How do I fit a channel drain across a drive or a patio?

A channel drain is the answer when a hard surface sheds water correctly but has nowhere for it to go — at a garage threshold, at the bottom of a sloping drive, across a patio that drains into a dead end, or at the boundary with a pavement. The job itself is straightforward. The three things that decide whether it works are the load class, the level relative to the paving, and where the outlet goes.

Safety

What you'll need

• Channel drain units and gratings of the correct load class
• End caps, outlet unit and any silt box
• Concrete for the bed and haunching
• Underground drainage pipe and fittings
• String line, pegs and a long level
• Spade, mattock and a wheelbarrow
• Cable and pipe detector
• Angle grinder with a diamond blade for cutting channel (optional)

Step by step

1. Work out where the water is going before you buy anything.The channel is a collector; it has to discharge somewhere. A surface water drain, a soakaway or a lawful outfall. Never a foul drain. If there is no destination, that is the problem to solve first — a channel that drains into nothing simply moves the puddle.
2. Choose the load class honestly.Channel drains are sold to load classes. A path or a patio is a light duty; a domestic drive with cars needs a substantially higher class; anything a delivery vehicle crosses needs higher still. The rating covers the channel body and the grating together, and a plastic channel with a light grating will not survive a car wheel.
3. Set out the line and the falls.The channel runs across the direction of flow, at the low point. Set a string line at the finished grating level, and plan a fall along the channel to the outlet — many channels have a built-in fall in stepped units, otherwise you build the fall into the concrete bed at around 1 in 100.
4. Excavate wider and deeper than the channel.You need at least 100mm of concrete under the channel and 100mm either side for the haunch, more under a drive. Excavate to firm ground and remove all soft material.
5. Lay the channel on a concrete bed.Bed each unit into fresh concrete, checking level and fall as you go, and butting the units together with their interlocking joints properly engaged. This is not a job to do on sand — the channel has to be held rigidly.
6. Set the grating below the finished paving.3 to 5mm below the adjacent surface, all along its length. Level with the paving and water runs across rather than in; proud of the paving and it is a trip hazard and a wheel-catcher. This dimension is the one that decides whether the drain collects anything.
7. Haunch both sides in concrete.Concrete up both sides of the channel to the underside of the paving, sloped away. Without haunching, the channel floats when the concrete cures, moves under traffic and cracks the paving alongside it.
8. Fit a silt box or an accessible trap at the outlet.Everything that washes off the drive ends up in the channel. A silt box you can lift the lid on and empty is the difference between a five-minute maintenance job and rodding an underground pipe.
9. Connect the outlet properly.Underground drainage pipe, laid with a fall, bedded and surrounded in pea shingle, to the surface water drain or the soakaway. Include an access point where the pipe changes direction so it can be rodded.
10. Lay the paving up to the channel and test it.Paving butted to the channel with the correct level difference, then a hosepipe test over the whole area. Water should reach the channel from every direction and leave through the outlet without backing up.

Tips

• Buy the whole system from one range — channel, end caps, outlet, silt box and gratings. Mixing manufacturers gives joints that do not seal and gratings that do not seat.
• A slightly longer channel than you think you need costs very little more and gives real capacity margin in a storm.
• Composite or ductile iron gratings on a drive are worth the extra over plastic. They take the load, they do not deform and they look better for longer.
• Cut channel units with a diamond blade and support the offcut. Snapping a unit leaves an edge that will not seal to its neighbour.
• Photograph the outlet pipe run before you cover it, with measurements from two fixed points.

Common mistakes

• Bedding the channel on sand or hardcore — It moves as soon as it is loaded, cracks the paving alongside, and the joints between units open and leak into the sub-base.
• Setting the grating flush with the paving — Water flowing across a surface needs a small step down to be captured. Flush gratings shed most of the water straight over them.
• Connecting to a foul drain — It overloads the sewer, causes pollution and flooding downstream and is an offence. Surface water goes to a surface water drain, a soakaway or a lawful outfall.
• No silt trap — Grit, leaves and moss from the whole drive go straight into the underground pipe, where clearing them is a rodding job rather than lifting a lid.

If it doesn't work

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