Blocked Drain: Do You Need a Plumber or a Drainage Engineer?

If only one sink, bath or toilet is blocked, it is almost always a blockage in that fitting's trap or waste pipe, which a plumber or a competent householder can clear. If every ground-floor fitting is slow or backing up, or water is rising in an outside gully or manhole, the blockage is in the underground drain and you need a drainage engineer. That single question, one fitting or several, answers most of the plumber-or-drainage decision before you pick up the phone.

Internal and external drains: where one ends and the other starts

Internal drainage is the pipework you can see or reach inside the building: traps, waste pipes, the toilet outlet and the soil stack. External drainage is the underground pipework from the foot of the stack or the gully out to the sewer, with manholes or inspection chambers along the way.

The split matters because the two need different tools. A blocked basin trap needs a spanner and a bucket. A blocked 100mm underground drain needs rods or a jetting unit, access through a manhole, and often a camera to see what caused it. The soil stack sits between the two: it is inside or on the outside wall of the building, but a blockage in it behaves like a drain blockage because every toilet above it is affected.

Signs the blockage is internal

A blockage is internal when the problem is confined to one fitting or one small group of fittings on the same waste pipe, and everything else in the building drains normally.

• One basin, bath or shower draining slowly, while the toilet and other fittings work fine. Hair and soap in the trap are the usual cause.
• The kitchen sink and the washing machine backing up together, while bathrooms drain normally. They usually share a waste pipe, and fat is the common cause.
• One toilet not flushing away, while the other toilet in the house and all the sinks drain normally. The blockage is in the pan or its connection to the stack.
• Outside gullies and manholes are empty or flowing. If you can lift an inspection chamber cover and see water running through, the underground drain is clear.

For a single slow basin or bath, cleaning the trap is a reasonable first step for anyone comfortable with it, since hair, soap and fat usually collect within the first few centimetres of pipe. A plumber is the right call if the trap is clear and the waste pipe behind it is still blocked.

Signs the blockage is external

A blockage is external when several fittings are affected at once, the lowest fittings in the house are the worst, or water is standing where it should be running outside.

• Ground-floor toilet, shower and sink all slow or backing up at the same time. They meet in the underground drain, so a blockage there affects them together.
• The toilet water rises when you run the bath or washing machine. Water from one fitting has nowhere to go and is coming out of another.
• An outside gully is overflowing, or a manhole is full. The blockage is at or beyond that point.
• Gurgling from the toilet or drains when other fittings empty. Air is being pushed back through water seals because the pipe below is restricted.

An external blockage will not be fixed from inside the house. Plunging a toilet or pouring chemicals into a sink only pushes more water into a drain that cannot take it. This is the point where a drainage engineer with rods, a jetting unit and a camera is the right answer, and our emergency drain clearance is set up for exactly that.

The soil stack: the grey area between the two

If an upstairs toilet backs up while the ground floor is fine, or waste appears in a bath when a toilet on the floor above is flushed, the soil stack or its branch connection is the likely problem. That needs either a plumber with drain rods or a drainage engineer, depending on where the blockage sits. Our guide to a blocked soil stack covers the signs and what to do first.

Stacks are usually reached through a rodding eye near the bottom, an access plate behind a boxed-in panel, or from the top at roof level. In London conversions, the stack for several flats often runs inside one flat's walls, which makes access the first problem to solve. Where the blockage is in the section of stack serving toilets rather than basins, we treat it as blocked toilet and internal drain clearance and clear it from the nearest access point.

Who pays for an internal or external blockage?

Internal pipework is always the property owner's responsibility, or the landlord's in a rented home. External drains are the owner's responsibility while they serve only that property and sit within its boundary. Drains shared with neighbours, and those outside the boundary, have generally been the water company's responsibility since October 2011.

That means an external blockage is sometimes free to you, provided it is in the shared section. Lifting your own inspection chamber is the quickest guide: if it is empty, the blockage is on your side; if it is full and the neighbours' chambers are too, the problem is further down. Our guide to who is responsible for a shared drain blockage explains how to check before you pay anyone.

When to call tonight and when to wait

Call tonight if the only toilet is out of action, if sewage is coming up inside the house, or if an external drain is overflowing near a door, a basement or an airbrick. A single slow sink can wait for a plumber in the morning.

Our rates are set by the clock rather than the day: 7am to 5pm is the standard rate, 5pm to midnight the evening rate, and midnight to 7am the night rate. Current figures are on the pricing page, and the price is agreed on the phone before anyone travels. If what you describe sounds like a trap or a single waste pipe, we will tell you a plumber is the better and cheaper call.