How to Install Underground Drainage

Installing underground drainage requires knowledge of strict building regulations.

The fitting is fairly straight forward but you will be required to adhere to building regulations. In this guide we provide you with practical information, advice and tips for installing an underground drainage system.

Step 1

If you intend to alter or install a new drainage system then you will be required to provide plans for the installation to your local Building Authority and this work needs to be inspected and approved. Note that if you intend to just replace a damaged section then there is no requirement to provide plans or inform the Building Authority.

Step 2

Note that there are two types of drainage water system,

A) Foul water, which is waste water from the house, such as kitchens, bathrooms, etc., and;

B) Surface water, which is simply rainwater.

If you live in an older property it is likely that the rainwater pipes, that is the guttering and down pipes, are channeled into the foul pipes.

These types of arrangements will have gully traps which stops foul air from escaping from the drains. In new homes you will find that both the systems are kept separate.

Take note of the Building Authority regulations – you cannot connect the foul water system to surface water drainage system. If you are unsure then speak to your local Building Authority for advice.

Step 3

Lubricating Pipes – Anywhere that a pipe requires fitting you should never use Washing Up Liquid, Grease or Oil as these can damage the seals which will result in a leak. Use a Silicone Lubricant that can be purchased at any good DIY Store.

Step 4

Pipes that are suitable for underground drainage are colour BROWN and this colour clearly distinguishes them from other types of pipes that may be buried.

You should NEVER use black colour soil pipes for any type of underground drainage system. If in doubt talk to your local Building Authority Inspector. They will be able to tell you the required pipes used in underground drainage systems.

WARNING: If you do use another type of pipe the Building Inspector will ask you to remove it and replace it with the appropriate type. Therefore it is better to get this right the first time around.

Step 5

Fitting Rest Bends – These are large radius bends and are used in connecting the soil stack and sanitary appliances to the drainage system.

Rest Bends are manufactured so that it is easy for waste from toilets, etc., to easily pass through the drainage system. These are also able to accommodate drainage rods to pass through in the event of a blockage.

The regulations require that the bottom of the bend should be equal to or greater than 450mm below the lowest point in the round that connects to the soil stack.

For this to work effectively the Rest Bend, between the outlet, and the bottom of the Rest Bend should be less that 1.5m.

Step 6

Gullies – The Gullies retain water pretty much the same way as a waste trap and prevents foul smelling air from rising up through the pipe work.

It’s advisable to fit a Gully that has a Rod Grate so that if it does become blocked it is simply a matter of removing the grate and pushing rods through the pipe work.

Alternatively you can use a Tick Gully, which is a square hopper and grating and acts as the inlet to the gully drainage trap.

These can be joined to the Gully Traps using short pieces of piping of diameter of 110mm. A glue can be used to seal the joints. Note that the waste pipe should be inserted into the grating but remain above the water like at an 45 or 90 degree angle.

Step 7

When it comes to preparing the trench for the drainage pipes you must ensure that the trench does NOT have any adverse effects of the foundations of the building.

If you are intending to lay pipes running parallel to the foundations you must ensure that the foundations are NOT affected in any way. Again, if you are in doubt ask your local Building Authority Inspector. When digging the trenches you don’t want to make them too wide.

The trenches should be constructed just wide enough for the pipe work and a minimum depth of 300mm above the top of the pipe. As soon as you are finished installing and testing the pipes for leaks you should cover the pipes as soon as possible.

Step 8

Laying the pipes, through a wall of a building requires careful attention. You should place a steel or concrete lintel above the opening for support and give the pipe a height clearance to a minimum of 50mm. Also you will need to ensure that the hole is sealed or covered so that soil and rodents cannot enter.

Step 9

You will need to include a Manhole, so that you have access to the underground drainage, in order to clear any blockages that may occur.

Step 10

When it comes to testing the underground drainage system you must comply with the Building Regulations Documentation. These can be obtained from your Local Building Authority. Once you have completed installing the system a Building Authority Inspector will visit the site to provide a certificate of approval, providing all regulations have been complied with.

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