Installing Drainage Channels into Concrete

The job starts with setting out the line and the levels. The installer stretches a string line or uses a laser level to fix two things: where the top of the grating will sit and which way the water will flow. Digging only starts once these are clear.

The trench has to be wider and deeper than the channel itself, because concrete goes underneath and along the sides. The bottom is compacted. Depending on the ground and the expected load, a layer of compacted gravel goes in next. A bed of concrete goes on top of that, and the channel sections are set into it one by one.

The sections are laid from the lowest point, the outlet or gully, working upstream against the flow of water. Each section is lined up with the string, the joints are pushed together, and an end cap goes on the first section at the top. Then concrete is packed along both sides. Installers call this the haunching. With plastic channels the gratings stay in place while the concrete goes in. Otherwise the pressure of the wet concrete can push the side walls inwards.

Finally, the channel is connected to the drain and the surface around it is finished, for example with block paving or concrete. The concrete has to cure before a car drives over it. Depending on the weather and the mix, this usually takes several days.

The main question is what will walk or drive over the channel. Channels and gratings are sorted into load classes under the European standard EN 1433. The lowest class, A15, is meant for footpaths and patios. Driveways and car parks for ordinary cars usually need B125. Heavier vehicles need a higher class still. The manufacturer's documents state the class.

• Plastic channels are light, easy to cut and suit patios, paths and driveways for ordinary cars, as long as the manufacturer approves them for that load class
• Polymer concrete and concrete channels are heavier and stiffer, carry loads better and are used on driveways, parking areas and yards with traffic
• Galvanised steel gratings are common on footpaths and patios
• Cast iron gratings are chosen where cars drive, because they are strong and lock firmly into the channel

The rule is simple: the channel and the grating both need to match or exceed the load class of the area. A heavy-duty grating on a weak channel does not solve anything.

The water in the channel needs somewhere to go. Some channels have a fall built into the base. Others are laid level, and the water drains away because of the depth of the channel and the position of the outlet. If you create the fall by laying the channel at a slope, roughly 0.5 to 1 % is typical, depending on the manufacturer's advice. That is about 5 to 10 mm per metre. On a channel five metres long with a 1 % fall, the height difference is about five centimetres.

The surface around the channel matters just as much. Paving or concrete must slope towards the channel. If it does not, the water simply runs past it.

Where the water ends up depends on the site:

• A connection to a surface water sewer or a combined sewer
• A soakaway or infiltration crates, if the ground drains well enough
• A rainwater storage tank

Do not connect to a public sewer without agreement from the company or authority that runs it. Many councils and water companies now prefer rainwater to stay on the property and soak into the ground. Check with your local building authority whether larger drainage work or a new connection needs approval, and do not assume that it does not. In a block of flats, speak to the building management or the owners' association before you start.

The simplest case is a new surface, where the channel goes in before the paving and the gully or soakaway is right next to it. The work is then quick and straightforward.

More work is involved when:

• The channel goes into an existing surface, so paving or concrete has to be cut out and made good afterwards
• The run is long, with corners or branches, or the outlet is a long way from the channel
• A higher load class is needed, which means more concrete around the channel and heavier sections
• The ground is poor, for example clay or made-up ground that has to be dug out or stabilised
• There are pipes or cables in the ground, access is tight or the digging has to be done by hand

A common mistake is setting the channel too high. The grating then sticks up above the paving, water collects beside it and car wheels gradually break it. The top of the grating should usually sit a few millimetres below the surrounding surface. Manufacturers often give roughly 3 to 5 mm.

• Too little concrete along the sides: the channel shifts over time and the paving around it opens up
• Concreting plastic channels without the gratings in: the side walls bend and the grating no longer fits
• Uncompacted ground: the channel settles and a dip forms where water stands
• No movement joints in large concrete areas: the concrete cracks next to the channel
• A missing end cap or a leaking outlet: water gets under the channel and washes out the base
• A surface sloping away from the channel: the channel stays dry and the water heads for the house

You can lay a short plastic channel beside a patio or path yourself if you have a spirit level, a string line and some patience. For a driveway that cars use, or for a connection to the sewer, a professional installer is the sensible choice.

1. Decide where the water will go and prepare the outlet, gully or soakaway.
2. Mark out the line and stretch a string at the height of the top of the grating.
3. Dig a trench wider and deeper than the channel, following the manufacturer's instructions.
4. Compact the bottom, add gravel and compact it again.
5. Spread a bed of semi-dry concrete.
6. Lay the first section at the outlet and work upstream against the flow.
7. Line up each section with the string and check it with the spirit level.
8. Fit the end cap and seal the joint at the outlet.
9. Put the gratings in and pack concrete along both sides.
10. Let the concrete cure before laying the paving around the channel.
11. Pour a bucket of water into the channel and watch that it all drains away.

• The grating sits slightly below the surrounding surface along its whole length and does not stick up anywhere
• The channel runs in a straight line with no steps at the joints
• The gratings sit firmly, do not rattle and are locked in place on a driveway
• No water stands in the channel after you pour water in
• Water flows into the outlet and does not escape under the channel
• The concrete along the sides is solid, without cracks or hollows
• The surrounding surface slopes towards the channel
• The load class of the channel and grating matches the traffic on the area
• The channel is easy to reach for cleaning and the gratings lift out