Installing soakaways on new build
On UK new-build and self-build plots, soakaways remain a common way to dispose of surface water where infiltration is acceptable and a direct sewer connection is not preferred or available. Sizing comes from drained area, rainfall basis, and percolation performance — not from copying a neighbour’s crate count.
This page owns plot-scale soakaway installation briefing within the construction-drainage hub. Development-scale storage and controlled discharge belong on the attenuation systems flagship; adoption strategy belongs on the S38/S104 and drainage-adoption guides.
Keep roof and gutter connections capped until the system is ready, so early rainfall does not wash silt into new crates during the rest of the build.
review Groundworks Contractors in Wandsworth and Groundworks Contractors in Kingston.
When this drainage work is specified
When Building Control or the drainage designer requires on-plot infiltration; when replacing a failed soakaway on an existing dwelling; or when roof and hardstanding drainage must discharge to a new crate or granular chamber before occupation.
It also appears when percolation results arrive late and the installation window is tight against foundation backfill and landscaping.
What installation typically includes
Typical installation excavates to the designed plan size and depth, places geotextile and crates (or specified granular fill), connects inlet pipework from gullies and rainwater pipes, and backfills so the structure is not crushed by plant or later loading.
Location offsets from buildings, boundaries, and water sources come from the design and local practice — this page does not invent a universal UK setback.
Where infiltration is unsuitable (high water table, slow percolation, contaminated ground), the designer may switch to attenuated discharge or another SuDS component. That redesign must land before you dig the soakaway hole.
Coordinate soakaway installation with roof completion and hardstanding formation so the system is not asked to swallow construction runoff. Where crates sit under future trafficked areas, confirm cover depth and load class with the designer before backfill. Keep manufacturer data sheets on site with the drawing revision.
If this matches your project stage, send drawings and site details for a scoped quote — or call to talk through programme and access constraints.
Get an Accurate QuoteConstruction sequence
Step 1: Confirm design and tests
What this step delivers: Size, depth, and location approved.
Step 2: Excavate and form
What this step delivers: Chamber geometry matches the detail.
Step 3: Install and connect
What this step delivers: Crates/fill, geotextile, and inlets complete.
Step 4: Backfill and record
What this step delivers: Structure protected; as-built noted.
Common failure points and constraints
- Percolation test completed and referenced on design
- Crate or rubble soakaway on drainage drawings
- Roof and gully connections shown to the chamber
- Building Control expecting infiltration evidence
- Clay or high water table making infiltration doubtful
Pre-install and testing checklist
- Soakaway drawing and percolation results on the latest revision
- Service check in the excavation zone
- Inlet falls from drained areas verified
- Geotextile and crate specification available on site
- Maintenance access detail if required
Install reference
| Risk | What goes wrong | Prevention |
|---|---|---|
| Silt ingress | Crates clog early | Protect during landscaping and roof works |
| Wrong location | Too close to foundations or boundary | Set out from coordinated design |
| Over-compaction | Crushed crates / lost void | Backfill sequence per manufacturer / detail |
Coordination with structure and adoption
Install to detail, keep silt out of the system during construction, provide maintenance access where specified, and record as-built position and invert for the property file.
If landscaping follows immediately, brief that team on cover locations and load limits over crates. Early trafficking is a frequent cause of crushed void volume and callbacks.
What drives complexity on site
Dig volume, muck-away, crate specification, high groundwater, and restricted access drive cost. Failed percolation after excavation forces redesign and rework. See soakaway cost under /groundworks-costs. On clay-rich or seasonally wet plots, treat percolation results as site-specific evidence, not a transferable rate from a nearby street. If crates are specified, follow the manufacturer’s surround and compaction limits so void volume survives backfill. Where crates must be imported on tight access plots, crane or telehandler logistics can dominate a single-day install window more than the excavation itself.
Typical scheme contexts
- Self-build and small housing plots
- Extension roof and patio drainage
- Replacement of failed domestic soakaways
- SuDS components where infiltration is proven
Frequently asked questions
Can Mainline size the soakaway without percolation data?
No. Sizing needs design inputs including infiltration performance. We install to the engineer’s detail.
Is a soakaway the same as attenuation crates?
No. Soakaways infiltrate. Attenuation stores and controls discharge to a sewer or watercourse. See attenuation systems for developers for development-scale storage.
Can I use rubble instead of crates?
Only if the design specifies it. Many modern details use crate systems for volume and access; rubble fills still need correct geotextile and sizing from percolation evidence.
How soon can landscaping go over a new soakaway?
After correct backfill and any specified protection period. Avoid heavy plant over crates unless the design allows that load.
How This Issue Is Normally Diagnosed and Repaired
Follow the typical path from problem identification through to resolution:
We provide these services across the UK, including
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Discuss Your ProjectInstalling soakaways on new build FAQ
Can Mainline size the soakaway without percolation data?
No. Sizing needs design inputs including infiltration performance. We install to the engineer’s detail.
Is a soakaway the same as attenuation crates?
No. Soakaways infiltrate. Attenuation stores and controls discharge to a sewer or watercourse. See attenuation systems for developers for development-scale storage.
Can I use rubble instead of crates?
Only if the design specifies it. Many modern details use crate systems for volume and access; rubble fills still need correct geotextile and sizing from percolation evidence.
How soon can landscaping go over a new soakaway?
After correct backfill and any specified protection period. Avoid heavy plant over crates unless the design allows that load.
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