Underground Drainage Installation Guide

Drainfast’s experts have over 20 years of practical knowledge and experience in underground drainage, ensuring you have the correct information to make informed decisions regarding your underground drainage installation. Join our engineers for this explainer on everything you need to know, including planning, pipe sizes, gradients, system testing and more.

Table of contents

• Planning Your Underground Drainage System
• Buying Your Underground Drainage System
• How to Properly Bury Underground Drainage
• Trenching for Underground Drainage
• Installing the Drainage System
• Testing the Drainage System

Planning Your Underground Drainage System

Planning where your underground drainage will run is essential to a properly functioning and long-lasting system. There are several key points you must consider when planning how to install your underground drainage.

Getting Permission to Install an Underground Drain

All underground drainage systems must comply with Building Regulations Part H and Water Industry specifications. You may also need to contact your local authority Planning & Building Control department so that it can sign off on your project. 

This isn’t always essential – if it is a straightforward replacement of a damaged existing underground drainage system, for example – but if you are adapting an existing system or installing a new one, you are legally obliged to inform the local authorities. You will also be required to submit drawings of your plans and consent to an inspection.

To avoid any problems, we would always recommend you double-check with your local authority, whatever underground drainage work you are undertaking.

The Distance Between Your Drainage System and Nearby Buildings

One stipulation of the building regulations is that a trench dug for underground drainage should not be lower than the foundations of any nearby building. If this is unavoidable, any part of the trench that is within one metre of a building must be backfilled with concrete up to the level of the lowest point of the foundations.

Any part of the trench that is more than a metre from the building must be backfilled with concrete to a level that is the equivalent of the distance between the building and the trench minus 150mm (for clarification, look at Diagram 8 of the building regulations or get professional advice from an architect or drainage engineer).

The Presence of Existing Drains

Details of existing drains or sewerage systems may be found on building deeds if you are working on a previously developed site. If not (and possibly as well), you should contact your local water authority. Existing drains may need to be rerouted or otherwise protected to prevent costly or hazardous problems in the future, or you may need to revise your plans due to their proximity to public sewers. In some instances, you may also need to get permission from the water authority before starting work on your underground drainage installation.

Providing Sufficient Drainage Flow Capacity

The plans for your underground drainage system should take into account its required hydraulic capacity. If it is likely or you are planning future additions to the system, this demand should also be included. One way to arrive at an accurate result is to use an online calculator.

The disadvantage of not providing sufficient hydraulic capacity is a higher risk of blockages, flooding and structural failure. As well as preventing future problems, detailed drainage plans that show these calculations have another big advantage: they are an invaluable reference point to help you through the planning application process and installation.

What do OD and ID mean?

 Two definitions you’ll need to understand when specifying underground drainage pipe installation are OD and ID. How pipe sizes are measured affects compatibility with fittings and overall performance:

• OD (Outer Diameter): Refers to the external measurement of the pipe, including the wall thickness. This is commonly used for rigid drainage pipes and determines which fittings, seals and connectors can be used.
• ID (Internal Diameter): Refers to the measurement of the opening inside the pipe. This indicates the flow capacity and how much water or waste the pipe can carry.

Planning how you will access the drainage system

Good access to all parts of an underground drainage system is vital if it is to be kept in good working condition. Inspection chambers, rodding chambers, manholes and other access points should be integrated into the plan to enable testing, inspection and maintenance.

Planning Gradients for Efficient Drainage

Foul water drains should have a sufficient gradient to make them self-cleaning: this is not only desirable but a legal requirement. A combination of gravity and water force should be enough to drive the wastewater into the sewer system without additional assistance.

The recommended minimum gradients for foul water drainage are as follows:

• 1:40 (falls 1 metre per 40 metres of run) for foul water drainage if no toilet is connected
• 1:80 for foul water drainage if a toilet is connected
• 1:100 for surface water drainage 

It is also important to bear in mind that a gradient that is too steep can be equally problematic as one that is too shallow. In surface water drainage systems, this can result in flooding if the receiving drains cannot cope with the flow, while in foul water drains, the liquids and solids should move along the drain at approximately the same speed to enable complete flushing and prevent blockages.

Planning for Excavation

Building Regulations stipulate that trenches for underground drainage should be as narrow as possible while allowing 300mm clearance around the pipework for backfilling.

