Flat rooflight falls, drainage and ponding

The word flat describes how the roof looks from the garden, not how it is built. Every flat roof worth the name is laid to a fall, which is a deliberate slope towards the point where the water is meant to leave. The slope is described as a ratio: a fall of 1:80 means the surface drops one unit for every eighty it travels horizontally. Over a three metre span that is a drop of about 37mm, roughly a finger and a half. You would not notice it standing on the roof. The water does.

British guidance for flat roofing, set out in BS 6229, treats 1:80 as the minimum finished fall the surface should still have once everything has settled. The catch is the word finished. A roof deck deflects under its own weight and under a load of rainwater, timber is never laid to a perfect plane, and insulation and membrane add their own small errors. If you design to exactly 1:80, the tolerances eat it and parts of the roof end up dead level or falling backwards. So the sensible design fall is steeper than the minimum you want to end up with, and the common figure is 1:40, double the minimum, so that after deflection and construction slack the worst area still clears 1:80.

Here is where those ratios sit in practice.

Two falls often work together. A main fall carries water down the length of the roof, and a shallower cross fall in the gutter or channel walks it along to the outlet. The two combine into a diagonal path the water actually follows, and that path is the thing a rooflight has to be planned around, not the tidy arrows on a drawing.

Ponding is standing water that stays on the roof after the rain has stopped. Not the sheet of water during a downpour, which is normal, but the pool that is still there a day or two later because it has nowhere to go. The working definition in the trade is water that remains on the surface more than 48 hours in weather that would otherwise let it dry. If you can see the tide marks of old puddles as pale rings on the membrane, the roof is ponding whether or not it is wet on the day you look.

A shallow puddle sounds harmless. On a flat roof it is not, for reasons that stack up over years rather than showing on day one.

• Weight. Water is about one kilogram per litre. A pond 20mm deep across two square metres is 40 litres, 40 kilograms, sitting on one spot. That load deflects the deck further, which deepens the pond, which adds more water. It is a slope that runs the wrong way.
• Debris and growth. Standing water traps leaves, grit and pollen, and that silt holds moisture against the surface and feeds moss and algae. A dry membrane sheds all of this. A wet one farms it.
• Freeze and thaw. A pond that freezes expands, works at every seam and lap it is sitting on, then thaws and refreezes. Across an Essex winter that cycle repeats often enough to find any weakness in a joint.
• The seal around anything that penetrates the roof. An outlet, a pipe, and above all a rooflight upstand, are the details most likely to leak. A pond that sits against one of them keeps its junction permanently wet and under a small standing head of pressure, which is the one condition every roofing detail is least happy to live with.

A membrane can carry rain running across it all day without complaint. What it dislikes is water that never leaves. Designing the fall is how you make sure it always leaves.

A flat rooflight does not sit flush in the membrane like a tile. It sits on a raised kerb, an upstand, that lifts the glass clear of the roof surface. That height is not decorative. It keeps the vulnerable glass-to-frame junction well above the wettest zone, so that even in a downpour, with water sheeting across the roof, the level never climbs to the seal. A common minimum is an upstand of 150mm above the finished roof surface, and manufactured kerbs are built to give you that height in one piece.

The upstand solves one problem and creates another. It is a solid box standing in a moving film of water. Water arriving at the uphill face of that box cannot go through it, so it has to go around. If the fall has been set out so the water reaches the kerb and then keeps moving past it to left and right, the rooflight is just an island the stream parts around. If the kerb sits square across the flow with nowhere for the water to escape sideways, the uphill face becomes a dam, and a puddle builds against exactly the junction you least want underwater.

This is why you cannot design the falls and then decide where the rooflight goes as an afterthought. The two are the same decision. The position of the unit, the direction of the fall and the route the water takes to the outlet all have to be resolved together, before a single batten is cut.

There are three ways to put a slope into something that reads as flat, and a given roof often uses more than one.

Firrings

Firrings are timber battens cut on a taper, deep at one end and shallow at the other, laid along the top of the joists so the deck sitting on them slopes even though the joists are level. A carpenter cutting firrings to a 1:40 fall over a four metre extension is starting near 100mm at the high end and tapering to almost nothing at the gutter. Firrings are cheap and well understood, and on a cold-deck build they are the usual answer. What they cannot easily do on their own is form a fall in two directions at once, so a roof that needs both a main fall and a cross fall may need cross firrings laid over the first set, or one of the other methods.

Tapered insulation

On a warm-deck roof, where the insulation sits above the deck and below the membrane, the slope can be built into the insulation itself. Tapered insulation boards are manufactured as wedges to a set fall, cut to a scheme for the specific roof, and laid to a plan that walks the water diagonally to the outlet. This is the method that handles rooflights and internal outlets most cleanly, because a scheme can be drawn to fall away from an upstand on every side and to steepen slightly near the outlet so the water speeds up as it arrives. It costs more than firrings and it needs designing rather than eyeballing, but on a roof with a big rooflight in the middle it is often the honest answer.

The structure

Sometimes the fall is built into the frame, by setting the joists themselves to a slope or by sitting a flat deck on a structure that already leans. On a new extension this can be the tidiest route of all, because the fall is there before any covering goes on. On existing 1930s and post-war stock it is rarely available without rebuilding the roof, which is why firrings and tapered insulation do most of the work on the housing round here.

The single most important placement rule is simple: the rooflight must never sit at the low point. Water heads for the lowest part of the roof, and if that is where the unit is, everything the roof collects ends up parked against the upstand. The low point belongs to the outlet, and nothing else.

