How Do You Install Your Own Plumbing And Drainage System?

The first thing that you will need to consider when undertaking any DIY plumbing and drainage are the regulations issued by your Local Authority or State Department covering these.

If you are altering your existing drainage, or installing a new drainage system you must always inform the local Building Control Department at your local council offices. You will almost certainly need to present detailed plans of the work that you intend to undertake and it will need to be inspected as the work progresses making sure that it complies with local building regulations. If you are simply replacing damaged sections, in most circumstances they do not need to be informed. However, if in any doubt – check it out. A simply phone call should suffice.

To begin with we will take a look at drainage systems.

These can be described as Foul Water systems and Surface Water systems. Foul water is anything that comes from bathrooms, kitchens, utility rooms, car washing areas etc. Waste from these sources must always go to your foul water drain system.

Surface water is basically rainwater. This can discharged into a soakaway, watercourse, surface water sewer or, particularly in older properties, into the foul water drainage system. In a combined system, the rainwater pipes are discharged into the foul water drains via gully traps which stop foul air escaping from the drains. However, modern systems are designed to keep the foul water and the surface water apart. It is extremely important to ensure that you do not connect foul water to a surface water drainage system. If you are unsure about the drainage system around your house get advice from the Building Control Department before you start any work.

Remember – If in doubt – check it out!


Before you begin, you will need to plan the route of the waste pipes. The main things to consider when planning the route of a waste or soil pipe are to keep the route as straight and short as you can. You must avoid making your pipe runs too steep. A soil pipe is normally laid to a fall of 1:40. This won’t seem very much but it is plenty. The rule of thumb for pipe laying is ‘a touch on the bubble’. This means that the bubble on the spirit level has moved to the higher end, but part of the bubble is still contained between the level lines on the spirit level. Remember – this is only a ‘rule of thumb guide. You can calculate the fall of a drain over a distance using a surveyor’s site level. If you do not have one of these available, establish a datum point and use a hosepipe filled with water to establish levels and calculate the fall from the datum this way. (If a hosepipe is filled with water, the level of the water will equal out at each end, measure to the trench floor from the end away from the datum to enable your calculations.)

Your local authority will be able to give you the recommended falls for pipework.

Putting in drainage:

Preparing the trench:

When installing your drain trench, you will need to make sure that you do not impair the stability of the building. If you are laying a drain run parallel to the building, you must ensure that any foundations are not undermined.

Do not dig the trench too long before laying the pipe when installing a new drainage system. Make sure that you get the pipes laid as quickly as possible and then backfill the trench as soon as the system has been inspected and tested.

The trench may need to be supported depending on the depth and soil conditions. Do not take any risks. If in doubt – add support to the trench to prevent it from collapsing. Keep the trench as narrow as possible, but allow room to work in the trench – say the width of the pipe plus 150mm (6″) on each side. The base of the trench should be clean and even and free from protruding stones or bricks etc. You may need to import a suitable material for the base of the trench if the existing material is unsuitable. Your local inspector will advise you.

Never use bricks and/or other hard materials to support the pipe in the trench. This will damage the pipe and must never be used as temporary or permanent support. The bedding should be properly compacted with hollows made to accommodate the joints in the pipes. You will need to provide a continuous and uniform support for the complete length of the pipe. Once the laid pipes have been inspected, granular material should be evenly backfilled and compacted to a depth of at least 100mm above the pipe. 10 mm shingle has often been used as a backfill material as, not only is it a uniform medium but is also a visual warning to anyone excavating in the area of the drains that there are drainage pipes immediately below. Above the granular backfill material, the original dug material can be used to completely backfill the trench. This should be compacted in 300mm layers. Be sure not use heavy compactors until there is at least 300mm of cover. Light vibratory tampers could be used sensibly to help with the compaction.

You will need to protect any part of the drain system that is less than 600mm below a garden. This is because of potential damage from garden activities and gardening tools. A 50mm layer of lean concrete laid 100mm above the pipe will be adequate to do this. However you could use preformed concrete slabs as an alternative.

It is important that all parts of your drainage system are designed so that all parts of the pipework are accessible to a set of drain rods. Therefore, a run of drains should be as straight as possible between two points. As a general rule, any change in direction of the pipework should be provided with an inspection chamber to allow rodding access. This will also apply to changes in gradient, all drain junctions that are inaccessible to a set of drain rods, changes in pipe diameter and at the head of each run of drain. Where a junction between two drains does not have a manhole, access should be sited on the branch drain within 12 metres of the junction.

DIY plumbing and drainage is certainly within the bounds of most DIY enthusiasts. However, you must remember that it is important to follow local building control regulations and, as always, it is ‘hard graft’!

Remember if in doubt – check it out!

Whilst the author endeavors to ensure the accuracy of the information contained within this article, you are reminded that this is only a guide and you should always check with local professional before undertaking any work based herein.