Flat Roof and Pitched Roof Rooflights Compared

Two roofs, two problems

A rooflight in a pitched roof and a rooflight in a flat roof are solving different problems, and the products reflect that. The pitched unit belongs to a surface that gets rid of water by gravity, in lapped courses, within moments of the rain landing. The flat unit belongs to a surface with no such advantage, where a continuous covering holds water on the deck and walks it slowly towards an outlet. Almost everything that separates the two categories follows from that one fact.

It is also why so little crosses the line between them. A hinged roof window is made for a slope and does not behave sensibly laid nearly level. A lantern is made to stand on a deck and has nowhere to stand on a slate roof. Even where one manufacturer offers what looks like similar glass in both ranges, the frame around it, the way the covering is joined to it, and the order in which the two are built are different pieces of work.

That is why the comparison is not a contest. There is no better of the two, because nobody chooses between them in the abstract. The roof decides, and the only useful question is what your roof actually is. What follows is what changes, in the order it changes, so that a quotation makes sense when it arrives and the parts of it that look expensive are recognisable as the parts doing the work.

In the plane, or above it

The clearest way to hold the difference in mind is this. A pitched-roof unit sits in the roof. A flat-roof unit sits on it.

On a slope, the unit is built into the structure. Rafters are cut and trimmed to form an opening, the frame is fixed to that timber, and the slate or tile is worked up to the frame on all four sides so that covering and unit form one continuous surface. Whether the frame stands a little proud, as a standard roof window does, or lines up with the covering, as a flush conservation-pattern unit does, it remains an element inside the thickness of the roof rather than an object placed on top of it.

On a flat roof there is no thickness to sit within, and setting glass down level with the deck would put it at the low point of everything the roof collects. So a kerb is built: a raised timber upstand off the deck, insulated, with the roof covering dressed up its face and over its top edge, and the unit seated on that. The rooflight is therefore not really attached to the roof at all. It is attached to a small structure built off the roof, and that structure is the part keeping the room dry.

Shedding water against holding it

On a slope, water is a passenger. It lands, it runs, and it is gone. The covering is not waterproof in the sense a bath is waterproof; it is a set of overlapping pieces arranged so that gravity carries water from one to the next without it ever having time to find a way back up. A rooflight in that system has to join the same arrangement, taking water from the courses above, passing it round both sides, and returning it to the courses below. Where these installations fail it is almost always because water was pushed sideways or upward by wind, into a lap too short for the pitch it was asked to work at.

A flat roof does the opposite. The covering is a continuous skin, welded or bonded into one surface, and it works because there are no laps for water to get behind. Water is not a passenger here. It sits, moves slowly, and stays on the roof for a while after the rain has stopped. Anything projecting through that surface has to be lifted clear of the water rather than joined to it, which is exactly what the kerb is for.

So the two failure questions are different. On a slope you ask whether the laps and the flashing can cope with wind-driven rain. On a flat roof you ask whether the covering has been carried high enough up the kerb and terminated properly at the top, and whether water can ever reach that termination.

What the flat side needs

Three things dominate the flat-roof job, and none of them is the glass.

The first is the height of the upstand. It is set by water rather than by appearance: the covering has to run up the kerb and finish above anything the roof will ever hold, including a wet spell with a partly blocked outlet, and including snow lying against the kerb before it melts. A low kerb looks tidier from a bedroom window and is the single most common shortcut in this trade.

The second is insulation. A kerb is a timber box standing above an insulated deck, and if it is left uninsulated it becomes a cold ring around a warm opening. What follows is not a leak but it is often reported as one: moisture in the room condensing on the coldest surface it can find, running down the inside face and appearing at the ceiling line. The kerb is insulated and the vapour control layer is carried through, or the room gets water it did not come from outside.

The third is falls. Flat roofs are not level; they are laid to a shallow fall towards outlets. Water still lies in places after rain, and a certain amount of standing water on a flat roof is normal rather than a defect. What matters is where it lies relative to the kerb and the outlets. Our flat roof skylight installation page carries the detail on build-ups, warm roofs and terminations.

