Problems & Repairs
Septic Tank Overflowing
The most common causes of a septic tank overflowing, what you can safely check, and when to call a specialist.
In short: an overflowing septic tank almost always means the tank is full and not draining away properly — usually because it needs emptying, or because the drainage field it discharges to has failed. It’s worth acting quickly, as continued overflow can cause pollution and ground damage.
Most likely causes
- The tank simply needs emptying — the most common cause, and the easiest to fix
- A failed drainage field — if the field can no longer absorb water, the tank backs up; see failed drainage field
- A blocked inlet or outlet pipe
- Excess water entering the system — such as guttering or surface water wrongly connected to the drainage system
- The tank is undersized for the property’s current occupancy
What you can safely check
- When the tank was last emptied, and whether it’s overdue
- Whether recent heavy rain coincided with the overflow (can point to surface water intrusion or field saturation)
- Whether water use in the property has increased recently (guests, a new appliance, an extension)
When to call a specialist
If emptying the tank doesn’t resolve the overflow within a short period, or it recurs shortly after emptying, the cause is very likely the drainage field or a blockage rather than simply capacity — at that point, a specialist assessment is the right next step rather than repeated emptying.
Preventing it happening again
- Keep to a regular emptying schedule appropriate to your tank size and occupancy
- Ensure surface water (roof drainage, driveways) isn’t connected into the septic system
- Avoid excessive water use spikes where possible, and space out heavy laundry days
Is it safe to keep using water if my septic tank is overflowing? + −
It's best to minimise water use until the cause is identified, since continuing to add water to an already-full system will make the overflow worse.
Can heavy rain cause a septic tank to overflow? + −
Yes, particularly if the drainage field is already struggling or if surface water is wrongly connected into the system — saturated ground reduces how much the field can absorb.
How urgently do I need to deal with an overflowing septic tank? + −
Reasonably promptly — ongoing overflow can pollute the surrounding area and, if left long enough, cause more expensive damage to the drainage field.
Sources
- British Water — Code of Practice: Septic tanks and treatment plants