Guide · 7 min read
Drainage for New Builds in Ireland
A new build needs two separate systems: foul drainage to a sewer or treatment system, and storm drainage for roof and surface water to a soakaway, attenuation feature or storm sewer.
The short answer
Irish new builds use separate foul and storm systems. Foul drainage carries wastewater from the house to a public sewer or to an on-site wastewater treatment system and percolation area. Storm drainage carries roof and hard-surface water to a soakaway, attenuation feature or storm sewer.
Mixing the two overloads treatment systems and is not permitted under current practice.
What good drainage installation looks like
Most drainage failures come from installation rather than design. The controls that matter are consistent falls, correct bedding and surround, properly made joints and accessible inspection points.
- —Trench excavated to the design invert level, with falls checked as work proceeds
- —Pipes bedded and surrounded in granular material, not laid on trench spoil
- —Joints made correctly and pipes protected before backfilling
- —Manholes and inspection chambers at junctions and changes of direction
- —Air or water testing before covering up, where specified
- —As-built records of runs, levels and connections for certification
Storm water, soakaways and attenuation
Where ground conditions allow infiltration, roof and surface water can discharge to a soakaway sized from a percolation test. Where infiltration is poor or discharge rates are restricted, attenuation storage is used to hold water and release it slowly.
Planning conditions increasingly require SuDS features — permeable surfaces, swales, filter drains — and these need to be built as designed, since they are inspected at compliance stage.
One-off houses and treatment systems
A rural house without a sewer connection needs an on-site wastewater treatment system and percolation area, sized and located from the site characterisation report prepared by a suitably qualified assessor.
Installation must follow that report and the manufacturer's instructions — depth, invert levels, distribution and the extent of the percolation area are all specified rather than chosen on site.
Frequently Asked Questions
Can foul and storm water share one pipe?+
No. Current Irish practice requires separate systems, and combining them can overload treatment systems and breach planning or connection conditions.
When should drainage be installed?+
After the platform is formed and before slabs, roads and hard surfaces go on, so nothing completed has to be broken out for pipe runs.
Do I get records of the drainage installed?+
Yes — as-built records of runs, levels, chambers and connections are produced for the engineer's certification and for future maintenance.