Flat roofs make good places for daylight. The roof is horizontal or nearly so, there is no rafter pattern to work around, and the room below may have few windows. A skylight there can transform a kitchen extension, a landing or a hallway.
The catch is water. A pitched roof throws rain off by gravity alone. A flat roof relies on a continuous waterproof membrane and a gentle fall, and every hole cut through it is a place where that system can fail. A skylight needs a raised frame, called a kerb, to lift the glass above the membrane, and the membrane has to be turned up the sides of that kerb to form an upstand.
This guide explains how that works. It covers what a kerb and an upstand are, why the fall of the roof matters round the opening, what Part L of the Building Regulations asks of a rooflight and of the upstand, and what the planning position is. It is a reading guide, not a recipe, because the build depends on the roof, the product and the maker's instructions.
Where a skylight is going into an existing flat roof, settle the details at the same time as any re-covering of the roof.
Can a flat roof take a skylight at all?
In principle, yes. Skylights are sold specifically for flat and low-pitch roofs, and the techniques are well known. Whether yours is suitable depends on four things.
The roof structure
A flat roof is carried on joists. Cutting an opening means trimming those joists around the hole and supporting the ends. That is a structural alteration. The remaining joists must be able to carry the load, and the opening may not suit the existing spacing. A roof built with deep joists at close centres is more forgiving.
The deck and its condition
If the surface is old, soft or patched, the skylight should wait until the roof has been renewed. Fitting a new rooflight into a tired membrane often makes both fail.
The room below
A skylight can warm a south-facing room in summer. The glazing specification, blinds or opening vents can manage that. Venting also helps with condensation on the glass, a topic covered in our guide to condensation on roof windows.
The product
Fixed rooflights, opening ones and walk-on glass each have their own installation instructions, and they vary in how they sit on a kerb. The manufacturer's data sheet governs, and a competent installer follows it to the letter.
Where the roof deck is sound and the joists can be trimmed, a flat roof skylight is a routine job. If not, the answer may be to renew the roof first and add the skylight as part of the same project.
Kerbs and upstands
These two words are often confused, though they describe different things.
The kerb
The kerb is a raised frame fixed to the roof deck around the opening. It lifts the skylight clear of the surface so that water running across the roof cannot lap over the edge of the glazing frame. It may be timber, built on site, or a factory-made kerb supplied with the rooflight.
The upstand
The upstand is the part of the waterproof membrane that is turned up the outside of the kerb. It makes the roof covering continuous from the flat surface up to the rooflight frame. The membrane is then finished at the top of the kerb with a capping, flashing or cover provided with the skylight.
Height
The right height depends on the product and the membrane, so the instructions for both govern. A kerb too low lets standing water reach the frame. One too high looks heavy and complicates the flashing. Ask for the height to be stated, and for the product instructions to be followed on site.
Insulation of the upstand
Approved Document L treats this part of the roof carefully. A kerb or upstand built on site may not exceed 0.35 W/(m2K), so it must be insulated and not left as bare timber forming a cold bridge. Factory-made kerbs usually come with insulation built in.
Why details matter
The corners, where the membrane turns up and round, are the weakest points. Welded or well-formed corners last. Cut and stuck ones fail early.
This is where a firm that works with flat roofs routinely earns its keep. See our flat roofing page for the membranes themselves.
Falls: keeping water moving round the opening
A flat roof is not truly flat. It is built with a gentle fall so that water runs to the gutter or outlet. A skylight changes how that water travels.
The standard
BS 6229:2025 governs flat roofs finished with flexible, fully supported waterproof coverings at a pitch of 10 degrees or less. The 2018 edition was withdrawn on 16 December 2025. The standard retains a finished fall of 1:80 as the minimum, and the membrane maker IKO says to design for 1:40. The design figure is higher than the finished one because building tolerances and deflection reduce the fall in practice.
What the kerb does to the fall
A kerb is a dam. Water running down the roof meets the up-slope side of the kerb and has to divert round it. Without help, it can pond against the kerb. A small raised ridge shaped like a saddle or cricket, built up behind the kerb, splits the flow and sends it round both sides.
Location
Placing the skylight near an outlet or at the high end of the roof avoids the worst of the problem. A skylight in the low corner, near the drain, can obstruct drainage.
