Blocked outside drain: how to clear it and when to call

Signs the outside drain is blocked

Outside drains rarely block without warning. The early signs are easy to miss because they show up indoors first: sinks, baths and showers draining slowly at the same time, a toilet that rises before it empties, or gurgling from a plug hole when another fitting is used. Outside, you may notice a smell near the gully or a grid that holds water after a downpour.

Once the drain is fully blocked, the water has to go somewhere. It usually comes up at the lowest open point, which is often the outside gully or a manhole cover in the garden or front path. In some houses it backs up into a ground floor toilet or shower first.

• Water pooling around or overflowing from the gully by the back door
• A manhole cover in the garden or path lifting, leaking or surrounded by water
• Several fittings indoors draining slowly at once
• Toilet water rising when the bath or washing machine empties
• Gurgling from sinks or the toilet
• A sewage smell outside that does not go away

Gully or underground drain: work out where the blockage is

Most London houses have two kinds of outside drain point. The gully is the small grid at ground level where waste pipes from the kitchen or bathroom end. The inspection chamber, usually called the manhole, is a square or round cover over a deeper chamber where underground drains meet and change direction.

Lift the manhole covers you can safely reach and look inside. If a chamber is full of water, the blockage is further downstream, beyond that chamber. If a chamber is empty, the blockage is upstream of it, between that chamber and the fittings that are backing up. Working from the house towards the road, the blockage sits between the last full chamber and the first empty one. That tells you, or the engineer, where to rod from.

If water only pools around a rainwater gully after heavy rain while the sinks and toilets drain normally, the problem may be a soakaway not draining rather than a blocked drain.

Lift heavy or damaged covers with care, keep children and pets away from open chambers and never climb into one. Deeper chambers can hold harmful gases, and a broken cover is a fall risk.

How to clear a blocked gully

A blocked gully is the one outside drain job most people can deal with themselves. Kitchen gullies tend to fill with fat and food waste, and gullies under trees and downpipes with leaves and silt.

Wear long rubber gloves and have a bucket and a bin bag ready. Lift off the grid and scoop out everything you can reach, down to the water in the bottom of the trap. A small trowel or an old plastic cup helps. Then pour a bucket of warm water with washing-up liquid down it and watch whether it drains away quickly. If it does, clean the grid and put it back. If the gully empties slowly or fills again as soon as a sink is used, the blockage is past the trap and in the drain itself.

How to unblock an outside drain with rods

Drain rods are flexible lengths that screw together, with a plunger or corkscrew head on the end. They can clear soft blockages such as wipes, fat and paper from an underground drain, if you can get at it from a chamber.

Start at the full chamber nearest the blockage, fit the plunger head and feed the rods into the outlet pipe at the bottom of the chamber, adding lengths as you go. When you meet resistance, push and pull firmly to break it up. Once the water level starts dropping, keep going a little further, then flush the drain with plenty of water from a hose or the taps inside.

• Only ever turn drain rods clockwise. Turning them anticlockwise can unscrew the joints and leave a rod stuck in the drain.
• Do not force rods against a hard obstruction; it may be a collapsed pipe or a root mass.
• Wash the rods and your boots afterwards and keep them away from the house.
• If you have rodded twice and the water will not drop, stop and get a professional in.

What not to pour into a blocked drain

Drain chemicals are designed for sink traps, not for underground drains full of standing water. Poured into a flooded gully or chamber, they sit in the water, rarely reach the blockage and make the job more dangerous for whoever clears it. If you have already used one, tell the engineer which product.

Boiling water will not clear a blocked drain either. It may soften fresh grease at the gully, but that grease sets again further along the pipe, and very hot water can damage plastic gullies and seals. Caustic soda and bleach are no better. Thames Water's advice is to keep fat, oil, grease and wipes out of the drains in the first place.

Is the blocked drain your responsibility?

In England, the pipes that serve only your property and sit within its boundary are your responsibility. Since October 2011, drains shared with neighbours and pipes beyond your boundary have generally been the responsibility of the sewerage company, which in most of London is Thames Water. Our guide to shared drain and sewer responsibility explains how the split works.

So if your neighbours have the same problem, or every chamber on your property is full, report it to Thames Water before paying for a clearance. If the blockage is on your own pipe, you can clear it yourself or call a plumber. Check your home insurance too, as some policies cover drainage. If you rent, tell your landlord straight away.

Why outside drains block, and how to stop it happening again

The biggest causes are what goes down the pipes: fat and food from the kitchen, wet wipes and sanitary products from the toilet, and hair and soap from bathrooms. Outside, leaves, silt and building debris wash into gullies, and tree roots find their way into old clay pipes through the joints.

Scrape plates into the bin, collect cooling fat in a jar, bin wipes even if the packet says flushable, and keep gully grids clear of leaves. If the same drain blocks every few months, clearing it again is only treating the symptom. Our guide on why drains keep blocking covers the usual reasons, and a CCTV drain survey shows whether a cracked, sagging or root-damaged pipe is the cause.

Is a blocked outside drain an emergency?

It becomes one when sewage is coming up inside the house or into the garden, when a ground floor toilet cannot be used, or when water is spreading towards the house walls, an airbrick or a basement. Stop using water indoors, keep people away from the waste water and call for help.

A slow gully with no smell or overflow is not an emergency, but it is worth sorting before it becomes one. Blockages rarely clear by themselves, and a partly blocked drain tends to fill up further every time a bath or washing machine empties.

Call a drainage engineer when DIY has not worked

A drainage engineer brings powered rods, a high-pressure jetter and a drain camera, and can reach blockages in sections a hand rod cannot, such as the pipe under the house or a long run to the boundary. Our drain cleaning methods compared guide explains when each is used. If the pipe is damaged, drain relining can often repair it without digging up the garden.

Loyal Plumbers clears blocked drains across London and everywhere inside the M25, with 10 vans working 24 hours a day. Call 020 4652 8585 or send an enquiry and tell us which chambers are full, which fittings are affected and whether sewage has come up anywhere. Our call-out charge is £60, and the engineer explains what clearing the drain involves before starting.