VELUX flat roof window systems

Fit a roof window into a 40 degree slate roof and it is one object: a frame with glass in it, tiled around, tilted enough that rain runs off on its own. A flat roof gives you none of that help. The deck is level, or close to it, water sits where it lands, and a window installed flush would be a puddle waiting to happen.

So the VELUX flat roof window separates into two components. The first is the base unit, an insulated frame that sits on top of the roof and lifts the glazing clear of the deck. The second is the top cover, the part you actually see and that keeps the weather out. The base handles the upstand, the insulation and the mechanism if the window opens. The top cover handles the light, the heat and the drainage. You buy one of each and they clip together into a finished unit.

This split is the thing to understand first, because almost every other choice follows from it. The base unit decides whether the window is fixed, manual or electric. The top cover decides how it looks, how it drains and what the glass does to the summer sun. Two houses can have the same base unit under completely different covers, and the room below will feel entirely different in July.

VELUX offers the flat roof window with three families of top cover, and they are not interchangeable in the way a showroom photo might suggest. Each solves the drainage problem differently and each behaves differently under load from the sun.

The flat glass cover is a laminated glass pane held a couple of degrees off level so water runs towards one edge. It gives the cleanest look from the garden, close to a frameless sheet of glass lying in the roof, and it takes the full solar-control glazing options. The curved glass cover puts a shallow dome of glass over the opening. Rain sheets off the curve and takes dirt with it, so it stays cleaner on very low pitches where a flat pane would hold standing water. The third option is the opaque or access cover, an insulated acrylic or polycarbonate top used where the opening is a roof exit or a smoke vent rather than a daylight window, and where clear glazing is not the point.

The values below are typical published figures for the current range. Exact numbers move with the glazing option and whether a figure is quoted for the glass alone or the whole assembled window, so treat them as the shape of the range and ask for the data sheet on the specific unit you are quoted.

Read the G-value column and the point jumps out. A curved glass cover on a north-facing landing at 0.55 is doing the right thing, letting daylight in where there is no strong sun to control. The same 0.55 sitting over a south-facing kitchen is a mistake, because on a flat roof that glass is aimed straight at the summer sun. That is why the flat glass cover with a solar-control coating, at a G-value nearer 0.34, exists at all. It is the answer to the problem that a flat aspect creates.

The VELUX flat roof window comes in a fixed set of module sizes, quoted as the base unit dimensions in centimetres. The common single-unit sizes run roughly like this:

• 600 by 600 mm, the smallest, for a hallway, an ensuite or a utility
• 600 by 900 and 900 by 600 mm, a common size over a stairwell or a small kitchen
• 800 by 800 and 900 by 900 mm, a square unit for a bathroom or a landing
• 1000 by 1000 mm, a popular all-round size for a rear extension
• 900 by 1200 and 1000 by 1500 mm, for a larger single opening over a dining area
• 1200 by 1200 mm, a big square that reads as a proper feature from below
• 1500 by 1000, 2000 by 1000 and 1200 by 2000 mm, the largest single units, for open-plan spaces

Where you want more glass than a single unit gives, the range is designed to gang. Two or three base units set side by side, with a slim upstand between them, cover a wide extension roof with a run of matching squares. This is the honest alternative to a rooflight the size of a table, and it is often the better structural answer, because each module carries its own load into the deck rather than asking one enormous opening to span the room. If you want a single unbroken sheet of glass over a big space, that is really a roof lantern or a bespoke rooflight question rather than a standard flat roof window one.

Whatever size you choose, the structural opening in the roof has to be formed to suit, with trimmers around the aperture carrying the joists you have cut. The base unit sits on that opening and on the upstand built up around it, which brings us to the part of the system that does the quiet work.

The flat roof window is rated for roof pitches from 0 to 15 degrees. In practice a genuinely level roof is not what you want under any rooflight. A minimum fall of around 1 to 2 degrees across the roof, built into the deck or the firrings, lets water find its way to the outlet instead of ponding against the upstand. The window can cope with a dead-level deck, but the roof around it should not be one.

The upstand is the low kerb the base unit sits on, lifting the glass clear of the roof surface. Good practice, and the figure Building Control will expect, is an upstand that holds the underside of the glazing at least 150mm above the finished roof level. That height keeps the seal above any standing water, splashback and the depth of a settled snow load. You have two ways to achieve it. Either the roofer builds a timber kerb as part of the roof structure and the base unit lands on it, or you buy the VELUX insulated kerb accessory, which is a ready-made insulated upstand that saves building one on site and comes with the thermal performance already sorted. On a warm-deck flat roof, where the insulation sits above the joists, the insulated kerb keeps the line of insulation unbroken around the opening, which matters for both heat loss and condensation.

Drainage is where the top covers earn their keep. The flat glass cover is set at a slight angle so water runs to the low edge and away. The curved glass cover sheds water off its dome from any direction and rinses itself in the process, which is why it holds up better on the flattest roofs and on units you will never reach to clean. Neither cover wants to be sitting in a pool, which loops back to that minimum fall across the roof.

The base unit sets how, and whether, the window opens. There are four routes.

