Below-ground drainage: pipes, falls and access

Foul water and surface water

Foul water is the used, contaminated water from sanitary appliances - WCs, basins, baths, sinks. Surface water is rainfall runoff from roofs and paving; it can pick up pollution, so the route and any treatment must suit its quality. A separate system keeps the two apart in different pipes: surface water goes, in order of preference, to a soakaway or other infiltration system, then a watercourse, and only then a sewer. Where the two systems are separate, every connection has to be proven to reach the system it belongs to: a foul pipe joined to a surface water drain carries sewage towards a soakaway or a river.

Gullies and soakaways

A trapped gully provides a water-sealed inlet to the drain at ground level, or occasionally basement level. A branch pipe carrying only wastewater can discharge to a gully, ending between its grating and the top of the water seal; a branch carrying WC discharge cannot. A gully can still take water after losing its seal. If it smells, check the water in the trap and remove any deposits: a gully that flows freely can still have either problem. A soakaway is a different fitting again: it lets surface water disperse into the surrounding ground on site, rather than carrying it away as a surface-water drain does.

Approved Document H keeps soakaways and other infiltration devices out of:

• ground within 5 m of a building or road;
• unstable land;
• ground where the water table reaches the bottom of the device at any time of year;
• places where contaminated runoff could pollute groundwater.

Drain or sewer?

A drain serves one building, or buildings and yards within the same curtilage. A pipe serving buildings in more than one curtilage is a sewer.

Foul drainage should join a public foul or combined sewer wherever that is reasonably practicable. For a small development, that means wherever a public sewer lies within 30 m and the developer has the right to lay the drain across any land in between. The sewerage undertaker needs at least three weeks' notice of the intended connection.

A sewer is normally at least 100 mm in diameter while it serves no more than 10 dwellings, and 150 mm once it serves more. Its access points go where they are easy to reach and plainly visible in an emergency: a highway, public open space, an unfenced front garden or a shared driveway. Keep a new sewer away from the parts of a house where an extension is likely, such as the rear.

Building near an existing drain or sewer has its own guidance, under requirement H4. It applies to building, extending or underpinning over, or within 3 m of the centreline of, a drain or sewer shown on the sewerage undertaker's records, and the owner is consulted first: for a public sewer, the undertaker. Without the owner's permission, no more than 6 m of drain or sewer should run under a building. Nothing should be built over, or within 3 m of, a sewer more than 3 m deep or larger than 225 mm. A building should not cover a manhole or other access point on a sewer serving more than one property.

To connect to an existing drain, use purpose-made junctions entering obliquely or in the direction of flow, and cut any hole in a pipe with a drilling tool so that the pipe is not damaged. Where a length of the existing pipe is cut out to fit a junction, repair couplings make the new joints watertight, and the junction is packed firmly so that it cannot settle on its own.

Sizing and falling the underground pipe

A drain carrying only wastewater needs an internal diameter of at least 75 mm; one carrying WC or trade effluent needs at least 100 mm. Read both the peak-flow band and the appliance footnotes in Approved Document H Table 6. Below 1 litre a second, its minimum gradient is 1 in 40 for 75 and 100 mm drains. Above 1 litre a second, it gives 1 in 80 for 75 mm drains; the 100 mm entry is also 1 in 80 and requires at least one WC. The 150 mm entry is 1 in 150 with at least five WCs. These figures set minimum gradients, not a maximum fall. A steep drain also needs a suitable receiving chamber: high-speed flow can deposit debris where the channel layout does not guide it smoothly.

Lay drains in straight lines and to even gradients where you can. Keep any bend at or near an inspection chamber or manhole, or at the foot of a stack, and give it as large a radius as practicable. The system is ventilated by a ventilating pipe at or near the head of each main drain; a ventilated discharge stack can do this.

Protecting the pipe in the ground

A drain can pass beneath a building if it is bedded in at least 100 mm of granular or other flexible fill all round, which protects it from settlement. If the top of the pipe would come within 300 mm of the underside of the floor slab, it also needs special protection.

Where a pipe is built into a wall, a footing or an inspection chamber, the aim is to stop movement damaging or misaligning it. There are two ways. One builds in the shortest possible length, with its joints no more than 150 mm from each face, linked on each side to a rocker pipe of up to 600 mm with flexible joints. The other forms an opening that leaves at least 50 mm clear all round the pipe. The opening is masked with rigid sheet to keep out fill and vermin, and the gap is filled with a compressible sealant to keep out gas.

Do not dig a drain trench deeper than the foundations of a nearby building unless you fill it with concrete. Within 1 m of the foundation, the concrete comes up to the foundation's lowest level; further away, it comes up to a level below the building's lowest level equal to the distance from the building, less 150 mm. Trench concrete needs movement joints of compressible board at each pipe joint.

