What a site characterisation involves
A percolation test on its own is not the deliverable. The EPA calls the whole exercise a site characterisation, and it has three parts: a desk study of what is already known about the site, a trial hole dug to examine the subsoil profile and establish the depth to bedrock and the water table, and the percolation tests themselves. Three tests are carried out and the average is used.
The output is a site characterisation report. It records the conclusions, and where the site is suitable it recommends the most appropriate system and discharge route. That report is what a planning authority reads.
Why this page exists
Search for a percolation test in Ireland and the results are strange. A UK pipe supplier ranks near the top. An American university extension page ranks on the first page. Wikipedia and YouTube are there. What is largely missing is a plain Irish explanation of what the number means and what happens next — which is what this page and the guides are for.
What the result opens and closes
The percolation value maps onto a specific set of permitted systems, set out in Table 6.4 of the Code. A value in the low range leaves everything open, including the simplest and cheapest option. A higher value progressively narrows it to secondary treatment with particular discharge arrangements. Above 120 the ground is out, and the alternative — discharge to surface water — needs a Water Pollution Act licence from the local authority.
The full breakdown is on the replacement page, because in practice the two questions are one question: what does the ground allow, and therefore what gets built.
Before the assessor arrives
Know where your site boundaries are, know where any existing system and well sit, and have any paperwork about previous assessments to hand. Separation distances are one of the gates that can rule a layout out, so the position of a neighbouring well or watercourse can matter as much as the soil itself.
Frequently Asked Questions
01 What is a percolation test actually for?
To find out whether the ground on a site can treat and disperse the effluent from a domestic waste water treatment system, and if so which kind. It is part of a wider site characterisation that also includes a desk study and a trial hole. The result is recorded on a site characterisation form, and it decides what may legally be installed — compliance with the EPA Code of Practice is required under the Building and Planning Control systems.
02 What does the number mean?
It is the percolation value, sometimes called the T-value. Lower means faster-draining ground. A conventional septic tank with a percolation area needs a value between 3 and 50. Between 50 and 120 you are into secondary treatment systems, with the permitted option depending on exactly where in that range you land. Above 120 the site cannot take a system discharging to ground. Below 3 the ground drains too fast to treat the effluent at all.
03 Which Code applies to my test?
The 2021 Code of Practice applies to site assessments and installations carried out on or after 7 June 2021. The older 2009 code may still be used where the assessment or installation commenced, or planning permission was applied for, before that date. If your site was assessed and ruled out under the old code, the options available on slow ground have since widened.
04 What happens if the trial hole and the percolation tests disagree?
The Code addresses this directly. The subsoil classification from the trial hole should broadly match the expected percolation results, and where the two do not correlate, further examination should be carried out to establish which assessment accurately reflects the site. The design is then based on whichever of the two is more appropriate — not simply the more convenient one.
05 What does a percolation test cost?
We do not publish a figure. Ranges circulating on Irish self-build sites cite no source, and the site assessors we checked do not publish prices. Ask two assessors for a written quote and confirm what it includes — in particular whether the excavator for the trial hole and the written report are in the price or extra.
06 Can I do anything to improve a poor result?
Sometimes, within limits the Code sets. Site improvement works are not acceptable where the slope exceeds 1:8, where the average percolation value from the three tests is greater than 120, or where the required separation distances cannot be satisfied. Within those limits, importing soil or subsoil with a different percolation rate can be considered — most usefully where the ground drains too fast rather than too slowly.
Where We Do This Work
Percolation Testing & Site Assessment is available across Galway, Mayo, Roscommon, Sligo and Leitrim — Galway, Mayo, Roscommon, Sligo and Leitrim. Each county page covers what is specific to it, from published prices in Galway to slow-draining ground in Leitrim.