. A kerb that is a parallelogram puts the frame in wind and loads the corner of the glazing seal.
- Level or laid to the specified fall. Out of level in two axes puts the frame in wind again. The kerb head is set level, or to the manufacturer’s fall, and the height is varied at the base to follow the roof.
- The right kerb dimension. Manufacturers publish either an external kerb size the frame sits over or an internal daylight opening. The two are not interchangeable, and confusing them on a made-to-order unit is the most expensive mistake available on this job.
This is why we build the kerb to the confirmed unit, get the manufacturer’s kerb detail in writing, and check the diagonals both before and after the kerb is fixed down. The full specification is on rooflight kerb construction.
The kerb is the job. The glass is the easy bit. A flush glass unit is only as good as the square, level, insulated kerb it sits on. A perfect unit on a racked kerb leaks and condenses. A good kerb makes the glass almost impossible to get wrong.
Fixed, opening, or walk-on
Most flat glass rooflights are fixed, and for pure daylight that is the right call: fewer moving parts, better thermal performance and a lower price. You open one for ventilation and summer heat, not for daylight, and that option is covered on opening and hinged flat rooflights. Where a flat roof is also a terrace or a light well and the glass has to carry foot traffic, that is a completely different structural unit, covered on walk-on rooflight installation.
Coastal specification
Within a mile of open sea at Felixstowe, Harwich or Frinton, standard flush units need upgrading. We specify 316L stainless fixings throughout, marine-specification powder coating on the frame, a 200mm upstand and isolation between dissimilar metals, because chloride from salt-laden air pits standard fittings within a few winters. Estuarine locations like Woodbridge carry moderate salt loading rather than open-sea exposure, so upgraded fixings make sense without the full marine package. The full coastal detail is on our Felixstowe page.
Common questions about flat glass rooflight installation
Do flat glass rooflights leak more than domes?
No, provided the kerb is built correctly. A flush glass unit sheds water off a bonded overhanging edge and sits on a square, level kerb dressed over with the covering. Domes shed by shape, which is forgiving on a poorly drained roof, but they yellow and lose light. On a properly built kerb a flush glass unit is at least as watertight and stays clear for decades.
Is a self-cleaning coating worth paying for?
On a flat rooflight you cannot easily reach, yes, for organic grime and general film. It needs rain and daylight to work, so a shaded unit keeps less benefit, and it does not shift salt deposits near the coast, where a hose rinse twice a year is still needed. It is a sensible upgrade, not a substitute for occasional cleaning.
Can a flat glass rooflight be laid completely flat?
It is laid to a shallow fall, typically two to five degrees, so water sheds off rather than pooling on the glass. Some units build the fall into the frame while the base sits level on the kerb. Water that sits still on flat glass leaves dirt rings and, over time, works at the perimeter seal, so a designed fall is part of the specification, not an option.
What size flat glass rooflight do I need?
As a working guide, aim for the glazed area to cover roughly 15 to 20 per cent of the floor area of the room below, since a flat unit lights from directly overhead. A 12m² kitchen wants around 2m² of glass. On a narrow roof, two smaller units often light more evenly and drain better than one large one. The survey confirms what the structure and drainage will take.
Get a price for a flat glass rooflight
Send us the opening size or the room dimensions, whether it is a new opening or a replacement, the existing covering if you know it, and roughly where the property sits. We will survey it, check the fall and the structure, specify the glass properly with a stated U-value and g-value, and give you one written fixed price. Every installation carries a 10-year workmanship guarantee, and the installers have 25 years fitting rooflights behind them.
Request a fixed quote for flat glass rooflight installation in Ipswich, Suffolk or north Essex, or see the published flat roof rooflight installation cost first.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.