While a new drain is being laid, cover any open end whenever work stops so rats cannot get in; disused and defective drains are known to harbour them. Keep construction traffic away from the drain line, and never stack heavy materials over it. On a previously developed site, ask the local authority whether special measures against rats are needed. One is sealed drainage, where the pipework passes through the inspection chamber under access covers rather than along an open channel; like an intercepting trap, it belongs only in chambers that can be maintained from the surface without anyone climbing in.

Approved Document H Table 10 gives plastic drains of 100 to 300 mm at least 0.6 m of cover when laid in fields, and 0.9 m under roads. Where a pipe cannot be laid that deep, protect it where necessary with a reinforced concrete slab bridging over it on flexible filler, with at least 75 mm of granular material between the top of the pipe and the filler.

Testing a new drain

Once a gravity drain or private sewer has been laid, with any haunching or surround, and backfilled, Approved Document H has it tested for watertightness with an air test or a water test. The figures here are for pipes up to 300 mm:

• Air test: pressurise to 110 mm water gauge and hold it for about 5 minutes; the pipe should then hold an initial 100 mm with no more than 25 mm loss on the manometer in 7 minutes.
• Water test: fill to no more than 5 m above the lowest invert of the section and at least 1 m at its highest invert, and leave it to condition (an hour is generally enough). Then hold the level for 30 minutes, topping up to stay within 100 mm of it.

In the water test, the water added is the loss. It may be at most 0.15 litres per square metre of internal surface for pipelines alone, 0.20 litres where the length includes manholes, and 0.40 litres for manholes and inspection chambers on their own.

Septic tanks and cesspools

Where there is no sewer connection, foul water can go to a septic tank or a cesspool. A septic tank settles the sewage and passes the liquid on to a drainage field, mound or constructed wetland to be treated. A cesspool is a sealed tank with no outlet and no treatment: it is ventilated, but its contents are simply held until a tanker empties them. Both should sit at least 7 m from any habitable part of a building, preferably downslope. For a septic tank, the usual 30 m tanker-access distance assumes an invert no more than 3 m below the access level; greater depth may require the tanker to stand closer. The cesspool guidance also places it within 30 m of vehicle access, at levels allowing safe emptying. Check the arrangement and provide a clear hose route that avoids carrying the contents through a dwelling or workplace. A septic tank's drainage field needs its own clearance: at least 15 m from a building, 10 m from a watercourse and 50 m from a groundwater supply. The two tanks are sized very differently. A septic tank holds at least 2,700 litres below its inlet for up to four users, and 180 litres more for each additional user, because it passes its settled liquid on. A cesspool keeps everything until it is emptied, so it holds at least 18 m³ (18,000 litres) for two users and 6.8 m³ more for each additional user. Either way, a notice fixed inside the building describes the maintenance. Approved Document H's example wording for a septic tank asks for a monthly look at the outlet chamber or distribution box, to see that the effluent is free-flowing and clear. The tank is emptied at least once every 12 months by a licensed contractor.

Access along the drain

Provide access where the drainage layout requires it, including changes of direction, gradient or size and the necessary junctions and intervals along runs. It must let the relevant length be inspected and cleared. Access can avoid unnecessary excavation, but damaged pipework or an unreachable obstruction may still need repair. The water levels in the chambers help you find a blockage once you have traced the route and run water from an appliance. Water backed up in a chamber points to a blockage further downstream; a chamber the water never reaches points you upstream. An empty chamber does not tell you that there is a blockage, or where. A surcharged sewer downstream can also back water up. Establish which route is affected, and who is responsible for it, before arranging clearance.

Inspection chambers and manholes take removable, non-ventilating covers strong enough for where they sit. Small lightweight covers are fixed down, with screws for example, so that children cannot open them. A chamber inside a building needs a mechanically fixed airtight cover unless the drain has watertight access covers of its own.

Access comes in four types: rodding eyes, access fittings, inspection chambers, worked from ground level, and manholes, deep enough to work in at drain level. A shallow inspection chamber, 0.6 m or less to the invert, can be as small as 225 × 100 mm or 190 mm across; one up to 1.2 m deep needs at least 450 × 450 mm or 450 mm across. Where a chamber has half-round channels, a branch of up to 150 mm enters in the direction of flow, at or above the level of the channel's horizontal diameter. The benching is built up at least to the top of the outgoing pipe at a slope of 1 in 12, rounded into the channel. Any manhole more than 1 m deep needs metal step irons or a fixed ladder. An access fitting, small or large, belongs only where the depth from cover level to invert is 0.6 m or less, unless it sits inside a chamber. An inspection chamber deeper than 1.2 m stays at least 450 × 450 mm inside, but the clear opening of its cover is restricted to 300 × 300 mm, or 350 mm across. Approved Document H restricts it for health and safety reasons, to deter anyone from climbing in.

Long runs need access at intervals. Table 13 sets the greatest spacing on drains up to 300 mm: from the start of a drain, 12 m to an access fitting, 22 m to an inspection chamber and 45 m to a manhole, and 90 m between manholes. The table's 200 m between manholes is only for drains and sewers big enough for a person to enter.