Laying underground drainage pipe properly starts well before the first length goes into the trench. The route and levels need checking, the trench needs suitable bedding and the pipe needs to hold the required fall from one end of the run to the other.
This guide focuses on how to lay drainage pipe rather than designing a complete drainage system. Drainfast supplies underground drainage systems for groundworks, domestic and commercial projects. If you need guidance on the wider installation, including chambers and system planning, see our Underground Drainage Installation Guide.
What Do You Need To Lay A Drainpipe?
The exact kit will vary from one job to another, but for a typical underground drainage run you'll need:
- The correct underground drainage pipe and fittings
- Suitable pipe bedding
- A shovel, excavator or other excavation equipment
- A spirit level or laser level
- String line and stakes
- Measuring equipment
- Pipe cutting and chamfering tools where required
- Suitable PPE and trench safety equipment
The pipe diameter should already be confirmed from the drainage design before work starts. Drainfast stocks both 110mm underground drainage and 160mm underground drainage, along with matching fittings.
How To Lay A Drainage Pipe Step By Step
1. Plan The Drainpipe Route
First, confirm where the pipe run starts, where it finishes and what sits between the two.
Check the route for bends, junctions, inspection points and connections to other drainage. The required invert levels and fall should also be established at this stage.
Before digging, make sure the proposed route works with the site as it stands and locate any existing underground services. If the drawings and conditions on site don't match, deal with that before excavation rather than attempting to work around it later.
2. Mark Out The Drainage Trench
Transfer the drainage route onto the ground using marker paint, stakes or string line.
Mark junctions and changes of direction so there is no ambiguity once excavation starts. Check your start and finish levels at the same time and set any intermediate reference points needed along a longer run.
Good setting out gives the excavation team something definite to work to and reduces the risk of having to alter the trench afterwards.
3. Excavate The Trench
Dig along the marked route, allowing enough room for the pipe, bedding and surrounding material.
The trench base doesn't have to provide the finished pipe level on its own, but it should be reasonably even and free from anything likely to create a hard pressure point beneath the pipe.
There is no universal trench depth for underground drainage. It will depend on the invert levels, required cover, ground conditions, loading and where the run needs to connect.
Existing services should be identified before excavation begins. Trench stability and the position of plant and spoil also need to be managed in line with the site method of work.
4. Prepare The Drainpipe Bedding
The bedding is what gives the pipe a consistent base to sit on.
Laying pipe straight onto an uneven trench bottom can leave sections unsupported or resting on individual stones. Either can make maintaining the correct fall more difficult and place unnecessary stress on the pipe.
Place the specified granular bedding along the trench and level it before laying the first section. Follow the project specification and pipe manufacturer's guidance for the material and depth required.
It's worth getting this right the first time. Once the pipework starts going in, an uneven bed quickly becomes obvious.
5. Set The Correct Drainpipe Fall
The fall is the downward gradient that allows water and waste to travel through the pipe under gravity.
Set the required gradient from the drainage drawings or specification and transfer those levels into the trench. On longer runs, a laser level is usually the quickest way to keep control of the levels. A string line and suitable level may be enough for shorter sections.
Check the fall as the work moves forward. Don't rely solely on the start and finish levels because a pipe can be correct at both ends and still develop a high or low point in between.
If something isn't right, adjust the bedding while the pipe remains accessible.
6. Lay The Underground Drainage Pipe
Once the bedding and levels are ready, begin positioning the pipe.
Set each length onto the bed without forcing it into place. The barrel should be properly supported and follow the intended route without rocking or bridging across low areas.
As the run progresses:
- Keep the pipe aligned with the setting out
- Check the fall regularly
- Correct the bedding if a section sits too high or low
- Keep soil and debris out of open pipe ends
- Protect the pipe from damage while other materials are moved around the trench
With standard socketed underground drainage, installation commonly works from downstream towards upstream, leaving the sockets facing upstream. Check the instructions for the system being installed before starting.
7. Connect Each Length Of Drainpipe
Make sure adjoining pipes are properly aligned before connecting them. Keep sockets, spigots and seals clean and assemble the joint according to the manufacturer's instructions.
A joint shouldn't be used to pull badly positioned pipework into line. If the pipes don't meet cleanly, correct the support underneath them first.
For the full jointing process, including cutting and preparing pipework, see our guide to connecting underground drainage pipes.
8. Check The Drainpipe Run
Before anything is covered, inspect the whole run while adjustments are still easy to make.
Check:
- Pipe alignment
- The fall
- Joints and connections
- Bends and junctions
- Support beneath the pipe
- Access points
- Visible damage
- Any testing required by the project
Pay particular attention to areas around fittings, where movement during jointing can sometimes alter the level of the adjoining pipe.
If something needs correcting, now is the time to do it.
9. Backfill The Drainage Trench
Protect the pipe with the specified surrounding material before completing the main backfill.
Place material carefully around both sides of the pipe so it remains supported and doesn't move off line. Large stones, demolition waste and other unsuitable material shouldn't be placed directly against plastic drainage pipe.
Once the initial surround is in place and the pipe remains correctly positioned, continue backfilling in accordance with the project requirements and manufacturer's instructions.
