Self Build Tips – Drainage

The functional and planning requirements for soil- and waste-drainage are the same, whether the drainage is into the public sewer or into a private ‘cesspit or septic tank.

Generally the requirements of a system are that it is: self-cleansing; accessible for cleaning by rodding at all branches and changes in direction; ventilated at the head of the drain; water-sealed at all basins, baths, toilets and bidets: arranged so as not to break any water seals through siphonic action.

Underground drainage.

In addition to the drain which communicates with the main sewer, any toilets, basins or baths on the ground floor of a building are usually served by underground drainage. To be accessible for rodding, all underground connections into the drain have to be made in a manhole. It is advisable, if possible, to group all drainage to discharge into the same manhole. As it is expensive to extend a drain, it is best to install any new fittings as close as possible to the existing drain.


Note that bath, basin and sink’wastes may not discharge directly into a drain, but must be arranged to discharge via a “trapped gully”, which is a kind of sump. This may take the form of an external gully with a grating or an internal “back-inlet” gully, in which case the waste pipe is sealed into the fitting and the gully fitted with a screw-down, grease-sealed access plate.

The manhole into which the various connections discharge can be located inside the building, provided it is fitted with a double cover.

Soil stack drainage.

  • Upper-floor soil drainage consists of a soil stack pipe, open to the air at the top but with a vented cover in order to ventilate the drain. It is connected to the drain via an “easy” bend.
  • In addition to the toilet connection’, bath, sink and basin connections can be made into the soil stack. It is very much cheaper to make a connection into a soil stack than to make an underground connection into a drain. It is, therefore, advisable to attempt to group as many fittings as possible around the soil stack.
  • All connections must be swept in the direction of flow. To avoid siphonic action, there should be no branch connection closer than 8 in. (or 20 cm) below the entry of the toilet branch.
  • Also avoid pipe runs in excess of 10 ft (or 3 m). If it is absolutely necessary to make a connection to an appliance which is more than that distance away from the soil stack, a resealing trap must be fitted to the appliance. It is not possible to connect a ground floor toilet or bath to a soil stack.
  • Ideally, a soil stack should be vertical for its entire length, but it is possible to turn the pipe through 90 deg to achieve an offset. The restrictions are that the pipe must have a slight fall along its horizontal run and there must be access panels (a standard fitting) so that the horizontal run may be cleared if necessary

Extending existing drainage

In any project that involves forming a connection to an existing public drainage system, local authority permission must be sought.

1 Position your sanitary fittings and mark the outlet positions on the wall. Cut the holes. This may be done with a cold chisel and club hammer or, if you can hire one, an electric rotary hammer.

2 Connect up the appliances to the water supply. Holes can be started by drilling, using a masonry bit, round the circumference of the pipe size. It then becomes much easier and neater to knock out the holes.

3 Connect the soil and waste pipes to the fittings. Plastic pipes are preferable for soiland-vent pipes because these have a preformed joint and seal and are far easier to slot together. The other advantage is that there are a variety of bends and junctions to meet angles. The pipes should be fixed in the vertical position at every 3 ft (say 1 metre) with the brackets. At the top of the stack pipe you need a plastic balloon and, at the lower end of the pipe, an easy radius bend, which helps to avoid blockages.

4 The final job is to build the manhole, after excavating to the drainage outlet, so that your stack pipe is in the correct position.

Building the manhole.

To build the manhole:

1 Excavate all around the pipe the size of the manhole base. If you have one or two new connections, an internal manhole size of 3 ft by 2 ft 6 in. (say one metre by 76 cm) is sufficient. The base to the manhole should be at least 6 in. (or 15 cm) thick in 1:2:4 concrete.

2 Once the concrete has dried, you can extend your soil-and-vent pipe up to the existing soil pipe. Underground plastic or pitch-fibre pipe is acceptable in place of the older, glazed-stoneware pipe. The new connections should be formed with a channel bend. The angle of this depends on the angle of the existing soil drainage with your branch connection. The bottom of your angled channel-connection piece should be about half way up the depth of the existing soil pipe.

3 To make the connection, the top half of the existing soil pipe has to be removed. Take care at this point not to crack the whole of the drain run: otherwise these pipes will have to be replaced. Once you have removed the top portion with a sharp cold chisel and light hammer, tapping gently around the pipe, you can position your new connections and pipe runs and concrete them into place.

4 The walls of the manhole can now be built. These can be 23 cm Flemish bond, using the type of brick locally approved. Build up to the manhole cover frame level.

5 When the top of the brick wall is reached you can bed the manhole rim in a 1 :3 mortar and leave this to harden.

6 Next, bench the inside of the manhole with a 1 :3 or stronger mix. You can add a water retarder to the mix to increase waterproofing. The mortar should be benched up with a slope to the top of the benching so that, in the case of a blockage, the soil cannot feed back into the pipe.

6 After benching around the pipes, next render the inside of the manhole with about ½ in. (1.2 cm) thick render, using the same mix proportions as for benching. Once this has dried, you can test the manhole.

7 The soil pipes outside the manhole can now be covered in either concrete or pea gravel to a minimum thickness of 6 in. (or 15 cm) all round the pipe.

8 Finally, fit the manhole cover. The type of manhole cover depends on whether the manhole is indoors or outdoors. An outdoor cover needs only an ordinary single-seal metal rim. A bolt-down double-seal unit with brass bolts must be used inside the house.

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