Underground drainage pipe and fittings: what each part does

What is the 110mm underground drainage system?

It is the buried drain that carries foul water and rainwater from the building to the sewer or soakaway. The pipe is 110mm, in 3m lengths, plain ended or socket ended, in terracotta. Every other part in the underground category is made to go with it: bends, branches, couplers, gullies, hoppers and inspection chamber parts, plus adaptors to old pipe.

A socket is the flared end that the next piece pushes into. Each socket holds a captive rubber seal, so joints go together by hand. That makes the system quick to lay, and it also makes preparation matter more, which is covered further down.

Why is underground pipe terracotta, and where does soil pipe go?

Underground drainage and the soil pipe that runs up the wall are separate systems, and the colour is the quickest way to tell them apart. Underground pipe here is terracotta. Soil pipe here is white or black. One is made to be buried, and the other is made to be clipped to a wall and left visible. Soil pipe is for above ground and the terracotta pipe is for below ground.

The two meet at the foot of the stack, where the vertical soil pipe turns into the buried run. Ring us before you plan that joint, so we can check the parts with you. Our guide to soil pipe sizes and fittings covers the above-ground side.

Which fitting does what?

Fitting	Angles and sizes here	Used for
Pipe	110mm, 3m, plain end or socket end	The straight run
Bends	15°, 30°, 45°, 87.5°, single or double socket	Changing direction
Adjustable bends	0 to 45° and 0 to 90°	Angles between the fixed ones
Rest bends	87.5°, single or double socket	The turn at the foot of a vertical pipe
Branches	45° and 87.5°, double or triple socket	Joining a side pipe to the run
Couplers and plugs	Double socket coupler, socket plug	Joining two plain ends, closing a socket
Inspection chambers	320mm and 470mm bases, raising pieces, cover frames	Rodding access
Adaptors	4 inch clay, 4 inch cast iron	Joining to old pipe

How do I choose a bend?

Use the gentlest bend the layout allows. The 45° double socket bend and the smaller 15° and 30° bends turn the run gradually, which keeps solids moving. The 87.5° double socket bend makes a right angle. The two adjustable bends cover the angles in between, which helps where an old drain runs at an odd angle. A double socket fitting has a socket at both ends. A single socket fitting has one socket and a plain end that pushes into the next socket.

The rest bends are 87.5° bends sold for the foot of a vertical pipe.

Which branch do I need?

Branches join a side pipe to the main run at 45° or 87.5°, and they come in double and triple socket versions. The number is the sockets on the fitting, so count the pipe ends meeting at the joint and pick the one that matches. A 45° branch, such as the 45° triple socket branch, joins the flow in the direction of travel. The 87.5° branch joins at a right angle. If the layout needs checking, ring us with a sketch.

What do gullies, hoppers and traps do?

A gully takes a downpipe or waste pipe into the drain through a water trap, so smells stay down in the drain. The range has back inlet roddable gullies with round or square tops, and a plain ended bottle gully. Roddable versions give access for drain rods, so a blockage can be cleared from the surface. A lowback P trap holds a water seal on a pipe discharge. The square hopper and the integral hopper collect open discharge from pipes above. The aluminium rodding eye adds a rodding point.

Bed a gully solid before backfilling so the pipe joint stays tight.

How does an inspection chamber go together?

An inspection chamber lets you get drain rods to a blockage instead of digging for it. It is built from three parts, each sold separately: a base that connects the pipes, raising pieces to bring it up to ground level, and a cover frame to cap it at the surface. The 320mm base and the 470mm base each have their own raising pieces and cover frame, so keep to one size. Buy the depth you need.

The plastic cover frames are for foot traffic. For a driveway, ask us for a suitable cover.

How are the joints made, and what goes under the pipe?

Cut the pipe square, chamfer the cut end and lubricate it before you push it home. That keeps the seal seated in its socket. Mark the pipe so you can see it is pushed fully in. You also need pipe lubricant for the seals.

Bed the run on granular material such as pea shingle, support the pipe along its length so it rests along its barrel, and keep a steady fall towards the sewer or soakaway. A steady fall keeps solids moving with the water. The right fall depends on the pipe and the run, so check with building control for new connections to a drain or sewer, and leave the trench open until the joints and falls have been checked.

How do I connect to old clay or cast iron?

Use the 4 inch clay to plastic adaptor or the 4 inch cast iron to plastic adaptor, both in the underground category. Measure the old pipe before you order, and if you are unsure what you have, bring an offcut to the counter.

How do I get it?

Collect free from the Liverpool showroom at Unit 7 The Boxworks, Heysham Road, L30 6UR, open Monday to Friday 7:30am to 4:30pm and Saturday 7:30am to 1pm, to trade and public. Delivery is priced by postcode and shown on each product page and at checkout. Pipe comes in 3m lengths, so check delivery to your postcode before you order. Ring us on 0151 525 4488 if you want a layout checked against the parts list.