In a large old house, drainage rarely fails suddenly. It degrades for years, invisibly, and then presents itself on the worst possible evening: a ground-floor shower that will not empty, a smell in the utility room, water rising in the lowest gully in the garden. By then the question is no longer maintenance, it is emergency work, and the cost difference between the two is considerable.
Key points
- Wipes account for 80 % of the waste found in wastewater treatment plants, according to an ADEME figure from 2019.
- Collecting and treating them costs around 500 million euros a year in France, according to the French Biodiversity Agency.
- Since 3 July 2021, EU rules require wet wipes to carry a marking warning against flushing them.
- In older properties the recurring causes are root ingress at pipe joints, hardened fat, and settlement of the pipe itself.
- A camera survey before renovation costs a fraction of the excavation it avoids.
Three failure modes, and how to tell them apart
Blockages in a period property almost always belong to one of three families, and the symptoms differ enough to be diagnosed before anyone digs.
Root ingress. Old drainage runs were built in short lengths of vitrified clay with mortar joints. Roots find the joints, enter as fine hairs, then thicken inside the pipe. The signature is a blockage that returns on a seasonal cycle, clears easily when jetted, and comes back within months. Mature trees near a run, especially poplars, willows and figs, make this the first hypothesis.
Fat and grease. In a house with a working kitchen, fats cool on the pipe wall and build up in layers, narrowing the bore. The signature is progressive: the sink drains a little more slowly every month, then one day not at all. Unlike roots, this one is entirely preventable at the sink.
Structural failure. Ground settlement, a cracked or displaced section, or a partially collapsed pipe. The signature is a blockage that recurs at exactly the same point regardless of what is done upstream, sometimes with a hollow appearing in the ground above. This is the expensive one, and the only one that cannot be resolved by cleaning.
| Symptom | Most likely cause | Right first step |
| Seasonal, clears easily, returns | Root ingress | Camera survey, then cutting and relining |
| Gradually slower over months | Fat build-up | High-pressure jetting, then kitchen habits |
| Always the same point | Structural defect | Camera survey with location, then repair |
| Smell without slow drainage | Dry or defective trap, vent issue | Check traps and ventilation before the drains |
| Several fixtures at once | Blockage on the main run | Emergency intervention, not a plunger |
What goes down the toilet, and what it costs collectively
The single biggest change in domestic drainage over twenty years is what people flush. ADEME put wipes at 80 % of the waste found in wastewater treatment plants in a 2019 figure, and the French Biodiversity Agency estimated the cost of collecting and treating them at around 500 million euros a year nationally.
The regulatory response has been labelling rather than prohibition. Since 3 July 2021, under EU single-use plastics rules, wet wipes sold in the European Union must carry a marking indicating that they should not be disposed of in the toilet. Products described as biodegradable are included: biodegradability in a composting process says nothing about how a wipe behaves in a cold pipe over a few hours.
For a private household the collective figures matter less than the private consequence: a wipe caught on a root hair or a fat ridge is what turns a narrowing into a complete blockage. The two problems compound.
Survey before renovation, not after
The most avoidable expense in this field is discovering a defective drain after the finishes are in. A camera survey of the below-ground runs, carried out before a renovation starts, produces a plan of the network, the position and depth of the defects, and a clear separation between what can be cleaned and what must be replaced.
That work is regional and specialised, combining emergency response with civil works. Around Nantes, for instance, Dubillot Environnement has operated since 1974 across sanitation works, electromechanics and depollution, alongside emergency drain unblocking. Owners of large period properties typically need that whole range rather than a single trade, which is why these firms cover both the survey and the repair.
Two points make a survey worth its cost. The first is sequencing: a run that has to be replaced under a terrace is a modest job before the terrace is laid and a demolition afterwards. The second is negotiation: on a purchase, a documented drainage defect is a priced item rather than a vague suspicion.
What owners can actually do
- Never flush wipes, including those sold as biodegradable, and keep a bin in every bathroom so the rule is easy to follow.
- Collect cooking fat in a container rather than pouring it into the sink, especially in a house where the kitchen runs are long.
- Keep inspection chambers accessible. Covers get paved over, planted around or hidden under a deck, and the first cost of any intervention becomes finding them.
- Watch the trees. A drainage run within a few metres of a mature tree is a candidate for root ingress, and knowing where the run goes is the prerequisite.
- Keep the survey footage. A recording from five years ago makes the next diagnosis far quicker and shows whether a defect is progressing.
FAQ
Do chemical drain cleaners work?
They can shift a small soap or hair blockage close to a fixture. They do nothing against roots or a collapsed pipe, and repeated use on old metal pipework is not neutral. On a recurring blockage they postpone the diagnosis rather than solving it.
How often should drains be cleaned preventively?
It depends on the property rather than the calendar. A house with long runs, a working kitchen and mature trees benefits from a jetting and a quick camera check every few years; a small modern property may never need one.
Can an old pipe be repaired without excavation?
Often, yes. Trenchless relining techniques insert a resin liner that cures inside the existing pipe. They suit cracked or root-damaged runs but not a fully collapsed section, which is precisely what the survey determines.
Who is responsible, owner or utility?
As a general rule the private section up to the boundary connection is the owner’s responsibility, and the public network beyond it is the utility’s. The exact boundary depends on local rules, and it is worth establishing before an intervention rather than during one.
Is a smell always a drainage problem?
No. A dry trap in a rarely used bathroom is the most common cause and is solved by running water. If the smell persists after that, the vent stack or the drain itself becomes the next hypothesis.
Sources
- ADEME, 2019 figure: wipes represent 80 % of the waste found in wastewater treatment plants.
- French Biodiversity Agency: the collection and treatment of wipes represents a cost of around 500 million euros a year in France.
- EU single-use plastics rules: since 3 July 2021, wet wipes placed on the market must carry a marking warning against disposal in the toilet.