150 Litres of Rainwater Diverted by a Aco HexDrain Run Beside a Sloping Path

Why the 150 litres appears

A sloping path sheds water according to its plan area and gradient. An 8 metre run at 1.2 metres wide gives about 9.6 square metres of catchment. In a downpour delivering 15 mm in an hour, that surface produces close to 144 litres. Porcelain sends almost all of it downslope because its water absorption sits below 0.5 percent under the EN ISO 10545-3 test. The 150 litre figure for one storm is not an outlier; it follows from a low-absorption surface laid on a fall.

The Aco HexDrain range deals with that sort of flow through a 118 mm wide channel with a load rating up to Class A15, suitable for foot traffic and light garden equipment. Along the downhill edge, the run catches sheet flow before it reaches turf or a neighbouring boundary. Invert depth sets the installation height. A HexDrain channel is roughly 80 mm deep, so the concrete haunch has to sit low enough for the grating to finish 3 to 5 mm below the adjacent porcelain. At that offset, water drops into the slot instead of skating over the top.

Bedding the porcelain against the channel

Porcelain at 20 mm thickness is unforgiving when laid on spots of mortar. The spot-and-dab method that can survive under riven sandstone leaves voids below a vitrified slab. Those voids become pressure points, and a single load on a corner over a hollow can crack the tile.

A full mortar bed removes the hollow zones. The bed is usually 30 to 40 mm of 4:1 sharp sand to cement, mixed wet enough to hold a slump while staying short of slurry. Each slab goes down onto fresh mortar, with no gaps left under the body of the tile.

Adhesion fails easily on porcelain unless the underside is primed. Vitrified backs are close to non-porous, so an SBR-modified cement slurry is painted onto the underside immediately before setting. Without that primer, the slab can lift on the first frost cycle. With the primer wet, the back is buttered and the slab is beaten down into the mortar bed with a rubber mallet until full contact is achieved.

The fall across the porcelain needs to be formed in the bed, aiming toward the HexDrain line. A gradient between 1 in 60 and 1 in 80 is steep enough to move water, yet shallow enough that a wheeled trolley does not feel as if it is tipping across the path.

Level against the grating matters more than visual flatness across the last course. If the porcelain finishes proud of the grating by even 2 mm, runoff can collect against the tile edge and leave the joint there saturated after rain. The screed rail should reference the set grating height, so the paving and channel meet at the intended 3 to 5 mm offset.

That final course also controls how cleanly the sheet flow enters the slot. A small lip of mortar squeezed against the channel, a tilted edge tile, or a grating set too high can defeat the purpose of placing the drain on the low side. The channel may have the capacity, while the approach to it still holds water on the porcelain face.

MOT Type 1 and the concrete surround

MOT Type 1 sub-base is a graded crushed limestone or granite from 0 to 40 mm conforming to the Specification for Highway Works Clause 803. Under a pedestrian garden path on stable, well-drained subsoil, 100 mm compacted depth is the working minimum. On clay, on made ground, or where a ride-on mower will occasionally use the path, 150 mm is the figure to build to.

Type 1 must be laid and whacked in layers no thicker than 75 mm. A single 150 mm dump plated from the top compacts the upper 60 mm and leaves the lower material loose. That loose material settles later, taking the mortar bed and the channel down unevenly. An 80 to 100 kg wacker plate should be run over each layer until the aggregate stops moving under the plate. The resulting base holds the HexDrain haunch at its set level through freeze-thaw.

The channel needs its own concrete bedding within the same formation. A HexDrain is normally placed in a C20 concrete surround, with about 100 mm of concrete beneath the channel and a 100 mm haunch up each side to the underside of the grating seat. The concrete keys into the compacted Type 1. The sub-base depth therefore carries through continuously under the paving and the drainage line, instead of stopping at the channel edge.

This arrangement still assumes the ground beneath is able to drain. The HexDrain intercepts surface flow, yet water rising through the sub-base from a high water table or a spring line remains outside the job the channel performs. A run sized for 150 litres of storm runoff and a base built to 150 mm of Type 1 both rely on the soil below accepting water. The outlet then has to discharge to a point able to take the same volume, whether that point is a soakaway, an existing gully, or a lower drain.

Jointing where water slows down

Water caught by the HexDrain is water that no longer loads the slab joints along the lower edge. The joints still take rain that falls directly on them and any surface water that travels across them before reaching the slot. Polymeric jointing compound suits porcelain paths because it is a graded sand pre-mixed with a polymer binder that cures when wetted. It resists the washout suffered by plain sand and helps lock the slabs against lateral creep.

Porcelain creates a specific cleaning problem during application. The polymer film stains vitrified surfaces if it dries on the face. The compound has to be swept into 5 to 10 mm joints, struck off cleanly, and then the surface has to be blown or brushed free of every remaining grain before the activating water is applied. A smear that cures on a 20 mm porcelain slab will stay on the surface. A joint below 3 mm will not accept the graded sand fully, leaving a shallow, brittle fill that can pop out.

At the meeting line between porcelain and HexDrain, the joint needs a different material. A flexible polyurethane or MS-polymer sealant absorbs the differential movement between the concrete-set channel and the mortar-bedded paving. Rigid polymeric sand at that interface can crack within a season because the two elements move at different rates under thermal and moisture cycling.

Edging the open side

The uphill side of the path, away from the channel, still needs restraint. Without an edge, the mortar bed and outer course can migrate downslope over years of loading, and the joints there open first.

A haunched edging holds that perimeter. It can be a course of block or a concrete upstand bedded on and backed by the same C20 mix, either as a flush kerb or as a hidden restraint buried below finished grade so the porcelain reads cleanly into the border.

Cleaning the damp edge

Even with the HexDrain carrying most runoff, the shaded downhill edge of a path stays damp the longest. Within a couple of seasons it can develop a black or green algal film. Lithofin Algex is a contact cleaner formulated for that growth: applied undiluted to the dry surface, left to dwell, and then rinsed away, it kills the organic film without the surface etching that a strong hypochlorite solution can risk on jointing.

On polymeric-jointed porcelain, the cleaner removes the film without dissolving the polymer binder. That matters because repeated close-range pressure washing strips polymeric joints faster than ordinary weathering does. Reapplication frequency follows aspect and drainage. A north-facing run that the channel keeps dry across its face greens far more slowly than an edge where water continues to linger against the tile.