Drainage around a trafficked rooflight

Perimeter channels

The usual solution is a drainage channel around the perimeter of the unit, at or just below finished level, taking water away before it reaches the frame junction.

On a terrace with a paved or decked finish, the channel usually sits within the build up and discharges to an outlet. On a smaller light well it may be a linear channel on one or two sides.

What goes wrong

No channel at all. Water crosses the terrace, reaches the flush junction and sits there. Eventually it finds a way past, and the fix is not a sealant.

Channels that block. Leaves, grit and moss fill a channel that nobody thinks to clean, and a blocked channel is worse than none because it holds water against the unit.

Falls laid towards the unit rather than away, usually because the paving was set out from the building rather than from the drainage.

Falls

The surface around a walk-on unit falls away from it, not towards it. That sounds obvious and it is routinely got wrong on terraces where the fall was set for the building rather than the rooflight.

Maintenance

Clear the channel a couple of times a year, more under trees. It takes minutes and it is the single thing that keeps a walk-on installation dry.

Worth agreeing at handover who does it, because a channel that is out of sight under a decking board is a channel nobody cleans.

Detailing at the frame

The waterproofing is turned into the frame rather than up a kerb, so the junction is doing more work than on a standard rooflight. It is a detail worth executing properly rather than adequately.

Why drainage is the whole problem

A rooflight on a pitched roof sheds water by gravity. The pitch does most of the weathering work and the flashing only has to handle what is left.

A trafficked rooflight is horizontal, at ground or deck level, in a place where water arrives from every direction and has nowhere obvious to go. Nothing is helping. Every drop that lands on or near the unit has to be given a deliberate route away from it.

That is why the great majority of failures in walk on installations are drainage failures rather than glass or seal failures. The unit was fine. The water was not sent anywhere.

Where the water comes from

Three sources, and a good detail handles all three.

Rain landing on the glass. The glass sheds it to the edges. That is the smallest of the three and the one people design for.

Water running across the deck. On a terrace or a paved area, the surrounding surface collects far more water than the unit does, and it all moves according to the falls. If those falls point at the glass, the unit receives the drainage of the whole terrace.

Water rising from below. On a light well or a sunken area, water that cannot escape backs up. A blocked outlet turns a well into a tank, and the unit ends up partly submerged.

Falls, and getting the direction right

The deck must fall away from the unit, or fall to its own outlets that are not at the glass perimeter.

This sounds obvious and it is the single commonest error on these installations. It happens because the deck is laid by a different trade, after the unit is in, working to a level datum somebody set for the paving rather than to a drainage strategy that included the rooflight.

The fix is to establish the falls at design stage, on a drawing, and to make one party responsible for the finished levels. Not to hope it works out on site.

A fall of even a small amount is enough on a smooth surface. Zero fall is not, because no surface is laid perfectly flat and the low spots become puddles.

The perimeter channel

On a flush installation, where an upstand cannot turn water away, a channel around the perimeter does the job instead.

Three requirements, and all three are non negotiable.

It needs an outfall. A channel that connects to nothing is a trough that holds water permanently against the perimeter seal. It has to discharge into the roof drainage or a drain, with enough fall along its length to actually move water to that point.

It needs a removable grating. A channel that cannot be lifted and cleared will fill with leaves, grit and moss within two seasons, and from then on it behaves exactly like the previous failure.

It needs to be sized for the catchment. A narrow decorative slot handles the water off the glass. It does not handle the water off a terrace in a downpour.

The drained and ventilated frame

Good structural glazing systems assume some water will get past the outer seal, and design a route for it back out.

The frame has a chamber below the glass with weep paths to the outside. Water that penetrates the surface seal drops into the chamber and drains away rather than accumulating against the glass edge or finding its way inside.

It is a standard concept and a standard term. Ask whether the system is drained and ventilated. A supplier who cannot answer is not selling a system designed for this position, and the difference shows up as the distinction between a seal failure that produces a damp patch and one that produces water in the room below.