One risk associated with trenching is that of trench collapse. To reduce this risk, the sides of the trench should be supported during the pipe laying process; often this is done by using temporary sheet piling. The trench should be left open for the least time possible, with backfilling following quickly after pipe laying.

Another point to consider about excavation is how it is to be carried out. Machine excavation is much faster and easier than digging by hand, but you must ensure there is sufficient access and that the weight of any machinery used will not cause damage to surrounding structures or underground pipes and cables.

How Does it Connect to the Above-Ground System?

Thought must be given to how your underground drainage installation will connect to the existing above-ground waste pipes, which is achieved via transition fittings that are designed to accommodate changes in pipe direction and diameter. This is typically achieved using rest bends or adjustable adaptors to link the soil stack to the horizontal run. Correct alignment, sealing and support are essential to maintain flow, prevent leaks and ensure long-term system performance.

Buying Your Underground Drainage System

A Material Difference

Concrete and clay pipes are still preferred in specific applications and with some local water authorities, as they offer greater structural strength where soil movement may be more likely. They also require less backfill material to guarantee a sturdy installation.

However, as mentioned in our introductory blog, most systems installed today use PVC plastic pipes and fittings as this is lightweight, durable and easy to cut. We will therefore focus on PVC here.

Terracotta (Coloured) or Nothing

PVC underground drainage pipe and fittings are manufactured in a terracotta colour, which is the officially recognised colour for underground drainage systems (black, grey and white are used for above-ground pipework). Only terracotta-coloured products should be used underground (they are sometimes called brown or orange) as your local authority may reject your proposal if another colour is used or proposed.

Pipes, Connectors and Fittings for Underground Drainage

Let’s take a look at the various drainage fittings you are likely to encounter while assembling your underground drainage system:

• Drainage pipes are manufactured in two types – plain and socketed. Plain pipes feature a constant outer diameter along the entire length of the pipe and require separate connectors between lengths of pipe. Socketed pipes feature a wider section at one end, allowing the lengths to slot into each other.
• Drainage pipe sizes commonly include 110mm and 160mm for underground drainage pipe installation, with larger diameters specified where higher flow rates or shared runs are required.
• Couplers are used to create straight joints between plain pipe lengths. Like the wider end of a socketed pipe, these feature rubber seals to ensure a watertight and secure installation.
• Standard bends are used for changes in pipe direction and are manufactured at angles of 15, 30, 45 and 90 degrees. They are also generally available with a socket on one or both ends, also featuring rubber seals.
• Rest bends are used to connect above-ground sanitation to underground drains.
• Gullies are used to drain surface water into an underground drain and to filter gases and noxious fumes from the system.
• Inspection chambers and rodding points provide access to runs of pipework that are otherwise inaccessible for inspection, maintenance and unblocking. Inspection chambers are also installed where the pipework changes its direction, diameter or pipe material.

How to Properly Bury Underground Drainage

Correct burial of your underground drainage installation is vital for long-term performance and compliance. Before backfilling, pipework should be installed to the correct depth, laid on suitable bedding and set to the required gradient.

Key considerations include:

• Maintaining consistent gradients to support self-cleansing flow
• Providing adequate bedding and surround material to protect the pipe
• Ensuring joints, seals and connections are secure before covering
• Allowing for inspection and testing before final backfilling
• Protecting pipework from future excavation or loading where required

Trenching for Underground Drainage

Here are a few basic tips that will make trenching easier, safer and compliant with local regulations.

Lay Out The Site

As we mentioned earlier, you should leave your trenches open for as short a time as possible to prevent the trench sides from collapsing. Using ground-marking spray paint will help you see where the trenches are to be excavated without constantly double-checking against drawings.

Choose Your Weather (if you can)

While British weather is notoriously unpredictable, try to avoid digging trenches in bad weather. Heavy rainfall will soften the earth, increasing the risk of trench collapse.

Mind The Edge

Avoid standing near the edge of the trench, as it could collapse, even in dry weather. If needed, create a 45-degree ramp at one end to access a deep trench.

Work Safe

Never work alone in an unsupported trench. If it collapses, soil is extremely heavy and can quickly cause harm, so you will need urgent assistance.

