An old soakaway: should the rainwater go into a manhole?

We were contacted by a homeowner whose rainwater drainage system and soakaway were thought to be more than 70 years old. Another drainage company had tried to carry out a CCTV survey but, according to the homeowner, could not get the camera through because of the condition of the pipes. Their suggestion was to abandon the existing arrangement and connect the rainwater drains into an existing manhole, with a new or extended chamber nearer the garage.

The homeowner wanted an independent assessment before going ahead. Could the soakaway be restored, extended or replaced? We agreed that it was worth investigating the existing system before committing to a different drainage route.

Where does the manhole discharge?

Our first concern with the proposed connection was what that manhole actually served. A nearby chamber might be convenient, but we need to know where the water goes after it leaves it.

Foul drains carry wastewater from toilets, showers, sinks and domestic appliances. Surface-water drains carry rainwater from roofs and other drained surfaces. A combined system is intended to take both. Those distinctions matter when deciding whether an existing drain can accept a new connection.

We would check the available records and trace the drainage as needed, looking at ownership, pipe condition, levels, capacity and any existing approvals. Finding rainwater already entering a foul drain does not make it a combined system or show that another connection is acceptable.

Has the soakaway failed, or the pipe leading to it?

If a camera cannot get through a pipe, there is a limit to what that survey can tell us. A blockage, root growth, displaced joint or collapsed section could be stopping water from reaching the soakaway. The soakaway may still be usable once the connecting pipe is repaired.

Equally, the soakaway could be silted up, damaged, too small for the area now draining to it, or sitting in ground that can no longer absorb the water quickly enough. There may be faults in both the pipework and the soakaway. We need to distinguish between them before recommending work.

We would start by reviewing the previous report and footage, tracing the rainwater runs and locating the soakaway as far as practicable. A CCTV drainage survey from available access points, suitable clearance where the pipe condition allows it, or targeted opening-up may then help establish the cause. Any inaccessible sections and remaining uncertainties should be recorded. An incomplete survey cannot confirm the condition of pipework beyond the point reached.

Why rainwater can cause trouble in an old foul drain

Where the receiving drain is foul-only and of a similar age, it would not be unusual to find age-related defects. We cannot confirm its condition from its age alone, but the fact that the toilets and showers drain normally does not tell us that the pipework is sound.

A toilet flush or a shower passing through the system may cause no obvious problem. Small cracks, defective joints and partial restrictions can go unnoticed for years under normal domestic use. Heavy rainfall is different. Even a relatively small roof can send a considerable volume of water into the drains in a short time.

Where that additional flow exceeds the available capacity, water can back up and fill the pipe under pressure: this is surcharge. It can increase leakage through existing defects, cause chambers or gullies to overflow, and produce localised flooding. The symptoms may appear some distance from the new connection, at a restriction or a lower point elsewhere in the system. Additional water movement through defective joints can also affect the surrounding ground.

Moving the rainwater may therefore move the problem, or expose a fault that had not previously caused noticeable trouble. The receiving drain needs assessing as well as the existing rainwater system. Thames Water's guidance on hydraulic flooding explains the effect of excessive flows on sewer capacity.

Can the existing soakaway be restored or replaced?

Where the ground is suitable, we would look at keeping the rainwater draining by infiltration. England's Approved Document H puts an adequate soakaway or other infiltration system first in the rainwater discharge hierarchy, followed by a watercourse, then a sewer where the earlier options are not reasonably practicable.

Depending on the findings, the work might involve repairing the connecting pipe, restoring an accessible part of the existing arrangement, or designing a replacement or additional soakaway. Cleaning does not guarantee that a silted soakaway will recover, and extending one without checking the ground may simply create a larger system that still cannot drain.

A new infiltration system needs appropriate ground investigation and rainwater infiltration testing to BRE Digest 365 at the proposed location and depth. The design should consider the measured infiltration rate, contributing roof and paved areas, storage needs, design rainfall and applicable climate-change allowances. Groundwater, ground stability, nearby foundations, services and maintenance access also affect suitability. Replacing an old soakaway like for like is not a substitute for that assessment.

What if the ground is unsuitable?

Poor infiltration, high groundwater or a substantiated risk to structures may rule out a soakaway at a particular location. The evidence should inform the next option: a suitable watercourse, an appropriate surface-water sewer, or a combined sewer where acceptable. Feasibility includes discharge rights, capacity, levels, any flow-control or storage requirements and the relevant consents before work begins.

A foul connection is not an automatic fallback. Thames Water's sewer connection guidance says that surface water should not enter its foul system without specific approval, and that other surface-water disposal methods must be exhausted before it permits a connection to a combined or foul sewer. The relevant sewerage undertaker must assess the particular proposal; its acceptance should never be assumed.

A substantiated concern about subsidence may help explain why infiltration is unsuitable and may require further ground or structural investigation. It does not, by itself, give permission to connect to a sewer. Whether a proposed connection is acceptable depends on the receiving system, the circumstances of the property and the approvals obtained.

What we advised

We recommended investigating the existing rainwater drainage and the feasibility of retaining infiltration before agreeing to connect it to another manhole. We offered to review the other company's report or quote for a CCTV drainage survey, then take the next step from those findings.

For a homeowner facing this decision, the useful starting point is to find out what has failed and what can reasonably be repaired. Once that is established, the options for the soakaway and any alternative discharge can be compared properly.