Guide · Flat Roof Rooflight Installation
Flat rooflight falls, drainage and ponding
Water leaving a flat roof is a design problem, not an accident. It is solved on paper with a fall, an outlet position and a route, and every one of those decisions changes when a rooflight is dropped into the middle of the plane. A kerb is an obstruction. Water arrives at it, goes round it, and whatever the roof does at that moment decides how the detail behaves for the next twenty years.
This page covers how falls are built, how much fall is enough, where outlets belong once a rooflight is in the way, and what ponding actually means. It sits alongside the wider flat roof rooflight installation guide.
The fall a flat roof is designed to
British practice is a minimum finished fall of 1:80, which is 12.5mm of drop for every metre travelled. Because timber deflects, insulation compresses slightly and nothing on site is built to drawing tolerance, roofs are commonly designed at 1:60 or 1:40 so that 1:80 is what actually exists once the roof is loaded and finished.
That is the entire logic of the numbers. The design fall is a starting figure with slack built into it. The finished fall is what the water experiences. Anyone quoting 1:80 as the design figure is quoting the floor as though it were the target, and on a roof with a rooflight in it there is no slack left to give away.
Design falls against finished falls
The gap between the two is where most standing water comes from. A 4m deep extension roof laid at a design fall of 1:80 drops 50mm across its depth. Take off 10mm of joist deflection under a snow load, 5mm of settlement in the insulation and 5mm of accumulated tolerance in the firrings, and 30mm of drop is left across four metres. That is 1:133, and it is enough to hold water in a broad shallow film that never quite leaves.
None of that is dramatic on its own. It is cumulative, which is why the design figure is deliberately generous and why cutting it back to save 20mm of build-up height is a decision that reads well on a section drawing and badly on the roof.
Firrings, and how a fall is built in timber
The traditional method. Firrings are tapered strips of softwood, ripped from stock and fixed along the top of each joist so the deck sits on a sloping line while the joists themselves stay level. A typical set runs from 25mm at the high end to 75mm at the low end over a four metre span.
Firrings run in one of two directions and the choice matters around a rooflight. Laid along the joists, the fall runs down the length of the roof and the deck boards can be laid the natural way. Laid across the joists, on cross firrings sitting over them, the fall can run in the other direction, which is often what is needed to get water past a kerb rather than into it. The second costs a little more timber and rather more setting out.
Tapered insulation, and how a fall is built in a warm roof
The modern method, and the usual one where the deck itself is already level. Insulation boards are supplied cut to a taper, laid to a scheme drawn by the manufacturer, and the fall is created in the insulation layer rather than the structure.
The advantage around a rooflight is that a tapered scheme can be drawn to do clever things: it can fall away from a kerb on all four sides, it can create a valley that takes water round an obstruction, and it can build a raised diverter behind an upstand without any carpentry at all. The cost is thickness at the high point and a longer lead time, because the scheme is designed and cut to the specific roof rather than pulled off a shelf.
Which way the fall should run past a rooflight
The rule is simple to state and frequently ignored. Water should reach the kerb along the sides, not head on into the upslope face, and it should leave the area around the unit without changing direction more than once.
A rooflight sitting square in a roof that falls straight at it creates a dam. Water arrives at the upslope face, spreads sideways looking for a route, and slows down where the flow splits. Slow water drops the silt it is carrying, and silt is what builds the deposit that eventually stops the water leaving at all. Rotating the fall so it runs parallel to the long sides of the kerb removes the problem before it exists.
The dead area upslope of a kerb
Even with the fall running the right way, there is a zone immediately behind the upstand where flow is disturbed. On a wide unit it can be half a metre deep, and it is where debris collects on every flat roof with a rooflight in it.
Two things reduce it. Keeping the kerb reasonably narrow in the direction across the flow, so water has less distance to travel round it. And building the roof so that this zone still has fall in it rather than being the one flat piece of an otherwise sloping surface. On surveys across Harlow and Basildon, where flat roofs are the local default rather than the exception, this strip is where the deposit sits and where the covering has usually worked hardest.
Diverters, cricket boards and splitting the flow
Where the geometry cannot be changed, water is turned instead. A diverter is a raised wedge built into the roof upslope of the kerb, tapering to nothing at each end, that splits the flow and sends it round both sides of the unit. In tapered insulation it is a set of cut boards. In timber it is a small firred structure decked over before the covering goes on.
They are common on side extensions where the fall has to run towards the house before turning to the outlet, and on roofs where a rooflight was added to a plane that was never designed with one in it. Built properly a diverter is invisible under the covering and changes nothing about the appearance from the ground.
Outlets, and how many a roof needs
Two, wherever there is room for two. The reason is not capacity, it is failure. A single outlet on a domestic flat roof is a single point at which a tennis ball, a bird’s nest or a season of leaves can close the entire drainage system, and once it has closed, the water level rises until it finds the lowest edge of something. Frequently that something is a rooflight kerb.
Where the roof genuinely only has one outlet position, the alternative is an overflow: a second opening set 25 to 50mm above the outlet, discharging somewhere visible, so a blockage announces itself as a stain down the wall rather than as a rising level nobody can see from the ground.
