How Do I Size a Drainage Field? | Pro Sewage Treatment

Sewer System

If you live in a rural part of the South Coast — in areas like Sherborne, Dorchester, Dorset, Bath, Wimborne, Ferndown, Bridport, Corfe Mullen or Corfe Castle — and your property isn’t connected to the mains sewer system, then chances are you rely on a private wastewater solution like a septic tank or sewage treatment plant. One of the most important components of these systems is the drainage field, also known as a soakaway. But how do you work out how big that drainage field needs to be?

Sizing a drainage field correctly isn’t just a regulatory requirement — it’s key to keeping your system working properly, avoiding expensive repairs, and protecting the environment. In this guide, we’ll walk you through the basics of what a drainage field does, how it’s sized, and what you should consider before having one installed.

What Is a Drainage Field and Why Does It Matter?

A drainage field is a specially designed network of underground perforated pipes laid in trenches, allowing treated wastewater from your sewage treatment plant or septic tank to safely soak into the ground. It’s the final stage of your wastewater system and plays a vital role in filtering and dispersing effluent.

If your drainage field is too small or poorly designed, you could end up with surface flooding, bad smells, soggy ground, or even pollution. Too large, and you’ll spend more money and use more land than necessary. That’s why accurate sizing is essential — and it’s something we take very seriously at Pro Sewage Treatment Plant.

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Step 1: Estimate Your Daily Water Usage

Start by working out how much wastewater your property produces. UK building regulations use a simple estimate — 150 litres per person, per day. So if your home has four bedrooms and typically five occupants, you’d expect around:

5 people × 150 litres = 750 litres per day

This figure forms the basis for your drainage field sizing calculations.

Perform a Percolation Test

Before you can design a drainage field, you need to know how quickly water drains through your soil — this is called the percolation rate.

To perform a percolation test:

Dig a hole 300mm square and 300mm deep below the level of the proposed pipe.
Fill it with water and let it drain away completely.
Refill it and then measure how long it takes the water level to fall by 75%.

Repeat this in at least two locations, three times each, and calculate the average time taken in seconds per mm of drop (Vp). This gives you the percolation rate used in the next step.

Apply the British Standard Formula (BS 6297)

Once you’ve got your average Vp, use the formula below to calculate the trench area needed:

Drainage Area (m²) = Daily Flow (litres) × Vp × 0.25

So for our example — 750 litres per day with a Vp of 30:

750 × 30 × 0.25 = 5,625 square metres of trench area

Of course, most systems won’t require that much land — that’s why proper design is crucial. A qualified installer like Pro Sewage Treatment Plant will optimise trench layout and ensure all components meet legal and environmental standards.

Other Important Considerations

Getting the numbers right is just one part of the job. You’ll also need to consider:

Planning Permission: Not always required, but it’s vital to check with your local authority before installation begins.
Soil Type: Clay soils, for example, may drain too slowly or not at all. Alternative solutions like raised mounds may be necessary.
Regulations: Your drainage field must comply with British Standards (BS 6297), Environment Agency rules, and be located at safe distances from buildings and water sources.
Lifespan: A well-installed drainage field can last decades with minimal maintenance. But a poorly designed one may fail within just a few years, leading to costly repairs.

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What Happens If You Get It Wrong?

An undersized or incorrectly placed drainage field can cause sewage to back up into your home, foul smells in your garden, or even contamination of nearby watercourses — all of which are expensive and stressful to fix. In some cases, you could even face fines or enforcement action from the Environment Agency.

That’s why it’s so important to use a fully qualified installer like Pro Sewage Treatment Plant, with proper accreditation from bodies like British Water and SafeContractor.

Frequently Asked Questions

Can I build over a drainage field?

No — never. Drainage fields must remain undisturbed and open to air to work properly. Building over them could damage the system and violate regulations.

What if my soil fails the percolation test?

If your soil doesn’t drain well enough, don’t panic. There are alternatives, such as a raised drainage mound or even a different type of treatment system. Our team can assess your options and help you choose the best one.

Do I need a survey before buying a property with a septic tank?

Yes. We strongly recommend a homebuyer septic tank survey before purchasing any property with off-mains drainage. This helps uncover hidden issues, check legal compliance, and give you peace of mind.

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Call the Experts at Pro Sewage Treatment Plant

If you’re planning a new installation, replacing an old system, or just not sure where to start, we’re here to help. At Pro Sewage Treatment Plant, we specialise in:

Septic soakaway installation Dorset
Sewage treatment plant installations
Homebuyer septic tank surveys
Drainage field design
Advice on wastewater treatment plant regulations

With a friendly, experienced team and full accreditation, we provide the support you need to get the job done right — first time.

Contact us today for expert advice or to arrange a site assessment. Let’s make sure your drainage solution is safe, compliant, and built to last.

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