The membrane sequence

On a terrace or a flat roof, the waterproofing membrane is the primary defence and the unit sits within it, not on top of it.

The membrane is dressed up the upstand and terminated into the unit, then the unit goes on, then the deck build up goes over. Membrane first, unit second, deck third.

Where a membrane has been dressed over the top of an already installed unit, the lap runs the wrong way and water is being asked to climb. That cannot be corrected without lifting the deck and starting again, which is why the sequence is worth confirming before the work rather than after.

Light wells specifically

A light well is a sunken area beside a basement wall, and it drains to a single outlet at the bottom.

That outlet is the whole system. If it blocks, the well fills, and a full light well puts a standing head of water against the basement wall and against the underside of the glass.

So it needs to be accessible, it needs a leaf guard, and it needs checking. Where the outlet discharges below the level of the surrounding drainage it may need a pump, and a pump needs power, maintenance and ideally an alarm.

This is the part of a basement conversion that gets least attention and causes most trouble.

Freezing

Worth a thought in a channel that runs shallow across an exposed terrace.

Standing water in a perimeter channel freezes, and ice expands. Repeated over winters that works joints and seals. The answer is not heating, it is fall: a channel with enough gradient does not hold standing water and therefore does not hold ice.

The maintenance that prevents failure

Clear the perimeter channel twice a year, more often under trees.

Check the outlets run freely. Pour a bucket of water in and watch it go.

Look at the deck after heavy rain and note where water sits. A puddle that forms in the same place every time is telling you the fall is wrong there, and it is far cheaper to correct while it is only a puddle.

Inspect the perimeter seal annually for splits or gaps at the corners, which is where a sealant joint gives up first.

The diagnostic when it does leak

Water appearing after heavy rain points to drainage: falls, channel, outlet.

Water appearing in cold weather without rain points to condensation on the underside of a cold unit, which is a thermal and ventilation problem rather than a drainage one.

Water appearing steadily regardless of weather points to something below rather than above, particularly on a light well beside a basement wall.

Establishing which of the three you have, before anybody starts lifting deck, saves a great deal of money.

Sizing the channel for the real catchment

The commonest sizing error is treating the perimeter channel as though it only handles the water landing on the glass.

It does not. On a flush installation the channel intercepts water arriving from the deck as well, and the deck is frequently many times the area of the pane.

So the channel and its outlet have to be sized for the terrace, not the rooflight. A narrow decorative slot that looks elegant on a drawing will back up in the first heavy shower of the autumn.

Fall along the channel

A channel needs its own gradient, not just an outlet at one end.

Without fall, water sits along its length and only the section nearest the outlet drains. The rest holds standing water permanently against the perimeter seal, which is exactly the condition the channel was installed to prevent.

That gradient has to be set when the channel is installed, and it is one of the things worth checking on a drawing rather than trusting to the day.

The outlet, and where it goes

Into the roof drainage or a drain, with a route that will not block.

A channel discharging onto the roof surface a short distance away is not draining, it is relocating. On a roof with marginal falls that water finds its way back.

Where the outlet is below the level of the surrounding drainage, a pump may be needed, and a pump needs power, maintenance, and ideally an alarm. That is a design decision, not an afterthought.

Access, which decides whether any of it survives

Every part of the drainage has to be reachable and clearable by somebody with a bucket and a pair of gloves.

A removable grating. An outlet that can be rodded. A channel shallow enough to reach into.

Where the design makes clearing difficult, it will not be done, and a drainage system that is not maintained is a drainage system that fails. Designing for maintenance is not a refinement here, it is the difference between working and not.

The annual routine

Lift the grating, clear the channel, pour a bucket of water in and watch it go. Twice a year under trees.

And after heavy rain, look at where water sits on the deck. A puddle forming in the same place every time is telling you the fall is wrong there, and it is far cheaper to correct while it is still only a puddle.