Beyond that, a few principles decide a good position.

• Fall away on the uphill side. The roof should drop away from the kerb on the side facing up the slope, so water is turned aside before it reaches the box rather than gathering in front of it. On a tapered scheme this is drawn in deliberately as a back gutter or a diverging fall.
• Keep the flanks clear. The water parting around the unit needs a clean run down each side to the gutter. A rooflight crammed hard against a parapet or a wall pinches that route and invites a puddle in the pinch.
• Mind the outlet count. A big rooflight can cut a roof into zones, and a zone that used to drain to one outlet may now be split so that half of it has nowhere to go. Splitting the roof can mean a second outlet is needed, and that is a decision to make on the drawing, not on the roof.
• Leave working room. The junction between the kerb and the membrane needs space for the covering to be dressed up and over the upstand properly. A unit shoehorned tight against an obstruction leaves no room to form that detail well, and the detail is the roof’s weak point.

A run of several smaller flat units, rather than one large lantern, changes the sums again, because now the water has to find its way between the boxes as well as around them. That is workable, and often the right look over a long kitchen, but the gaps between units become drainage channels in their own right and have to be set out as such.

The upstand can be a builder’s kerb, framed on site in treated timber and then covered by the roofing, or a proprietary insulated kerb supplied to sit under the rooflight as a single component. The manufactured kerb gives a known height, a square top for the unit to seat on, and insulation built in so the upstand is not a cold ridge running round the opening. Either way, the covering has to be carried up the outside face of the kerb and finished under the rooflight frame, so the water can climb the upstand and still meet a sealed surface all the way up.

The covering itself decides how forgiving the roof is about the odd shallow area. These are the common choices on domestic flat roofs and how they behave.

No covering on that list is a cure for a roof that ponds. A good membrane laid over a bad fall is still a bad roof, it just takes a little longer to say so. The fall comes first, the covering second, and the detail where the covering meets the rooflight upstand is where the two either add up or let you down.

The reason this matters so much across south Essex is the shape of the housing. The large 1930s semi belt that runs through the area was mostly built with pitched main roofs, but the rooms people have added since, the rear kitchen extension, the utility off the back, the flat-roofed link between house and garage, are overwhelmingly flat-roofed. Those are the roofs that grow a rooflight, because the whole point of a single-storey rear extension is to get daylight into a room that has a house on one side and a boundary on the others.

Many of these flat roofs are decades old, laid to falls that were shallow to begin with and have flattened further as the timber has aged and taken up moisture. Adding a rooflight to a tired roof, or replacing the whole covering and the failed unit together, is the moment to put the falls right rather than to lay a new membrane over the same old slump. If the deck no longer falls properly, a new rooflight on top of it inherits every drainage fault the old roof had.

There is a second reason a south Essex flat rooflight deserves attention, and it is the one this business is built on. A flat rooflight faces straight up. It is the most exposed orientation there is, collecting more solar energy per square metre through a summer afternoon than any pitched or vertical glass on the same house. Southend faces south across the estuary with an open horizon, so a flat rooflight over a south-facing kitchen extension is a solar collector with a dining table under it. That makes the glass specification a separate question from the drainage, and just as important: the same unit that has to shed water in February has to keep the room under it from cooking in July. It is why the flat roof over the kitchen is the classic case for solar-control glazing, chosen for its G-value, and why we look at the compass on a flat roof as closely as we look at the falls. Drainage keeps the room dry. The glass keeps it usable.

You do not need to be a roofer to ask the questions that separate a flat-roof skylight that stays dry from one that gives trouble in its third winter. Raise these before anything is fixed.

1. What fall is the roof laid to, and how is it formed? You want to hear a design fall of around 1:40 aiming at a finished 1:80, and a clear answer of firrings, tapered insulation or structure. A shrug about the fall is the warning sign.
2. Which way does the water go around the rooflight? There should be a definite answer: it falls away from the kerb on the uphill side and runs down both flanks to the gutter. If the unit sits square across the flow with nowhere to escape, that is a dam.
3. Is the rooflight clear of the low point? The lowest part of the roof is for the outlet. Confirm the unit is not sitting in it.
4. How high is the upstand? Around 150mm above the finished roof surface keeps the glass junction well clear of any standing water. A kerb barely proud of the membrane is a leak looking for a wet week.
5. Does the drainage still work with the unit in place? A large rooflight can split a roof into zones and leave part of it stranded from the outlet. Ask whether a second outlet is needed once the box is in the middle of the roof.
6. Who notifies Building Control? A new or enlarged rooflight is notifiable, and the work has to meet the current regulations, including Part L for thermal performance and Part K for the safety glazing overhead. We make that notification to Southend-on-Sea City Council, or the relevant local authority for your address, as part of the job.

If the roof itself has failed, standing water sitting on a deck that has softened, a covering at the end of its life, that is a roofing job first and belongs with a roofer. Where the work is a new flat rooflight, or lifting out a failed unit and putting a properly specified one back on a roof that drains, that is our work, and it starts with the falls. We have been fitting flat-roof skylights and roof lanterns across south Essex for over fifteen years, and every one of them is planned around the water before it is planned around the view.

If you want the falls, the upstand and the glass thought through together for your own roof, read more on flat-roof skylight installation, look at the options for a roof lantern over a larger extension, or ask us for a specification that covers the drainage and the glazing on the same drawing.