Sizing outlets and the storm the roof is designed for
Rainwater design in the UK works from a rainfall intensity, commonly 0.021 litres per second per square metre for the two minute storm used on domestic roofs, multiplied by the effective catchment area. A 5 x 4m extension roof is 20 square metres, which gives a design flow of about 0.42 litres per second.
A 75mm outlet handles that comfortably; a 50mm outlet is marginal once a leaf grating is fitted, because the grating is the real restriction rather than the pipe. The number worth knowing is that a rooflight adds nothing to the catchment but takes area away from the drainage path, so the water arriving at the outlet is the same and the route it takes is narrower.
Gutters, spouts and where water leaves the roof
Not every flat roof drains through the deck. Plenty of Essex extensions, particularly single storey rear additions on Victorian terraces in Chelmsford and Colchester, drain over an edge trim into a gutter on the low side. That is a perfectly good arrangement and it has one property worth understanding: the whole low edge is the outlet, so a blockage cannot close it completely.
The complication comes when a rooflight sits close to that edge. Water rounding the kerb accelerates into a narrow strip between the unit and the trim, and that strip needs to be wide enough to carry the flow. Under about 300mm and the water backs up behind the kerb during heavy rain, which is the exact condition the upstand height was designed to survive but not the condition it should meet weekly.
Deflection under load, and the fall it eats
Timber flat roofs deflect. The permitted figure for a joist is span over 250 under full load, which on a 4m span is 16mm at midspan. Insulation and the deck add a little more. If the roof was built with the minimum fall and the rooflight sits near midspan, the loaded roof can be close to level in exactly the place it most needs not to be.
This is one reason a large flat rooflight often sits better near a bearing than in the middle of a span, and one reason the joist sizes get looked at during a survey rather than taken on trust. A stiffer roof holds its fall, and a roof that holds its fall drains.
Ponding, how it is measured and what it means
The working definition is water still standing 48 hours after rain has stopped, in conditions where evaporation should have cleared it. Anything shallower than about 5mm is a film and it dries. Anything that survives two dry days is a pond, and it is telling you the fall in that area is effectively zero.
Ponding is not automatically a defect in every covering, and some membranes are warranted with it. What it does reliably is find weakness: it keeps a joint permanently wet, it grows algae, it collects silt that then holds more water, and it freezes. Around a kerb it is the single condition most likely to expose a marginal termination. Where a survey finds ponding at the proposed rooflight position, the honest order of work is to correct the fall and set the glass into a roof that drains. The separate question of what to do with snow and standing water on a unit that is already in is covered on snow and standing water on a flat rooflight.
Water standing two days after the rain stopped is not weather, it is a fall problem with a date on it.
Debris, moss and the drainage path that closes itself
Most drainage failures are not built, they accumulate. Leaves from the sycamore three gardens away, moss washed down off a pitched roof above, grit from a covering that is ageing, and the silt that all of it traps. The path narrows over four or five autumns and nothing looks wrong from the ground.
Around a rooflight the accumulation has a specific signature: a dark crescent behind the upstand and a clean strip either side of it where the water still moves. Clearing that crescent each autumn is five minutes of work and it is the highest value maintenance available on a flat roof. Where the roof cannot be reached safely, that is worth solving at design stage rather than discovering later.
Self-draining glass and the fall built into the unit
The rooflight itself has a fall too. Flush glazed flat units are commonly supplied at 3 to 5 degrees, built into the frame or achieved by the kerb, so water runs off the pane rather than sitting on it.
Below about 3 degrees, water beads, sits, evaporates and leaves behind the dissolved minerals it was carrying. Over a couple of years that becomes a haze that no cleaning removes, because it has etched. Self cleaning coatings help by making the film sheet rather than bead, but they need a fall to work with. The coating manages the dirt; the angle manages the water.
Checking falls on a survey
The check is done with a long level and a tape rather than by eye, because a roof laid to 1:80 looks flat to everybody. A two metre level with a 25mm packer under one end reads level when the fall is exactly 1:80, and that is a quick field test anyone can run.
Alongside it: where the outlets are and what condition the gratings are in, whether there is an overflow, which way the deck boards run, whether there is an existing deposit line, and whether the proposed rooflight position sits in the drainage path or beside it. Those answers change the price and occasionally change the position of the unit, which is far cheaper to discover before anything is ordered.
What a drainage led design changes about the plan
Starting from drainage rather than from the ceiling plan usually moves things by a few hundred millimetres, and the moves are small enough that nobody notices them in the finished room. A unit slides sideways to sit beside the flow rather than in it. A pair of units is set with a clear channel between them rather than butted close. The fall is rotated ninety degrees at design stage, costing nothing, where rotating it later costs a roof.
The finished room does not care about any of that. It gets the same rectangle of sky in almost the same place. The roof cares enormously, and the roof is the half of the job that has to survive the next two decades of Essex weather without anybody thinking about it.
Keep reading
More on flat roof rooflight installation
Walk-on rooflights and where they make sense
Read the guide →Single-ply, felt and GRP roofs around a rooflight
Read the guide →Flat rooflights on a cold roof build-up
Read the guide →How many rooflights a flat roof extension needs
Read the guide →Flat rooflight standard sizes and made to measure
Read the guide →
Get a quote
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
- Surveyed before it is priced
- 10-year workmanship guarantee
- Building Control notification handled
- New installations and replacements