VELUX flat roof windows on a low pitch

How it differs

It sits on an upstand rather than in the plane of the covering, so it is weathered like a flat roof rooflight: the covering is dressed up the kerb and terminated under the frame.

No flashing kit, because there is no covering profile to lap into.

The curved top cover

Most systems offer a curved or domed acrylic cover over the glazed unit. It is not there for looks.

On a nearly flat unit, water sits on the glass and evaporates, leaving whatever it was carrying behind as spotting. The curved cover sheds water by shape rather than by fall, so the surface stays cleaner and self cleaning coatings have something to work with.

It also cuts rain noise noticeably.

The trade is a slight loss of clarity looking up, and the acrylic will age over decades where glass does not.

Where they suit

Shallow pitched post-war roofs, which there are a lot of in Colchester. Flat roofed extensions and garage conversions. Anywhere the pitch is between about 5 and 15 degrees, which is genuinely awkward for every other product.

Opening versions

Available manually, by mains electric and by solar. On a flat roof the unit is above reach by definition, so electric is the version people use.

The kerb still decides it

Everything about flat roof kerbs applies: adequate height, square, level along the top, insulated on the outer face, and the vapour control layer sealed to the frame.

Where a pitched roof window stops and a flat roof unit starts

Every pitched roof window carries a minimum pitch stated by the manufacturer, commonly around 15 degrees. Below that the flashing arrangement cannot cope: water does not shed reliably, it sits against the head detail, and the covering itself is frequently outside its own minimum pitch too.

Above 15 degrees a standard roof window works, with the caveat that the shallower the roof, the harder the flashing kit is working and the more the installation quality matters.

Below it, the answer is a flat roof rooflight sitting on a kerb, which is designed for exactly that situation.

The awkward band is 10 to 15 degrees, and several manufacturers make units specifically for it. That is worth asking about rather than assuming a shallow mono pitch extension roof has to take a flat roof product.

The band where most modern estates sit

Housing built from the 1980s onward frequently has main roof pitches of 25 to 30 degrees, which is comfortably within range for a pitched roof window and shallow enough to change three things.

The flashing does more work. Less water sheds under gravity, so a greater proportion of the weathering falls on the kit itself. The correct deep profile kit for a profiled interlocking tile becomes critical rather than merely preferable, and substituting a flat kit intended for slate leaves open voids under the tile channels that water finds within a season.

That is the most common installation fault we find on modern estate roofs, and it is invisible once the covering is back.

Headroom gain is smaller. A window in a 45 degree slate roof gains a lot of standing height. The same window in a 27 degree roof gains much less, and if the room below is already tight that is worth understanding before ordering.

The reveal is longer. This is the one that costs light. A shallow pitch means the window sits at the end of a longer, shallower tunnel through the roof construction, and a square boxed reveal throws a narrow beam straight down while blocking a surprising amount of what the window collects.

Splaying the reveal, which matters most here

Cutting the reveal back at an angle at the head and the sill rather than boxing it square opens the beam out and distributes light across the room instead of down onto one patch of floor.

On a steep roof it is a worthwhile improvement. On a shallow one it is the single best value decision available on the whole installation, because the reveal is longer and there is more to be gained by opening it up.

It costs a little more in plastering. It frequently means a smaller unit does the job, which more than pays for it.

Solar gain on a shallow slope

A shallower unit is closer to horizontal, and closer to horizontal means presenting more directly to a high summer sun.

So a 27 degree south facing rooflight collects more midday sun than a 45 degree one of the same size. Solar control glass matters more on a shallow slope than a steep one facing the same direction, and it is worth specifying at order because it cannot sensibly be added later.

The same logic taken further is why flat rooflights and lanterns should be treated as if they were south facing whichever way the house faces: a horizontal pane has no aspect to shelter behind at all.

Structure on a modern shallow roof

Almost always trussed rafters, and that decides more about the price than the choice of window does.

A trussed rafter is a single engineered component in which the rafter, the ceiling tie and the internal webs work together, connected with pressed steel plates. Cutting any member sends the load it carried somewhere it was not designed to go.

