Quick answer: internal drain clearance means the pipework inside your house, from a sink, bath, shower or toilet down to the point where it leaves the building. External means everything outside: the gully, the underground run across your garden, and the inspection chamber. The quickest way to tell them apart is to run water at more than one fixture. If only one is slow, the trouble is internal and often close to that plughole. If several go at once, or the gully outside is standing full, it's external and further down the line. It matters because they take different equipment, they cost differently, and in some cases the external one isn't even your bill.
What counts as internal, and what counts as external?
Internal is the pipework you can mostly reach without going outdoors. The trap under a sink or basin, the waste pipe running from it through the wall, the toilet's own outlet, the short branch from a shower or bath. These are narrow, usually 32mm to 50mm on waste and 110mm on a toilet, and they block with things that arrive from directly above: hair, soap, toothpaste, food, cooking fat, wipes.
External is everything past the wall. The gully outside your kitchen or bathroom where the waste pipe discharges, the soil stack's connection below ground, the underground run crossing the garden, and the inspection chamber where several runs meet before heading to the sewer. All 100mm or wider, all buried, and they block with what accumulates over time: silt, leaves, grease that set hard, roots, and occasionally a displaced joint.
The boundary between them is roughly where the pipe passes through the outside wall or drops into the ground. That is also, conveniently, where the diagnosis changes.
How do I tell which one I've got?
Five minutes with the taps will usually settle it. Work through it in this order.
- Run one fixture on its own. Fill the basin, pull the plug, and watch. If it drains normally, that fixture is fine.
- Do the same at every other fixture, one at a time. You are looking for a pattern, not a single result.
- Count how many are slow. One slow fixture with everything else normal means an internal blockage on that branch. Two or more means the problem is downstream of where those pipes join, which is usually external.
- Go and look at the gully outside. If the water in it sits above the outlet, or it overflows when someone runs a tap indoors, the blockage is external and you have just found roughly where it starts.
- Listen while you flush. Gurgling from a plughole when you flush the toilet means air is being displaced past a partial blockage on a shared run, which again points outside.
There is one combination worth taking seriously. If a downstairs toilet or shower backs up while an upstairs one still drains, the blockage is below the lowest fixture in the house and the next thing to fill is the ground floor. That is not a rodding-it-tomorrow job.
Clearing an internal blockage
Most internal blockages are within arm's reach of the plughole, which is why they are the ones people fix themselves.
Start by taking the trap off. Put a bucket under it, unscrew both collars by hand, and tip out what's inside. On a bathroom basin the answer is almost always a matted plug of hair and soap sitting right there. On a kitchen sink it's usually congealed fat. Clean it, refit it hand-tight, run the tap and check for drips.
If the trap is clear but the pipe behind it isn't, a plunger over the plughole with the overflow blocked off will shift a soft blockage. Beyond that, a hand auger fed into the waste pipe reaches a couple of metres in. What none of this handles is a blockage in the soil stack or below floor level, and that is the point to stop and get someone in.
Keep the caustic drain cleaner in the cupboard. It rarely clears a solid blockage, it damages older plastic waste pipes, and if it doesn't work you have now left a pipe full of caustic liquid for whoever opens it next.
Clearing an external blockage
External work is slower, dirtier, and needs actual equipment. The first step is always to lift the grid and empty the gully pot by hand, because a surprising share of external blockages are nothing more than that pot silted up to the outlet. Details on that in clearing a blocked outside drain.
If the gully is clear and water still won't go, the blockage is in the underground run. Rods will reach it if it's within a few metres and soft. Feed them in, turn clockwise only, and stop if steady pressure isn't moving anything. Compacted silt, hardened fat and root ingress do not come out on rods, and forcing them is how people put a rod head through an old clay joint.
That is what high-pressure jetting is for. It scours the pipe wall rather than boring a hole through the middle of the blockage, so the run comes back to full bore instead of narrowing again a fortnight later. It is also the only realistic option once roots are involved.
Across Sussex we do both ends of this, from a single slow basin to a fully blocked underground run, in Brighton, Hove and out through East Sussex and West Sussex.
Does it cost more to clear an external drain?
Usually, yes, though not always by much. An internal trap or waste pipe is quick and needs little more than hand tools. An external run means lifting covers, working underground, and often bringing a jetter, so it takes longer and costs more. An external blockage that turns out to be a silted gully, though, can be the cheapest job of the lot.
What actually moves the price is access and severity rather than which side of the wall it's on. A chamber that has been paved over, or a blockage at eleven at night, costs more than the same job in daylight with the cover in plain view. The ranges and what shifts them are set out in how much it costs to unblock a drain. We agree the price with you on the phone first, and there is no call-out fee either way.
Who is responsible for which part?
This is where the distinction stops being academic and starts being about money.
Internal pipework is yours. Every time, no exceptions, whether you own the place or the landlord does.
External is where it varies. A drain inside your boundary serving only your house is normally yours too. But a drain shared with neighbouring properties, and the public sewer it eventually joins, is normally the water company's, and Southern Water may clear it at no cost to you. In the Victorian terraces around Brighton and Hove, where one run often serves several houses, that shared arrangement is far more common than people assume. It is worth checking before you pay for anything, and who is responsible for a blocked drain walks through how to work out which you have.
When to stop and call someone
Call when the water is coming back at you rather than going away, when more than one fixture has gone at once, or when you have cleared the obvious and it returned within a week. A recurring blockage in the same spot is telling you something is wrong with the pipe itself rather than with what went down it, and clearing it again will buy you another fortnight at best.
Danny Ozoum at Drains 4 Brighton handles internal and external blocked drain clearance across Sussex, 24 hours a day, price agreed before any work starts.
Call 07459 599505