How to Build a Soakaway for Effective Surface Water Drainage

Managing rainwater runoff around a property is one of those essential but often overlooked tasks that can save homeowners considerable trouble down the line. When heavy rain falls on roofs, driveways, and patios, that water needs somewhere to go. Connecting everything to the main drainage network is not always possible or even desirable, which is where a soakaway comes into play — a simple underground pit that collects surface water and allows it to gradually percolate back into the surrounding soil. This method mimics nature’s own drainage cycle and reduces the burden on public sewer systems. Understanding how water demand in water supply system works at a household level also helps you appreciate why managing runoff locally makes sense from both an environmental and infrastructure perspective.

What Is a Soakaway and Why Do You Need One?

A soakaway is a buried structure designed to collect rainwater from roofs, patios, driveways, and other impermeable surfaces and release it slowly into the ground. Unlike connecting drainage pipes directly to a municipal sewer, a soakaway keeps water on your property where it can replenish groundwater supplies naturally. This is particularly important in areas where the main drainage system cannot handle additional surface water or where building regulations require sustainable drainage solutions.

Soakaways have been used for centuries in one form or another. Traditional versions were simply deep pits filled with rubble, stone, or loose brick, allowing water to pool and then seep away. Modern versions use purpose-built plastic crates that achieve the same result with greater efficiency and a smaller footprint. Regardless of the type, the principle remains the same: capture runoff and return it to the earth rather than sending it to a treatment plant. The concept of managing water at its source is also relevant when considering how instantaneous hot water systems a complete guide to tankless water heaters addresses demand-side efficiency in a different but equally resource-conscious way.

• Prevents localised flooding around foundations and paved areas
• Reduces the risk of water pooling on lawns and paths
• Lessens the load on public drainage infrastructure
• Helps meet sustainable drainage (SuDS) requirements for new builds
• Recharges local groundwater levels naturally

If your property experiences persistent damp patches, water collecting near the house after rain, or if you are extending a driveway or patio, a soakaway is likely the most practical and cost-effective solution. Many local planning authorities now require evidence of adequate soakaway performance before approving extensions or new paved areas.

Building Regulations and Planning Considerations

Before digging any hole on your property, it pays to check whether your soakaway project falls under building regulations or requires planning permission. In many jurisdictions, any drainage work that involves discharging water into the ground is subject to approval, particularly if it is part of a larger development such as a home extension or a new driveway. The key document in the UK is Approved Document H of the Building Regulations, which covers drainage and waste disposal. This document specifies that soakaways must be located at least five metres from any building foundation to prevent the softening of soil and potential structural movement.

You also need to consider the position of boundaries, neighbouring properties, and underground services. A soakaway should not be placed within two metres of a boundary line, and you must avoid any gas, water, or electrical cables that may be buried in your garden. If your property is in a conservation area or near a protected watercourse, additional restrictions may apply. Understanding these upfront prevents costly mistakes. As with any system that manages resources efficiently, the same principle highlighted by water heating is a system not just a water heater applies here too — the soakaway is not an isolated pit but part of a larger drainage strategy that must work with your soil type, rainfall intensity, and property layout.

If you are unsure about any of these requirements, contact your local building control office before proceeding. They can advise on site-specific conditions and may request a percolation test result before granting approval. Failing to obtain the necessary permissions can lead to enforcement action and require you to remove or relocate the soakaway at your own expense.

Testing Your Soil for Suitability

Not every garden is suitable for a soakaway. The single most important factor is the percolation rate of your soil — how quickly water can travel through the ground. Clay soils, for example, have very low permeability and may not drain fast enough to handle the volume of runoff generated by a typical roof during a heavy storm. Sandy or gravelly soils, on the other hand, drain quickly and are ideal for soakaway installation. Before committing to the project, you should carry out a percolation test, also known as a soakaway test.

To perform the test, dig a hole approximately 300 millimetres square to a depth of about one metre. Fill it with water and let it drain completely overnight to saturate the surrounding soil. The next day, refill the hole with water and measure how far the water level drops over a set period, usually thirty minutes. The rate of drop in millimetres per hour gives you the percolation rate. A minimum rate of around fifteen millimetres per hour is generally considered acceptable for a soakaway, though this can vary depending on the volume of water the system needs to handle. For a proper understanding of how water flows through different materials and pipework, it is worth reading about a guide on how to develop an efficient drainage system for a structure, which covers the hydraulic principles that apply to any drainage design.

• Step 1: Dig a test pit 300 mm x 300 mm to 1 metre depth
• Step 2: Fill with water and allow it to drain completely overnight
• Step 3: Refill and measure the water level drop every 30 minutes
• Step 4: Calculate the percolation rate in millimetres per hour
• Step 5: If below 15 mm/hour, consider an alternative drainage method

If the test reveals poor drainage, do not lose hope entirely. You may be able to improve performance by increasing the size of the soakaway, using deeper excavation to reach more permeable subsoil, or combining the soakaway with a French drain that spreads water across a larger area. In extreme cases, you may need to consider a different approach entirely, such as a rainwater harvesting system that stores runoff for garden use rather than dispersing it underground.

Building a Traditional Rubble-Filled Soakaway

The traditional method of constructing a soakaway involves excavating a pit and filling it with hardcore rubble, gravel, or crushed stone. This creates a void that can hold water while the gaps between the stones allow it to percolate into the surrounding soil. The minimum recommended size for the pit is one metre wide by one metre long by one metre deep, measured from the bottom of the incoming pipe downward. This 1 x 1 x 1 metre volume provides a baseline, but larger soakaways may be required for bigger roof areas or slower-draining soils.