A unit narrow enough to sit between two trusses at 600mm centres, touching neither, needs no structural work at all. That is the quick, inexpensive job and it is what most enquiries on these roofs turn into, which is why so many rooflights on modern estates are tall rather than wide.

Anything wider needs a structural trimmer designed by an engineer, and that is the point at which the quotation steps up. It is not a cost worth avoiding, and it is worth knowing about before setting a budget.

Going taller rather than wider is frequently the answer: lengthening an opening up the slope does not cut rafters.

The covering

Concrete or clay interlocking tiles on almost every roof in this band, and they are good to work with: robust, they lift cleanly, and a small allowance covers the proportion that breaks on any roof of age.

What they need without exception is the deep profile flashing kit matched to the tile. Ask for it by name and part number on the quotation. A supplier who cannot answer that has not written the price from a survey.

Where a flat roof unit is the better answer anyway

On a mono pitch extension roof below 15 degrees, obviously.

But also where the extension roof is nearly flat and the room wants more glass than a narrow between trusses unit will give. A flat roof rooflight on a properly built kerb is not constrained by rafter spacing in the same way, and two or three of them in a row light a long extension more evenly than one large unit ever will.

On those, the kerb decides everything: at least 150mm of upstand, the deck falling away from it so water cannot pond against the seal, rigid insulation on its outer face, and a continuous vapour control layer sealed to the frame.

That last pair is where the heat actually goes, and it is the first thing left off a cheap quotation.

The problem this product solves

Every pitched roof window has a minimum pitch below which it is not warranted, and below which it will not weather reliably. Water moves slowly on a shallow roof, driven rain has time to work under the covering, and a flashing designed for a steeper slope is being asked to do a job it was not built for.

A great many roofs in this area sit below that threshold: shallow rear extensions, garage roofs, dormer tops, and the low mono pitch roofs common on post war housing.

The flat roof window range exists for exactly those, and it is a different product rather than a standard window used optimistically.

What sets the flat roof product apart

It sits on a kerb rather than being flashed into the covering. The waterproofing is dressed up that kerb and terminated into the unit, and the unit sits on top, so the weathering is a flat roof detail rather than a lapped covering detail.

The glass is usually supplied with a curved or pitched top element rather than a flat pane, so water runs off rather than sitting on it. That matters more than people expect: a truly horizontal pane holds water, dulls over the years as deposits build up, and defeats self cleaning coatings entirely because they need run off to work.

The kerb, which is where the job is won or lost

Height. High enough to clear any water the roof can hold, including with a blocked outlet, and on a ballasted or green roof, high enough to clear the finished surface by a real margin.

Insulation. On the outer face, continuous with the roof insulation, meeting the underside of the frame with no break. An uninsulated timber kerb runs cold on its internal face, sits at the top of a warm humid room, and produces condensation that is reported as a leak every single winter. This is the most common defect on these installations.

Square and level. The unit is made to the finished kerb dimensions and the tolerance is small. A unit on an out of square kerb does not sit down evenly, and the gap gets taken up in sealant, which is a temporary arrangement from the day it is built.

The roof underneath

Two things to establish before anything is ordered.

Cold roof or warm roof. Most flat roofs built more than twenty years ago are cold roofs, with insulation at ceiling level and a ventilated void above. A kerb interrupts that ventilation path, and unless air can pass around it a stagnant damp pocket forms beside the unit and rots the deck silently.

The fall. A kerb is an obstruction in the water path. On a roof with marginal or lost fall, water ponds permanently against it, which is the condition that ends waterproofing details. Correct the fall first, with tapered insulation or firrings, because doing it afterwards means the unit comes out.

Powered and manual options

Available manually operated, solar, or mains powered, and on a low pitch roof over a habitable room the opening version earns its place, because venting at the highest point in the room is where stack ventilation actually works.

A rain sensor is close to essential on a powered unit here, since a low pitch roof is frequently over a kitchen or an extension where an open rooflight in a downpour matters.

What to check on a quotation

That the unit specified is the flat roof product rather than a pitched window, that the kerb is included and stated as insulated, that the fall has been assessed, and that the build up has been established by inspection rather than assumption.