Will I See the Kerb of a Flat Rooflight From Inside?

Every flat rooflight sits on a raised collar, whatever the trade calls it on the day, and that collar is there to keep the glass above the level water and driving rain reach on the roof. Height is set by drainage rules, not by what looks tidiest from the sofa, so a kerb is never designed to vanish the way a slim frame or a hidden capping can be.

That is the first thing worth separating out. A frame edge and a glazing bar sit at glass level, right where the eye lands. A kerb sits below that, forming the walls of a short shaft between the roof and the ceiling, and it only enters the view once the angle of sight crosses its inner face rather than clearing straight past it.

Stand directly under a flat rooflight and look straight up, and the kerb rarely shows at all: your eye travels through the middle of the opening and meets glass before it meets anything else. Move off to one side, towards a pillow against a wall or a sofa set back from the opening, and the geometry changes. The near lip of the kerb, and the lining below it, starts to intrude on the lower edge of the view before the sky does.

How far you can move before that happens depends on how deep the shaft is and how far the kerb sits back from the room below. A tall kerb, forced by a steep fall or a thick layer of warm-roof insulation, creates a deeper shaft and a narrower cone of sky that stays kerb-free. A shallow one, the kind a low-profile or well-detailed upstand achieves, lets you sit further off-centre before any part of it appears.

• Directly under the glass: the kerb is normally out of sight altogether.
• A metre or so off-centre: a shallow kerb still clears the view; a tall one starts to show at the near edge.
• Well off to one side, as from a sofa against a far wall: even a shallow kerb can appear as a thin band unless the lining below it is shaped to open the view back up.

Between the kerb and the ceiling sits the reveal, the lined surface your eye actually meets first from an angled seat. A reveal built square, with the lining running straight down at ninety degrees to the ceiling, keeps the shaft the same width all the way up, which is exactly what turns a deep kerb into a visible collar from off-centre.

A splayed reveal angles that lining outward as it drops, wider at the ceiling than it is at the kerb, so the same shaft opens like a shallow funnel rather than a straight-sided well. From an angled seat, a splay lets the eye travel past more of the lining before it reaches the kerb's inner face, which is often enough to keep the kerb out of an ordinary seated view even where a square-cut reveal of the same depth would show it plainly. It is one of the few changes made entirely below the roofline, with no effect on the glass, the frame or the kerb height itself, and it is usually the cheapest way to answer this question without changing the specification of the rooflight at all.

A frameless flat rooflight sinks the structural frame down inside the kerb, under a slim capping, so the border you would otherwise see around the glass is removed from the sightline entirely. That is a real change to the view, but it answers a different question from this one: a frameless unit hides the frame, not the kerb sitting beneath it.

Where frameless designs do help with the kerb is indirectly. Because they are usually specified with a low-profile, integrated kerb built into the unit itself rather than a taller site-built or factory box, the shaft below a frameless pane tends to be shallower than the shaft below a standard framed one on the same roof. A shallower shaft is, by definition, less likely to cross an off-centre eyeline in the first place. The saving comes from the shorter kerb paired with the design, not from the capping working on the kerb directly.

A site-built timber kerb, a factory-made insulated upstand and a low-profile integrated kerb can all meet the same drainage rule while finishing at different heights, and that difference matters more to this question than almost anything else about the unit. A factory upstand is usually slimmer than a timber kerb built to the same insulation value, and an integrated low kerb slimmer again, though it is limited to roofs with little else competing for depth, a garage conversion more often than a new single-storey extension.

None of these choices is made in isolation from the roof itself. The fall the covering needs, the insulation depth on a warm roof, and the room's ceiling void all narrow down which kerb type is even available before the question of view comes into it, which is why kerb type is a survey conversation rather than a catalogue pick.

The only reliable way to answer this for a particular bed, sofa or worktop is to draw the sightline from that exact spot rather than from the middle of the floor. A sightline sketch, included with every quote, plots the eye position that matters, the kerb height the roof allows, and the reveal shape proposed, and shows on paper whether any part of the kerb crosses that line before the sky does.

Where the drawing shows a band of kerb intruding on a seat that matters, the answer is rarely to abandon the room's layout. A shallower kerb where the roof build-up allows it, a splayed rather than square reveal, or moving the opening a short distance over the seating rather than the room's centre line, can each close the gap on their own, and together they usually close it completely. We install to current Building Regulations and handle the Building Control notification where the work is notifiable, whichever combination the roof and the room end up needing.

See the full flat roof rooflight installation service for how kerb, frame and reveal fit together, or read more on kerbs and upstands for flat rooflights, internal reveal finishes, and frameless flat rooflights. If a sofa across the room is the seat in question, what you will see through a flat rooflight from the sofa works through that eye position on its own terms, and why flat rooflights are not quite flat explains the fall that sets kerb height in the first place. For placing the glass over a deep single-storey extension rather than the room's centre, see where to place a flat rooflight in a deep extension. Our page on what affects the cost of a rooflight covers where kerb type sits against the other specification choices, and the sightline sketch page sets out how we draw all of this before anyone reaches for a saw. When you are ready, book a home visit and we will bring a level to the actual seat, not just the roof.

Does every flat rooflight show its kerb from somewhere in the room?

Only from certain angles, and only where the kerb is tall and the reveal is cut square. A shallow kerb with a splayed lining can keep the kerb out of view from most ordinary seats, though standing right at the edge of any room will eventually find an angle where some lining shows.

Is a splayed reveal always better than a square one?

For opening up the view, yes, a splay generally shows more sky from an off-centre seat than a square reveal of the same depth. A square reveal can still suit a hallway or a room with no fixed seating, where the difference matters less.

Can I ask for a shorter kerb just to reduce what shows?

Only down to the minimum the roof's fall and covering allow. Going shorter than that risks water getting past the seal, so any request to reduce kerb height has to be checked against the roof first, not decided from the room alone.

Will replacing an old rooflight change how much kerb I see?

It can, if the new unit is specified with a different kerb type or a splayed rather than square reveal. A straight swap onto the existing kerb keeps the same shaft depth, so the improvement usually comes from changing the reveal lining rather than the glass itself.