An outside drain that blocks once is a nuisance. One that blocks every few weeks or months is telling you something. The good news is that there are only a handful of common causes, and each has a clear fix.
1. Leaves and debris getting into the gully
Open gullies (the square grids where downpipes and waste pipes discharge) collect leaves, moss, silt and litter. In autumn, a gully near trees can fill up within days.
Signs: the blockage is at or just below the grid, and it is worse after windy or wet weather.
What helps: a gully guard or finer grid to stop leaves getting in, and clearing the top of the gully regularly.
2. Grease from the kitchen sink
Kitchen waste pipes often run into an outside gully. Fat, oil and grease are liquid when warm but set as they cool, coating the gully and the pipe beyond. Food scraps then stick to the grease until water cannot pass.
Signs: the blocked gully is the one the kitchen drains into, with white or grey greasy deposits visible.
What helps: keeping fat and food out of the sink, and having the gully and drain properly cleaned. Where the build-up has spread along the pipe, drain jetting cleans it back to the full bore, whereas rodding may just make a hole through it.
3. Wipes and sanitary products
If the blocked drain carries waste from a toilet, wipes are the most likely culprit. Many wipes, including some sold as flushable, do not break down quickly, and they snag at bends and junctions.
Signs: wipes visible in the inspection chamber, blockages that return after heavy use.
What helps: binning all wipes, cotton buds and sanitary products.
4. Tree roots
Roots are drawn to the moisture inside drains. In older clay pipes with mortar joints, they find their way in through tiny gaps and then grow, catching paper and debris.
Signs: blockages that return every few months, a drain near large trees or shrubs, roots visible when the drain is cleared.
What helps: roots can be cut or jetted out, but they usually return through the same gap. A CCTV drain survey shows where they are getting in, and a repair to that joint stops them coming back.
5. A damaged or sagging pipe
Pipes can crack, joints can shift and sections can sink over time, especially under driveways, near trees or after building work. A dip (sometimes called a belly) lets water and solids settle; a displaced joint creates a lip that catches debris.
Signs: recurring blockages at the same point, even when the drain is used carefully.
What helps: a survey to confirm the defect, then a drain repair that deals with it.
6. The drain is shared
If your neighbours have the same problem at the same time, the blockage may be in a shared section of drain. In England, shared drains and those beyond your boundary are generally the water company’s responsibility.
How to find out which it is
If you have had the drain cleared once or twice and it keeps coming back, the most reliable next step is a camera survey. It replaces guesswork with footage of what is actually inside the pipe. Read more in what is a CCTV drain survey?
If your drain is in Southampton, see blocked drains in Southampton, or contact us to arrange a visit.