A fixed unit does not open at all. It is the cheapest, the best insulated because there is no mechanism to interrupt the seal, and the right choice where the window is there purely for light and the room is ventilated some other way. A manual opening unit is worked with a control rod from the floor, which is fine on a single-storey extension where you can reach a low ceiling, and tiresome on a high one. An electric unit opens at the press of a switch on the mains, which suits a ceiling nobody is going to reach with a pole. A solar-powered unit does the same job with a small photovoltaic cell charging a battery in the frame, so it needs no wiring run across the roof and can be retrofitted where getting a cable to the opening would mean lifting finishes.

Any of the opening versions can carry a rain sensor that shuts the window automatically when the first drops land, which matters more on a flat roof than a pitched one, because a flat opening left ajar in a downpour is pointing straight up at the rain. On a tall or double-height space, an opening rooflight at the very top of the room is the most effective way to clear the hot air that collects up there, far better than a window at head height, which is the reasoning behind fitting electric opening units where a manual rod is never going to be used twice a day.

Here is the thing about a flat roof window that a brochure will not lead with. It faces straight up. A window in a pitched roof is tilted, and at the height of an Essex summer the midday sun strikes it at an angle that throws a good share of the energy off. A flat roof window has no tilt to help it. At solar noon in late June, with the sun high over Southend, a level pane is close to square-on to the beam and collects about as much solar energy per square metre as glass on this coast ever will. Of every rooflight geometry you can fit, the flat one is the hardest worked by the summer sun.

That is not an argument against flat roof windows. It is an argument for choosing the glass with the aspect in mind, and it lands harder in south Essex than almost anywhere in England, because Southend faces south across the Thames Estuary. The flat-roofed rear extension on a 1930s semi, of which the patch has an enormous number, usually sits behind the house with the garden and the afternoon sun to the south. Put a run of flat roof windows over a knocked-through kitchen like that, glaze them with a G-value of 0.55, and you have built a solar collector over the room the family actually lives in. Glaze the same openings at 0.34 and you keep the daylight while turning off a large part of the heat.

The general figures worth carrying in your head: clear double glazing sits around a G-value of 0.6, a solar-control double around 0.3 to 0.34, and the point of a solar-control coating is that it blocks the near-infrared that carries the heat while letting most of the visible light through, so the room stays bright without cooking. On a south or west-facing flat roof that is the specification to ask for. On a north-facing one, where there is no direct beam to fight, clear glass and the highest G-value you can get is the better call, because every bit of northern daylight is worth keeping. This is the whole basis of how we specify, and there is more on it under solar-control glazing.

Installing a flat roof window means forming the opening, trimming the joists around it, building or fitting the upstand, seating the base unit, dressing the roof covering up and over the upstand to make it watertight, and clipping on the top cover. On a warm-deck roof the insulation and the vapour control layer have to be carried around the kerb properly, or the coldest ring of the whole roof will be the frame of your new window and it will show as condensation in winter. It is careful work and it is a large part of why a properly fitted flat roof window costs what it does.

A new or enlarged rooflight is notifiable under Building Regulations. Part L governs the thermal performance, and it sets a maximum U-value that any unit worth buying beats comfortably, which is why the whole-unit figures in the table above sit where they do. Part K covers safety glazing, and because this glass is overhead, the pane facing down into the room has to be laminated, so that if it ever breaks the interlayer holds the fragments rather than dropping them. VELUX flat roof windows are built to meet this as standard, but it is worth knowing why the inner pane is laminated and not simply toughened. We make the Building Control notification to Southend-on-Sea City Council, or to the relevant local authority for your address, as part of every installation.

On permitted development, a rooflight on a house is generally allowed without a planning application if it projects no more than 150mm above the roof plane and sits below the ridge, which most flat roof windows on their upstands do comfortably. Flats and maisonettes, listed buildings, conservation areas and the Article 4 zones over parts of Leigh are the exceptions, where a planning application is needed even for work that would elsewhere be permitted. Treat that as general guidance and check your own address with the council before you order, because the cost of getting it wrong is far higher than the cost of a phone call. There is more detail on the flat-roof job itself on our flat-roof skylight installation page.

Put together, the decision comes down to a short sequence. Settle each one in order and the specification writes itself.

1. Which way does the roof face? South or west points you at a solar-control flat glass cover. North points you at clear glass and the highest G-value you can get. This comes before anything else.
2. Fixed or opening? If the room is ventilated elsewhere, fixed is cheaper and warmer. If the rooflight is your ventilation, decide between a manual rod on a low ceiling, mains electric on a high one, or solar where running a cable would mean lifting finishes.
3. Flat glass or curved? Flat glass for the flush modern look and the full solar-control options. Curved glass for the flattest roofs and for units you will never reach to clean.
4. One unit or a gang? One module for a single room, two or three ganged for a wide extension, and a lantern conversation instead if you want one unbroken sheet of glass.
5. What is the whole-unit U-value and the G-value? Ask for both, and ask whether the U-value is centre-pane or whole-unit, because centre-pane always flatters the number. Building Control cares about the whole-unit figure and so should you.
6. How is the upstand being built? A site-built timber kerb or the VELUX insulated kerb, and either way, is the insulation being carried around it unbroken? On a warm-deck roof this is the difference between a dry frame and a wet one.

Get those six answers on paper and you have a real specification rather than a product code. If you want the numbers worked out for your own roof, plane by plane, with the G-value written down next to the U-value rather than left off, ask us for a specification and we will start from which way your roof faces.