Percolation Tests
Before a drainage field is designed, the ground must be shown to absorb effluent at a suitable rate. UKDP carries out percolation tests to assess soil suitability and calculate the drainage area required for the property.
Percolation Tests for Drainage Field Design
A percolation test measures how quickly water drains through the soil where a new or replacement drainage field is proposed. It establishes whether the ground can absorb effluent at a suitable rate, not so slowly that wastewater collects, or so quickly that it passes through without sufficient treatment.
UKDP carries out the test over two days using multiple trial pits, then uses the recorded drainage rate and the property’s design occupancy to calculate the drainage area required.
Percolation testing applies to drainage fields serving septic tanks and sewage treatment plants, but not to sealed cesspits.
Benefits of Percolation Testing
A percolation test shows whether the ground can absorb water at a suitable rate before a drainage field is designed. This helps to avoid installing a system that is too small for the property or unsuitable for the surrounding soil.
The results help UKDP to:
- Confirm whether the ground is suitable for a drainage field.
- Measure how quickly or slowly water drains through the soil.
- Calculate the drainage area required.
- Identify groundwater conditions that could prevent installation.
- Recommend another discharge solution if the site is unsuitable.
UKDP combines the measured drainage rate with the property’s number of bedrooms and the British Standard formula to calculate the required drainage-field size. This provides a reliable basis for planning the installation before substantial work begins.
What UKDP’s Percolation Test Includes
UKDP’s percolation test examines how water moves through the ground and provides the measurements needed to assess suitability and calculate the required drainage-field size.
Multiple trial pits
UKDP typically tests three trial pits to build a reliable picture of the ground conditions. At least two pits are used for every percolation test.
Correct testing depth
Each trial pit is excavated 300mm below the proposed invert level of the drainage field pipe, allowing drainage to be measured at the relevant ground level.
Soil saturation
On the first day, the trial pits are filled with water to saturate the surrounding soil. They are then left overnight before measurements are taken.
Measured drainage rate
If the water has drained, UKDP returns on the second day, refills the pits and measures how quickly or slowly the water level falls through the soil.
Ground suitability
The findings establish whether water drains at an acceptable rate. Ground that drains too quickly, too slowly or fills with groundwater may be unsuitable.
Drainage-field calculation
UKDP combines the measured drainage rate with the property’s number of bedrooms and the British Standard formula to calculate the drainage area required.
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The UKDP Percolation Test Process: Step-by-Step
UKDP carries out the test over two days, using a series of trial pits to measure how effectively water drains through the ground.
Trial pits excavated
UKDP excavates several trial pits for the test. Three pits are usually used, with at least two tested at every site.
Correct depth established
Each trial pit is dug 300mm below the proposed invert level of the drainage field pipe.
Ground saturated
On the first day, the pits are filled with water to saturate the soil and prepare the ground for accurate measurement.
Pits left overnight
The saturated trial pits are left overnight, allowing time for the water to drain naturally through the surrounding soil.
Drainage rate measured
If the water has drained, UKDP returns on the second day, refills the pits and measures how quickly or slowly the water level falls.
Drainage area calculated
The result is combined with the property’s number of bedrooms and the British Standard formula to calculate the required drainage-field size.
Who UKDP Helps
UKDP provides percolation testing for those who need clear evidence about soil suitability and the drainage area required for a new or replacement drainage field.
Homeowners
Percolation tests that show whether the ground at your home can take a new soakaway or drainage field.
Learn moreLandowners
Confirm whether the ground is suitable before committing to a new or replacement drainage field. The results also establish the area required for the property.
Learn moreInsurers
Where an accepted claim requires a replacement drainage field, obtain measured ground data to support decisions about its suitability and required size.
Learn moreEstate Agents
Give buyers and sellers clearer information when a property transaction involves a proposed drainage field or changes to an existing off-mains drainage arrangement.
Learn morePercolation Test Cost
A UKDP percolation test costs £950 + VAT. UKDP will confirm the details for your site when you book.
Testing before a drainage field is designed can help avoid spending money on an installation that is unsuitable for the ground or incorrectly sized for the property. The results provide the evidence needed to make informed decisions about the proposed system and the drainage area required.
From Ground Uncertainty to Clear Test Results
A percolation test replaces assumptions about the soil with measured information for planning a new or replacement drainage field.