What the pitched side needs

The pitched job turns on two things: the perimeter and the timber.

At the perimeter, the unit is weathered with a flashing arrangement made for the covering around it. Slate, plain tile, interlocking concrete tile and stone slate each behave differently, and each takes a different kit or a different piece of leadwork. Side soakers are woven into the courses as the covering is laid, an apron takes water away below, and the head is dressed so that everything arriving from above is thrown clear. All of it depends on laps being long enough for the pitch. A shallower slope needs longer laps, because water moves more slowly and wind has more chance to drive it back up. Every product also carries a minimum pitch below which the manufacturer will not stand behind it.

Below that, the rafters decide what is possible. A unit that drops between existing rafters at their present spacing is a light structural job. A unit wider than that spacing means cutting a rafter and trimming the opening so the loads go round it, which is a designed alteration rather than a detail. Trusses are stricter again.

Then there is how the finished unit reads from the ground, which on a slope is never irrelevant. Our pitched roof skylight installation page covers coverings, trimming and flashing in full.

How each one reads outside

Appearance splits the two categories more sharply than anything except water.

A pitched-roof unit is on an elevation. Somebody standing on the pavement can see it, and so can anybody looking across from further off, because roofs are read from further away than walls are. That is why the choice between a standard roof window and a flush conservation-pattern unit is a real choice on a slope, why the finish matters, and why the position of the unit on the slope becomes a subject in its own right. Where a building is older, in a conservation area or listed, the visible slope is where the specification narrows, and that is settled before anything is ordered.

A flat-roof unit is usually invisible from the ground. On a single-storey rear extension the roof sits below every sightline a passer-by has, and the appearance question quietly disappears. That is genuinely freeing: it is why flat roofs carry large glazed areas, walk-on panels and lanterns that would be unthinkable on a front slope.

Usually is not always, though. A flat roof is seen from the upper windows of the house itself, from neighbouring upper windows, and from any rising ground behind. A lantern in particular is an object standing above the roof line rather than a panel lying in it, so it has a shape, a frame colour and a height that somebody will notice. That is worth thinking about at specification rather than after installation.

Which roof takes which product

Some products only exist on one side of this divide, and knowing which is which saves a good deal of wasted looking.

The reasons are structural rather than commercial. A lantern gains its light by carrying glass on several planes above the roof line, which requires a level base to stand on. A walk-on panel has to be a floor as well as a window, and a floor cannot be sloped. A conservation-pattern unit exists to disappear into a slate or tile plane, which is not something a flat roof has.

Fixed and opening versions exist on both sides, and glazing choices such as coatings and solar control run across both. The categories above are the ones where the roof rules the product out entirely, and they are the ones worth checking before falling for a photograph.

Knowing which conversation you are in

Most people know which roof they have. The ones who do not usually have a good reason, and it is worth naming the cases.

The commonest is the shallow slope. A lean-to over a rear addition, a garage roof, or a low extension roof can look flat from the garden and still be a pitched roof carrying a lapped covering, or it can be a genuinely flat deck laid to a fall. The test is not the angle you perceive. It is the covering. Overlapping pieces, whether slate, tile or sheet, mean a shedding roof and a pitched-roof product. A single continuous membrane, felt, single-ply or asphalt, means a holding roof and a kerb.

The second case is a house with both, which is very common: a pitched main roof over the original house and a flat roof over a rear extension. These are two separate specifications, often ordered at the same time, and it is entirely normal for the two units to look nothing alike.

The third is a shallow pitch that falls below a product’s stated minimum. It is a real slope, but the range you were looking at will not go on it, and the honest answer is a different product rather than a smaller version of the same one.

When you enquire, say what the covering is and roughly how steep the slope runs. Those two pieces of information settle which half of this page applies, and everything after them follows quickly.