Flat roof VELUX units on a kerb, which is a different product

Why a pitched roof window cannot be laid flat

A roof window is designed to shed water down a slope. Almost everything about it assumes that water arrives at the top and leaves at the bottom under gravity.

The flashing kit interleaves with a lapped covering. The frame drainage runs downhill. The gaskets are arranged to shed rather than to sit in water. Every one of those assumptions fails on a surface that is essentially level.

Manufacturers therefore state a minimum pitch for every pitched roof window, and below it the product is outside its design range. Fitting one flatter than that is not a compromise, it is an installation that will leak.

What a flat roof unit is instead

A flat roof rooflight is designed for a level or near level installation and it sits on a kerb rather than in a covering.

The kerb lifts the frame clear of the water on the roof. The covering is dressed up the face of the kerb and terminated above the water line. The unit sits on top of that, sealed to the kerb rather than interleaved with anything.

There is no flashing kit, because there is no lapped covering to interleave with. The waterproofing is the covering taken up the kerb, and the unit is what closes the top of it.

The two products compared

The kerb is the job

On a pitched roof the difficult part is the flashing. On a flat roof it is the kerb, and it decides everything about how long the installation lasts.

It has to stand high enough above the finished surface at the lowest point of the fall, be level across the top even though the roof falls, be square, be insulated on its outer face, and carry the vapour control layer up its inner face to seal against the frame.

Each of those has its own page. What matters here is that none of them is a consideration on a pitched roof, so a customer whose experience is a loft window is meeting all of it for the first time.

Fall, and why flat is not flat

A flat roof is built with a small deliberate slope so that water leaves it. That slope is what makes a level unit workable.

A unit sitting on a kerb over a falling roof is itself effectively level, and the water that lands on its glass runs off onto the roof, which then takes it to an outlet. The unit is not doing the draining; the roof is.

Where the roof has lost its fall, through deflection or poor construction, water stands against the kerb and the whole arrangement is compromised no matter how good the unit.

Domes, and what most people have

A great many flat roofs around Worthing carry an acrylic or polycarbonate dome from the 1970s or 1980s sitting on a timber kerb.

Replacing one with a modern flush glazed unit on the same kerb is one of the most worthwhile jobs we do. The light quality improves out of all recognition, the heat loss falls substantially, and the rain noise drops noticeably.

The trap is measurement, because domes were sold by nominal sizes that matched neither the kerb nor the frame. The old unit tells you almost nothing and the kerb tells you everything.

Sizing is completely different

A pitched roof window is ordered against a size code that refers to a frame in the manufacturer range, and the opening is formed to suit it.

A flat roof unit is ordered against the measured kerb, and the kerb already exists on most jobs. That reverses the relationship: the opening decides the unit rather than the unit deciding the opening.

It also means the twelve kerb measurements matter far more than any code, and it is why we survey the kerb rather than reading a plate.

Which dimension the supplier means

The commonest ordering error on flat roof units, and it is entirely avoidable.

Manufacturers quote either to the kerb size the unit sits over, or to the external frame size, and the two differ by the kerb wall thickness all the way round. They do not all use the same convention, and the same maker sometimes differs between ranges.

We confirm which one applies for that specific product before the order goes in. It is a two minute question and it is the highest value check in the process.

Opening flat roof units

Nobody reaches an overhead unit on a flat roof by hand, so an opening flat rooflight is electric rather than manual.

A chain actuator drives the sash open against its own weight and pulls it closed. That is a different mechanism from the pivot or friction hinge on a pitched window, and it needs a supply, which means the cable route is decided before the ceiling closes.

On this coast a rain sensor with it earns its place, because weather arrives quickly off the Channel and an open rooflight in an empty house is a wet floor.

Ventilation and escape

An opening at the highest point of a room is the most effective vent position there is, because warm moist air collects there.

Where a flat roof unit is the means of escape, which happens on some flat roofed extensions with habitable space, the clear opening and the operation both have to satisfy the requirement, and that is settled with Building Control rather than assumed.

Most flat roof rooflights are fixed, and a fixed unit is cheaper, quieter in rain, better sealed and has no mechanism to age. Where ventilation comes from elsewhere it is frequently the better specification.

Glass, and the overhead case

A flat unit faces straight up, which makes it a more efficient daylight collector than a sloping one and a more exposed one in summer.

It takes the high summer sun at close to its most efficient angle in the middle of the day, whatever direction the room faces, so solar control is a more general consideration than it is on a pitched window where orientation does some of the work.

The inner pane is laminated on any overhead glazing as standard, for safety, and it does useful work on rain noise as well.

Rain noise

A flat unit is louder than a pitched one of the same glass, because rain arrives close to perpendicular and every drop transfers its full momentum into the panel.

An old polycarbonate dome is the worst case: light, thin, flexible and drum-like. Replacing one with a laminated glass unit is a substantial improvement, and people frequently mention the quiet before they mention the light.

Walking on it

A flat roof looks walkable and a rooflight in it looks like part of the floor. Neither assumption is safe.

Unless a unit is specifically specified and certified as non-fragile or walk-on, it is treated as fragile without exception. A fall through a rooflight is a fall to the room below with the glazing following.

Walk-on units exist and they are a structural product with a different specification entirely, which is a different conversation again.

Upstands, and the version without a separate kerb

There is a middle case worth knowing about, because it turns up on newer extensions and it changes the survey.

Instead of a timber kerb built on top of a finished deck, the upstand is framed as part of the roof structure itself, so the insulation and the vapour control layer are continuous through it by design rather than by somebody remembering to make them so.

It is the cleanest arrangement and it is what we would build where a roof is being formed as part of the job. On a survey it is worth identifying, because an integral upstand and an added kerb look similar from above and behave quite differently once you open them up.

Two units on one flat roof

Where a flat roof takes more than one rooflight, they are two separate kerbs rather than one shared arrangement, and the space between them matters.

Water arriving from above has to pass between them, so the gap has to be wide enough to drain properly rather than becoming a channel that dams at the top. Setting them in a line down the slope rather than across it lets water past both with the shortest diversion.

Two smaller units also frequently avoid the structural step that one large opening would trigger, which is covered on the structural openings page. On a narrow extension it is regularly the better arrangement for both reasons at once.

Kerb heights on older extensions

What we find on the 1970s and 1980s flat roofs here is consistent.

• A kerb built to a constant height on a falling roof, so the top is not level.
• Clearance measured at the high corner and inadequate at the low one.
• No insulation on the outer face at all.
• No vapour control seal to the frame.
• Soft timber at the top where a failed dome seal has been letting water in for years.

All of it is correctable while the unit is off, and that is the only moment it is reachable.

Which product for which roof

The rule is simple and it gets broken regularly.

Above the manufacturer minimum pitch, a pitched roof window in a flashing kit. At or near level, a flat roof rooflight on a kerb. On the shallow slopes behind a parapet on a Brighton or Hove terrace, which are frequently below the minimum for any pitched window, a kerb mounted unit is the correct product even though the roof does not look flat.

The pitch is measured at survey rather than judged by eye, because the difference between twenty degrees and fifteen decides the product.

What we measure on a VELUX

We measure the pitch before recommending a product, and where it is marginal we measure it properly rather than estimating. On a flat roof we survey the kerb fully, confirm which dimension the manufacturer quotes against, and order only from a kerb that is level, square and high enough. We correct the kerb while the unit is off, because that is the moment.

They look like the same thing from the ground. They are two different products doing the same job in two different ways.