Before percolation testing
- Ground suitability is unknown
- The drainage rate is based on assumptions
- Excessively fast or slow drainage may be overlooked
- Groundwater conditions have not been tested
- The required drainage area is unclear
- Installation planning lacks site-specific evidence
After a UKDP percolation test
- Soil percolation rate has been measured
- The drainage rate has been measured
- Unsuitable drainage conditions are identified
- Groundwater entering the trial pits is identified
- The drainage-field size is calculated
- Decisions are supported by the test result, bedroom count and British Standard formula
Recent Percolation Tests by UKDP
Why Choose UKDP for Percolation Tests
UKDP combines practical off-mains drainage knowledge with a defined testing method and clear calculations, giving you evidence that can be used to plan the drainage field.
Off-mains expertise since 2009
Our experience with septic tanks, sewage treatment plants and drainage fields helps us understand how the test findings affect the complete system.
A thorough two-day test
We test ideally three trial pits, with a minimum of two, allowing the ground to saturate overnight before measuring the drainage rate.
British Standard calculations
The measured rate is combined with the property’s number of bedrooms and the British Standard formula to calculate the drainage area required.
Clear starting cost
UKDP percolation tests cost £950 + VAT, giving you a clear figure when planning the assessment and wider drainage project.
Substantial business cover
UKDP carries £15 million in Public Liability and £15 million in Employers’ Liability insurance, with Professional indemnity cover of £2 million.
Nationwide, locally delivered
UKDP coordinates percolation testing across the UK through our engineers and trusted local partners, supported by our central technical team.
Understanding Percolation Test Results
The result sheet records how quickly water drains through each test pit. These readings are used to calculate the site’s Vp value, measured in seconds per millimetre.
A very slow rate may mean wastewater cannot disperse effectively, increasing the risk of pooling or the system surcharging. If water drains too quickly, it may not receive sufficient treatment as it passes through the soil. Results from several pits also help identify whether drainage conditions are consistent across the proposed area.
UKDP uses the final Vp value, the number of bedrooms and the relevant British Standard calculation to determine whether a drainage field is suitable and the area it requires.
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Read articlePercolation Tests: Frequently Asked Questions
Ask our teamWhen is a percolation test required?
A percolation test is required when a new or replacement wastewater drainage field is being planned. It establishes whether the soil can accept wastewater at a suitable rate and provides the value needed to calculate the required drainage area.
Is a wastewater percolation test the same as a rainwater soakaway test?
No. UKDP’s percolation tests assess ground intended to receive wastewater from a septic tank or sewage treatment plant. Rainwater soakaways serve a different purpose and are assessed using different design guidance. The correct test depends on what the proposed system will discharge.
How long does a UKDP percolation test take?
The test is completed over two days. On day one, the trial pits are excavated, filled with water to saturate the surrounding soil and left overnight. If they have drained, UKDP returns the following day to refill them and record the drainage rate.
How many trial pits are needed?
A minimum of two pits is required, although UKDP recommends three where possible. Testing several points provides a better picture of conditions across the proposed drainage field, as the soil and drainage rate may vary within the same site.
What does the Vp value mean?
Vp is the percolation value, measured in seconds per millimetre. It shows how long the water level takes to fall through the soil. A lower value indicates faster drainage, whereas a higher value indicates slower drainage. UKDP uses the calculated average when assessing the site.
What is considered a suitable percolation test result?
Approved Document H states that drainage field disposal should generally only be used where the average Vp value is between 12 and 100 seconds per millimetre. Ground outside this range may drain either too quickly or too slowly for a conventional wastewater drainage field.
Can rain affect a percolation test?
Yes. Heavy rain or already saturated ground can distort the readings by filling the test pits or slowing drainage. Testing should therefore be arranged when conditions allow the natural drainage characteristics of the soil to be measured reliably.
What happens if the ground fails the test?
A failed result means a conventional drainage field may not be suitable in the tested location. UKDP can assess the findings and consider other compliant options, which may include a different discharge location, an alternative drainage arrangement or a sewage treatment plant discharging to an appropriate watercourse.
Can the test be carried out anywhere on the property?
The trial pits should be positioned within the area proposed for the drainage field. Soil conditions can vary considerably across one property, so results obtained elsewhere may not accurately represent how wastewater will drain at the intended installation location.
Arrange Your Percolation Test
Get reliable ground data before committing to a new or replacement drainage field. UKDP carries out the two-day test, records the drainage rate and provides the calculations needed to guide the system design.