Guide · 7 min read
Drainage Systems Explained
Irish sites use separate foul and storm systems. Foul goes to the sewer or a treatment system; storm goes to a soakaway, watercourse or attenuated connection. Mixing them fails at certification.
Foul versus storm — always separate
Foul drainage carries waste water from toilets, sinks, showers and appliances. Storm (or surface-water) drainage carries rainwater from roofs, driveways and yards. In Ireland these must be separate systems: combining them overloads treatment plants in wet weather and is refused at connection stage by Uisce Éireann.
On site that means two independent pipe networks, two sets of manholes, and clear marking so nothing gets cross-connected during fit-out.
Getting the falls right
Gravity drainage works on gradient. Typical practice is 1:40 for 100mm foul runs and 1:60 for 150mm, with storm runs laid to suit the flow. Too flat and solids settle; too steep and liquid outruns the solids, leaving blockages behind.
Pipes are bedded and surrounded in granular material, not dropped on a rough trench bottom. Bad bedding causes the joint deflections and dips that show up on a CCTV survey years later.
Where the storm water goes
Discharge options depend on the site and what the local authority permits:
- —Soakaway — infiltration into permeable ground, sized from a percolation test
- —Attenuation tank or crates — stores peak flow and releases it at a restricted rate
- —Watercourse outfall — requires consent and usually a headwall and non-return protection
- —Public storm sewer — where one exists and capacity is confirmed
- —SuDS features — permeable paving, swales and filter drains, increasingly required on planning
Manholes, access and as-builts
Access chambers go at every junction, change of direction and change of gradient, and at intervals along long straight runs. Covers must be rated for the loading above them — a pedestrian-rated cover in a yard trafficked by delivery lorries will fail.
Before backfilling, invert levels and runs should be recorded on an as-built drawing. That drawing, plus an air or water test, is what the engineer needs to certify the system.
Frequently Asked Questions
Do I need a percolation test for a soakaway?+
Yes. The soakaway is sized from the measured infiltration rate. Building one without a test risks a system that floods in the first heavy winter.
Can you connect to an existing public sewer?+
Where a connection is permitted, we carry out the works to the standard required and provide the records for the connection application. The application itself is made to Uisce Éireann.
What is attenuation and when is it required?+
Attenuation temporarily stores storm water and releases it slowly, protecting downstream capacity. It is commonly a planning condition on developments that increase hard-surface area.