Toilets not flushing properly with septic tank

Whether slow-flushing toilets point to a simple blockage or a bigger septic tank problem.

Sam WarrenUKDP co-founder
Published4 May 2020
Updated26 Oct 2026
Read time4 min

Key takeaways

Toilets not flushing properly with a septic tank can mean a simple blockage, such as too much toilet roll, or a bigger problem.
A temporary blockage in the pipe from the toilet can often be cleared with a plunger or by an unblocking firm.
Lifting the manhole between the house and the tank shows where the problem lies.
A blocked manhole points to the tank or the pipe beyond it, while a clear one points to the pipe from the house.
If slow flushing is the only sign, call an unblocking company, but an overflowing tank or boggy garden needs a fuller investigation.

With your property’s septic tank tucked away out of sight underground, it can be easy to assume that everything is working as it should be. But if your toilets are not flushing properly, and your property has a septic tank, this could be a sign of a problem that needs checking out.

First, don’t panic, toilets not flushing properly aren’t always a sign of a significant problem. It could be something straightforward such as a temporary blockage in the pipework running from the toilet. This could be caused by too much toilet roll being flushed down the loo (after all, every house has that one inhabitant who uses too much loo roll!), which can be quickly remedied with a plunger or a friendly local unblocking firm.

But how will you know whether the cause of the toilets flushing properly is something simple to remedy, or a sign of a bigger problem that needs proper investigation? Here at UKDP we have a specialist team of engineers located across the UK who visit property owners suffering with problems such as this, and their first port of call would be to identify where the problem is located. This would usually involve lifting a manhole between the house and the septic tank. If the manhole is blocked then the issue is likely in the tank or between the manhole and the tank. If it’s not blocked, then it is a problem somewhere between the house and the manhole, which might simply be a soft blockage.

If the only sign of drainage trouble you have at the property is the toilets not flushing properly, then your first port of call should be a Google search of an unblocking company. If you have also noticed other problems such as an overflowing septic tank or signs of boggy patches in the garden, then a quick unblock may not be the answer. Here at UKDP we can help investigate and resolve all sorts of septic tank problems, so please get in touch with our friendly team today for advice and next steps.

If the fault is with the tank itself, a septic tank repair may be needed.

UKDP engineer inspecting a drainage system

A slow or failing flush is often the first hint that a septic tank is struggling, well before anything shows above ground. Whether it is a simple blockage or the tank itself, a quick check points you to the right fix.

Frequently Asked Questions

Can a full septic tank stop my toilet flushing?

Yes. When the tank is full or the soakaway has failed, waste can't flow away, so toilets flush slowly or back up. If the tank's overdue, get it emptied and see if that fixes it.

How do I tell if it's a blockage or the septic tank?

Lift the manhole between the house and the tank. If it's full of standing water, the problem is the tank or soakaway; if it's clear, the blockage is on the house side and a plunger or drain company can clear it.

Why are all my toilets flushing slowly at once?

One slow toilet is usually a local blockage; every toilet and sink draining slowly points to the tank or soakaway being full or failed. That's the point to book an inspection rather than keep plunging.

Can I fix a slow-flushing toilet myself?

If it's a simple blockage, yes, a plunger or a drain company will clear it. If the tank's involved, that needs emptying and possibly an inspection, which isn't a DIY fix.

Why is water coming up in the garden when I flush?

That means wastewater has nowhere to go, usually a failed soakaway, and is surfacing instead. Stop heavy water use, keep off the wet ground and get the system inspected.