The incoming drainage pipe should be at least 75 millimetres in diameter, though 100 millimetres is the preferred size for better flow. The pipe must be laid to a fall of 1 in 40, which means for every four metres of pipe, the trench drops by 100 millimetres. This ensures water flows by gravity without pooling or silting up. The hardcore infill should completely surround the pipe entry point and extend about 100 millimetres above the pipe. An impervious layer — such as heavy-duty polythene sheeting, tarpaulin, or a thin concrete cap — is then placed on top of the stone to prevent surface soil from washing down into the void. Topsoil is spread over this layer to restore the garden level. The mechanics of moving water efficiently through pipework tie directly into how pumps in water supply system maintain flow and pressure across a network, though in a soakaway gravity does all the work.

• Excavate a pit at least 1 m x 1 m x 1 m below the pipe inlet level
• Lay the drainage pipe at a minimum 1:40 gradient
• Fill the pit with clean hardcore, crushed stone, or gravel
• Cover the stone with an impervious membrane
• Restore topsoil and turf over the completed soakaway

One important detail often missed by first-time builders is the need to prevent fine soil particles from entering the voids between the stones. Over time, silt and clay can clog the gaps and reduce the soakaway’s capacity dramatically. Wrapping the stone fill in a geotextile membrane before adding the impervious cap adds an extra layer of protection and extends the life of the soakaway considerably.

Using Soakaway Crates as a Modern Alternative

Modern technology has refined the soakaway concept into purpose-built plastic crates, sometimes called aquablocks, water blocks, or soakaway drainage crates. These units arrive flat-packed and clip together to form a rigid box structure that creates a void for water storage. They offer several advantages over traditional rubble-filled pits. The crates provide a much higher void ratio — typically around 95 per cent of the crate volume is empty space compared to only about 30 to 40 per cent for loose stone. This means a smaller hole can hold the same amount of water, or the same size hole can handle a much larger volume of runoff.

Soakaway crates come in different grades, usually identified by colour. Green crates are designed for non-traffic areas such as gardens and lawns, while blue crates are load-rated to withstand the weight of vehicles and can be installed under driveways and parking areas. Always check the manufacturer’s specifications before purchasing, as ratings can vary between brands. The installation process involves lining the excavation pit with a geotextile membrane, assembling the crate, knocking out the pre-formed inlet to accept the drainage pipe, and then wrapping the entire crate in the membrane before backfilling. The connection between the crate and the incoming pipe must be sealed properly to prevent soil ingress. The same hydraulic principles that govern how short connecting pipes work at larger scales — as explained in when a drainage system is connected to a main drainage channel a segment of short pipe is used what is the reason of such arrangement — also apply to the transition between your downpipe, the underground drain, and the soakaway crate inlet.

For homeowners who already have leftover hardcore from other building work, the traditional method makes financial sense. For those starting from scratch with no stone on hand, the convenience and performance of crate systems often justify the higher upfront cost. Both methods are effective when installed correctly in suitable ground conditions.

Maintaining Your Soakaway System

A well-built soakaway requires very little maintenance, but it is not entirely fit-and-forget. Over years of operation, silt, leaves, and organic debris can build up in the pipework leading to the soakaway, gradually reducing flow. Blocked gutters and downpipes accelerate this process by allowing more debris into the drainage system. Inspecting and cleaning gutters at least twice a year, particularly after autumn leaf fall, is the single most effective maintenance step you can take.

If you notice water pooling above the soakaway location after heavy rain, or if the ground remains saturated longer than usual, this may indicate that the soakaway is silting up or that the surrounding soil has become clogged. In mild cases, rodding the drainage pipe from the downpipe connection can clear localised blockages. In more serious cases, you may need to excavate the pit and replace the geotextile membrane or wash and replace the stone infill. Understanding how water distribution infrastructure works at a larger scale can shed light on why even small blockages cause disproportionate problems — this is exactly what pumping stations in a water distribution system are designed to overcome when gravity alone cannot maintain flow. In a soakaway, gravity is always the driver, but keeping the pathway clear is equally critical.

• Clean gutters and downpipes twice yearly, especially in autumn
• Check that surface water from patios and drives flows toward the soakaway inlet
• Look for signs of pooling or soggy ground above the soakaway after heavy rain
• Rod the drainage pipe every two to three years as preventive maintenance
• Avoid planting deep-rooted trees or shrubs within three metres of the soakaway

One sign that often catches homeowners by surprise is a sudden increase in dampness in the garden that appears unrelated to recent weather. This can happen when a soakaway that has been working fine for years finally reaches the end of its useful life. Proactive maintenance and early intervention are far cheaper than excavating and rebuilding a failed soakaway.

Final Thoughts

A soakaway is one of the simplest and most effective ways to manage surface water runoff on your property. Whether you choose a traditional rubble-filled pit or a modern crate system, the underlying principles remain the same: capture rainwater, hold it temporarily, and release it slowly into the ground. The success of any soakaway depends on careful planning, correct sizing, proper soil assessment, and adherence to local building regulations. Taking the time to carry out a percolation test and consulting with building control before you start can save weeks of wasted effort and significant expense.

For anyone planning larger drainage works or looking to integrate a soakaway into a broader water management strategy, studying the methods of setting water distribution system layout provides valuable context on how water routing and dispersal principles apply at different scales. Whether you are diverting runoff from a single downpipe or designing drainage for an entire property, the goal is the same: keep water moving to where it can do no harm and instead contribute to the natural water cycle. A properly built soakaway achieves exactly that, quietly and reliably, for decades.