What Fall Should A Drainpipe Have?
Drainage fall describes how quickly the pipe drops along its length. A gradient of 1:80, for example, means a fall of one unit vertically for every 80 units travelled horizontally.
For foul drainage, we advise 1:40 where no WC is connected and 1:80 where a WC is connected. Surface water requirements can differ, and the correct gradient will also depend on pipe diameter and the drainage design.
Too little fall can affect flow, but making a pipe unnecessarily steep isn't the answer either. Work to the gradient shown in the design and keep it consistent through the run.
What Should You Put Under An Underground Drainpipe?
Underground drainage pipe needs even support beneath its barrel.
Granular bedding, such as suitable pea shingle, is commonly used to create a level base and prevent stones or other hard objects from pressing directly against the pipe. The exact material and depth should follow the project specification and manufacturer's requirements.
Ground conditions matter too. A prepared trench bottom may form part of the support in some installations, but only where the relevant guidance allows it.
Whatever method is specified, the aim is to support the pipe evenly rather than leaving individual sections carrying more load than others.
Common Mistakes When Laying Drainpipes
Incorrect Or Inconsistent Fall
Getting the start and finish levels right isn't enough if the pipe dips between them.
Check levels through the run as each section goes in. Uneven bedding and movement during installation are common causes of localised high and low points.
Poor Pipe Support
Pipe laid over an uneven base can bridge across gaps or rest on isolated hard spots.
Prepare the bedding properly and make sure each section sits securely before making the next connection.
Allowing Debris Into Connections
Soil or grit around a seal can interfere with the joint.
Keep open pipes protected while work is taking place and clean the socket, seal and spigot before making each connection.
Too Many Unnecessary Changes Of Direction
Bends have their place, but adding extra changes of direction to get around poor setting out can make the system harder to maintain.
Follow the planned route and provide suitable access wherever the drainage design requires it.
Backfilling Before Checking The Pipe Run
Once a trench is filled, even a simple correction becomes a bigger job. Check the line, fall, connections and pipe support before covering the work.
Laying Drainpipes Around A House
Drainage around a property can involve foul water, rainwater, gullies, downpipes, inspection chambers and connections into existing systems. The job therefore involves more than simply getting pipe into the ground.
Planning drainage for a residential property? Read our guide on how to install drainage pipe around a house.
Choosing Underground Drainage Pipes For Your Project
Drainfast stocks uPVC underground drainage pipe and matching components for construction and groundworks projects.
Pipe diameter should come from the drainage design. Once that's confirmed, the required lengths, fittings and socket configuration can be selected around the planned run.
110mm Underground Drainage Pipes
110mm underground drainage pipe is used across a wide range of foul and surface water drainage installations.
Drainfast stocks plain-ended and single-socket pipe alongside matching bends, junctions, couplers and other components.
160mm Underground Drainage Pipes
For systems designed around a larger diameter, Drainfast also stocks 160mm underground drainage pipe.
The range includes plain-ended and single-socket pipe in 3m and 6m lengths, with compatible fittings available for completing the installation.
Browse the full range of underground drainage pipes to find the products required for your project.
How To Lay A Drainpipe FAQS
How Deep Should A Drainpipe Be Laid?
There isn't one correct depth for every drainpipe. Required depth depends on factors such as invert levels, ground level, loading above the pipe, cover requirements and where the drainage needs to connect.
Work from the drainage drawings and installation requirements rather than relying on a standard trench depth.
What Should Underground Drainpipe Be Laid On?
Underground drainage pipe should have even support beneath the barrel. This commonly involves a suitable granular bedding material laid to the depth and profile required by the project or manufacturer.
Large stones and other hard objects shouldn't be left directly beneath the pipe.
How Do You Maintain The Correct Fall When Laying A Drainpipe?
Set the start and finish levels first, then check the pipe regularly as it is laid.
A laser level is useful for longer runs, while other suitable levelling methods can be used where appropriate. If a section is out, correct the bedding underneath it rather than trying to force the pipe into the required position.
Can Underground Drainage Pipe Be Laid Directly On Soil?
Sometimes, but only where the ground conditions and installation guidance allow it.
An uneven trench base containing large stones or hard material won't give the pipe reliable support. Where granular bedding is specified, it should be installed before the pipework is laid.
Do Underground Drainage Pipes Need To Be Laid In A Straight Line?
No. Changes in direction can be made using the appropriate drainage fittings.
The route should still follow the drainage design and unnecessary bends are best avoided. Inspection and maintenance access also needs to be considered where the system changes direction.
Should Drainpipe Sockets Face Uphill Or Downhill?
Standard socketed underground drainage pipe is commonly installed with the socket facing upstream and the spigot pointing downstream, in the direction of flow.
The precise recommendation should still be checked against the manufacturer's instructions for the Drainfast system being installed before publication or installation.
Can You Lay Underground Drainage Pipe Yourself?
Someone with the right knowledge and equipment may be able to undertake smaller private drainage jobs, but the installation still has to meet the relevant requirements.
Commercial groundworks, public sewer connections and more involved drainage schemes will usually need additional design, site controls or approvals. Where the requirements aren't clear, use a competent drainage contractor or check with the relevant authority.