Not for lead
BS 6229 covers flexible membranes. Lead roofs follow BS 6915, as Approved Document C points out. The two are not interchangeable.
Ask any installer how the water gets round the kerb. A clear answer is a good sign. A shrug is not, and a drawing or photograph from an earlier job is a fair thing to request.
Part L: the figures a rooflight must meet
Energy rules apply to the rooflight and to the roof around it. Both are written into Approved Document L, Volume 1, and the figures differ.
Rooflights
In Table 4.2 a rooflight is held to 2.2 W/(m2K) when it is new or replaces an old one in an existing home. A roof window, by contrast, follows the window figure: 1.4, or WER Band B. The two products are treated differently, and a flat roof skylight is normally a rooflight. If the product is described as a roof window, check which figure its data sheet claims.
The surrounding roof
Regulation 2(3) of the Building Regulations 2010 defines the thermal elements as walls, floors and roofs and leaves out windows, roof windows and roof-lights. So the rooflight is assessed against its own figure, while the roof around it is assessed as a roof.
Renovation of the flat roof
In AD L, a flat roof that gets a replacement waterproof membrane is an example of renovation. If that happens at the same time, the roof falls under Table 4.3, with a threshold of 0.35 and an upgrade to 0.16 where the existing performance is poorer, subject to the exceptions in paragraph 4.13. See our guide to re-roofing and Part L.
Editions
The 2021 edition applies until 24 March 2027, when the 2026 edition takes over, with the tables renumbered 3.2 and 3.3.
A manufacturer's data sheet gives the U-value of the rooflight. Ask for it, and keep it with your records.
Planning and building control for a flat roof opening
Cutting a hole in a roof brings in two systems, and they answer different questions.
Planning
For a house, Class C permitted development covers alterations to a roof other than those Class B deals with, and a rooflight is one. The conditions include the 0.15 metre limit on how far the alteration may project from the original roof plane, which affects the height of the kerb, so a taller kerb may tip a project out of permitted development. Class C has no conservation area exclusion. A flat, meaning a dwelling in a building with several, has no permitted development rights, so a rooflight there needs the council's view. An Article 4 direction can remove the right in a protected street. Test Valley Borough Council is the planning authority that covers Andover.
Listed buildings
A listed building needs consent where the work alters its special interest, and a concealed flat roof may or may not matter to the conservation officer. Ask first.
Building Regulations
Trimming joists is a structural alteration, and the opening, the kerb and the energy performance all come under the Building Regulations. Ask your building control body how the work is to be notified. In the Test Valley area, that means local authority building control, or a registered building control approver.
Party walls
Where the new opening touches a wall you share with a neighbour, the Party Wall etc. Act 1996 may come into play, and a party wall surveyor handles that.
Our guide to building regulations for a new roof window has the full list of checks.
Rooflight, lantern or sun tunnel for a flat roof?
A flat rooflight is the simplest answer, but it is not the only one. Three products suit a flat roof.
Flat rooflight
This is a glazed unit on a kerb, available fixed or opening. It is the simplest and least obtrusive option and suits a modest opening. The glass may be fixed or opening, and the dome or flat-glass options give different looks.
Roof lantern
A lantern is a larger, pitched glazed structure standing proud of the roof. It brings more light and a striking look, but needs a larger opening, a bigger structure and more care with the details. See our guide to skylights and roof lanterns.
Sun tunnel
A sun tunnel carries daylight down a tube from a small dome on the roof to a ceiling diffuser. It suits a landing, corridor or bathroom with no space for a full opening, and the roof windows and skylights page shows the range.
Choosing
Match the product to the size of the room, the structure, the budget and the look you want from outside. A small opening is simpler to trim and seal, with fewer risks.
Aftercare
Check the seals and the membrane upstand every year or two, clear leaves from the roof around the kerb, and do not stand on glass unless it is rated as walk-on. Moss and debris held against a kerb keep the membrane wet and shorten its life.
If your flat roof leaks around an existing rooflight, the causes of a leaking roof window carry over, and a survey finds which applies.
We survey the roof and structure, photograph the joists and deck, and give one price covering opening, kerb and waterproofing together.