If you cannot completely prevent access to the site, use temporary high-visibility plastic fencing to cordon off the trench area. This is particularly important if children or pets are around.

Get Deep Trenches Checked

Any trench that is deeper than 1200mm must, by law, be checked by your local authority.

Lay a Firm Bed in the Trench

While a huge range of materials is available, it is important to use the right one for your project; this will be determined by the soil type and the maximum weight load you anticipate will be placed on the drainage system. We recommend you refer to Building Regulations Part H to determine which is best for you.

• Pea shingle is probably the most commonly used material for pipe bedding
• In some rare instances, the soil of the trench itself can be used if its composition is suitable
• Material such as bricks and stones should never be used as trench bedding for underground drainage systems

We recommend using a laser or string to ensure your levels and gradients are correct while laying the bed.

Installing the Drainage System

Assembling a drainage system is relatively easy. Socketed pipes and all connectors and fittings feature a flange and rubber seal that is a simple push fit, although pipe lubricant may be required to make the connection easier. Push the pipe fully into the flange and then pull it back about 10mm to ensure a tight seal.

Ensure all connectors and fittings are correctly aligned and snugly fitting as you progress along the pipe route.

Cutting Pipe to Length

If a length of PVC pipe needs to be cut to length, this can be done with a fine-toothed saw. This could leave the cut end a little rough, possibly with a slight burr that can affect the quality of the seal. The burr should be removed by chamfering the pipe.

How to Chamfer PVC Drainage Pipe

Chamfering a PVC pipe removes material from the outer edge of a pipe cut, creating an angled bevel around the edge that makes it easier to insert the pipe end into a connector.

A specialist chamfering tool may be available for smaller pipe diameters, but to chamfer underground drainage pipes with a larger diameter, you can use either a manual deburring tool or sandpaper. Hold the blade of the deburring tool or the sheet of sandpaper at a 45-degree angle to the pipe and carefully scrape or abrade the plastic until all roughness or burrs are removed and a uniform bevel is created.

The same method can also be used to finish the inside edge of the pipe, although this will be only to remove any roughness or burring, as a chamfer is not required here.

Connecting PVC to Clay Pipe

If you are replacing an old, damaged drainage system or connecting a new system to an old one, you may find the existing pipe is clay, not PVC. The easiest way to connect these two is to use a durable, flexible adaptor specially designed for this situation. Made from a rubber-like material, it simply fits over the ends of the clay and plastic pipe and is secured in place using two worm drive hose clamps (jubilee clips).

Backfilling The Trench

Once you have completed the installation, carried out air tests and been granted local authority approval, you can backfill the trench. This will also require a suitable material that will help protect the pipe – once again, we recommend you refer to Building Regulations Part H for the most suitable backfill material for your project.

This will typically be a granular material such as pea shingle and should not contain stones larger than 40mm, and no lumps of frozen material or clay larger than 100mm.

Having laid this material, you may choose to add further protection if there is a risk of the pipe being uncovered or damaged by future excavation. This can be a physical form of protection, such as small concrete slabs, or visual protection, such as printed underground warning tape. The trench can then be backfilled completely and the surface finish applied.

When to Test the Drainage System

Testing should be carried out once the underground drainage installation has been sited and jointed, but before backfilling. This allows joints and connections to be tested and flow performance checked while the pipework remains easy to access. Testing may involve air or water, depending on the design of your system and any local authority requirements, and it may need to be witnessed by Building Control.

It’s always best to check with your local authority as soon as possible what the specific requirements for your project will be. Only once the system has passed full inspection and approval should permanent backfilling and ground restoration proceed.

Ground Restoration

Once testing is complete and the installation has been fully approved, the ground can be reinstated to its original condition. This typically involves backfilling and compacting in layers, followed by restoring the main surface, which could be concrete, paving, tarmac or grass, which may also require landscaping work for proper reinstatement of the original surface conditions.

Proper restoration not only protects the installed system below ground but also prevents future settlement and ensures the site is returned to safe use.

Underground Drainage Support From Drainfast 

We hope you have found this information helpful and interesting. If you have any further questions, you are always welcome to call the friendly team of drainage experts at Drainfast on 01420 555600 or email [email protected].

Also, look out for more articles on our blog, bringing you useful information, insights, guides and tips on all things drainage from the experts at Drainfast.