A septic tank in full crisis is genuinely unpleasant — sewage backing up indoors, smells in the garden, soggy ground over the soakaway. Don't panic, but don't ignore it either. This guide covers the immediate actions, what to avoid, and when to get help.

If sewage is actively backing up into the house:
  • Stop using all drains (sinks, toilets, washing machine, dishwasher)
  • Keep pets and children out of any affected area
  • Call an emergency drainage specialist immediately — don't wait until morning
  • Ring two or three local firms and say it’s urgent. We’re an independent guide and can’t send anyone out ourselves.

What's happening — quick diagnosis

Septic tank emergencies usually fall into one of three categories:

1. The tank is simply full

Most common. The tank hasn't been emptied for too long; solids and effluent have filled the tank to capacity; effluent now backs up the inlet pipes into the house. Solution: emergency emptying. Usually fixes the immediate problem in 1–2 hours.

2. The soakaway / drainage field has failed

The tank may be normal-full, but the drainage field can't accept any more effluent — usually because it's saturated (heavy rain over Cornwall is a frequent trigger) or clogged with solids that escaped the tank over years. Solution: emptying buys time, but you'll need a soakaway repair or rebuild. See our repairs guide.

3. There's a blockage in the pipework

The tank may be fine, but a blockage between house and tank (or tank and soakaway) is causing the backup. Common in older Cornwall properties where tree roots invade clay pipes. Solution: drain clearance and CCTV survey. See our drain clearance guide.

What to do — in order

  1. Stop adding to the problem. Don't run taps, don't flush, don't run washing machines or dishwashers. Every litre you add makes it worse.
  2. Identify the worst affected area. Is it indoors or outdoors? Which fixtures are backing up?
  3. Look at the tank lid (if accessible). Don't open it without protective gear. If you can see standing effluent at the inlet, the tank is full. If it's empty, the blockage is between house and tank.
  4. Check the garden. Soggy ground over the soakaway means drainage field failure.
  5. Call for help. Ring a local septic tank or drainage firm directly; whoever takes the waste away must be on the Environment Agency’s waste carrier register.
  6. Contain any indoor spillage. Towels, mops, gloves. Wear protective gloves and a mask — septic effluent is hazardous.

What NOT to do

These all make things worse:
  • Don't pour bleach or strong cleaners down drains — it kills the bacteria the tank needs and won't unblock anything
  • Don't use a plunger aggressively on backed-up toilets — risks splashing contaminated water
  • Don't dig over the soakaway while it's saturated — risks structural damage and exposes you to contaminated soil
  • Don't try to empty the tank yourself — illegal disposal of septic waste carries fines, and unprotected exposure is genuinely dangerous
  • Don't ignore it overnight — overflowing sewage causes structural damage to floors, contamination of porous materials, and serious health risks

How fast do you need help?

Be honest with yourself about the severity:

  • Sewage backing up into the house: Within hours. This is a genuine emergency — book the first available out-of-hours specialist.
  • Toilets slow or gurgling, no actual backup: Within 1–2 days. Probably emptying needed but not crisis-level.
  • Smells in the garden, no other symptoms: Within 1–2 weeks. Worth investigating but not urgent.
  • Soggy ground but no backup: Within a month, ideally during dry weather. Get the soakaway assessed.

Costs to expect

  • Standard emergency emptying: £200–£450 (versus £150–£300 for a booked job)
  • CCTV survey to diagnose: £150–£250
  • Soakaway rebuild: £2,000–£5,000
  • Tank replacement: £3,000–£15,000 depending on type
  • Indoor cleanup: can claim on building insurance for sudden damage; not for gradual

See our full cost guide for more detail.

Preventing the next one

Most septic tank emergencies are preventable:

  • Stick to a regular emptying schedule — see our frequency guide
  • Don't put things down the drain that don't belong — no wipes, sanitary products, cooking fats, paint
  • Get a CCTV survey every 5–10 years if your system is older — catches problems before they're emergencies
  • Service treatment plants annually if you have one
  • Don't drive over or build on the soakaway area — compaction kills drainage fields

Insurance considerations

Most building insurance excludes "gradual damage" — slow seepage, long-term wear. But sudden events (e.g., a tree falling and crushing a pipe, a flash-flood saturating the drainage field) may be covered. Document everything with photos and dates if you might claim.

Getting help fast

When it’s urgent, ring local firms directly rather than filling in a form: say it’s an emergency and describe the access. We’re an independent guide, not a septic tank company, so we can’t send anyone out. If it can wait, you can ask us for an introduction. Whoever you use, check them on the Environment Agency’s waste carrier register.

Common questions

What should I do if sewage is backing up from my septic tank?

Stop using all drains immediately. Keep pets and children out of affected areas. Call an emergency drainage specialist — don't wait.

Why is my septic tank overflowing?

Three common causes: (1) tank is full and needs emptying, (2) soakaway/drainage field has failed (often due to saturation or solids buildup), (3) blockage in the pipework between house and tank.

How much does emergency septic tank emptying cost in Cornwall?

Emergency or out-of-hours emptying typically costs £200-£450, compared to £150-£300 for a booked job. Weekends and bank holidays carry higher premiums.

Can I try to fix an overflowing septic tank myself?

No. Don't try to empty the tank yourself (illegal disposal carries fines), don't pour bleach or chemicals down drains (kills the bacteria), don't dig over a saturated soakaway. Get a specialist who is a registered waste carrier.

Will building insurance cover septic tank damage?

Usually not — most policies exclude 'gradual damage'. Sudden events (tree falls and crushes a pipe, flash-flood saturates drainage field) may be covered. Document with photos and dates if you might claim.