How To Clear Blocked Drains

Blocked drains are unpleasant. But it is also very expensive to clear blocked drains professionally, so it pays to know how to do the job yourself.


Before you attempt to unblock main drains, make absolutely certain what system you have and get to know how it works.

how drainage systems work

Two pipe systems are still very common on houses built before the Second World War. There are two separate waste stacks running down the outside of the house — one for waste water and one for soil (waste from the WC). The waste pipes from your plumbing fittings run into the waste stack either directly or via a hopper head (now obsolete, but still very common). Pipes from ground floor fittings often connect to the stack under-ground. But if they are far away from the stack they run instead to a separate gulley — a kind of underground U trap. This joins the underground pipe from the waste stack at an inspection chamber, covered by a manhole.

Soil from the WC always runs to the soil stack direct. The underground pipe from the stack joins the waste water pipe at the inspection chamber.

How To clear a blocked drain

Rainwater may be collected at a gulley to join the waste water system. It may run from the gulley to the inspection chamber via a separate pipe. It may be dispatched to a separate gravel-filled pit or soakaway. Or, in areas where water is in short supply, it may run to a separate stormwater drain.

From the inspection chamber, the combined waste and soil water flows towards the main

drain, normally in the road. Before-it gets there it may well pass through another chamber — the interceptor — containing a large U trap.

Interceptors Were once used to cut houses off from the main drains; this is no longer done, so they are no longer fitted.


Your first check is always to see if the trap on the fitting itself is blocked.

If you are sure the blockage is in the drains, adopt the follow- ing procedure.

Open the manhole nearest the house. If this is empty the fault is in the fitting waste pipe, the stack or the gully trap (in the case of a backed-up gully).


You have to lift the manhole cover to check the drains, so this is always the first job. It’s not easy: cast iron manhole covers are heavy, so get some help.

Frequently the cover is rusted in its frame. Scrape around the join with a screwdriver then tap the cover gently with some wood — the vibration should release it from its seating.

Special keys are available for lifting covers but if you don’t have one a strong hook or a piece of steel bent in a vice will do just as well.

Some covers have handles consisting of small bars across indents.

In this case loop several turns of string or wire through the bars and around stout pieces of timber. With someone on either side of the cover and using the timber as handles, lift the cover and swing it free.

As a last resort, use a garden spade to lever up the cover over a fulcrum made from wood or bricks. The easiest way is to lift one end first and support it across the opening on a broom handle or similar. You can then lift the other end onto another support and use them both as rollers to push it clear of the opening.

Some covers are secured by screw bolts, in which case soak If the handles are intact, use timber and rope to lift cover them in penetrating oil before attempting to undo them. Remove the bolts with a spanner or wrench.

On all manhole covers a little grease smeared around the frame before replacement will stop future rusting.


The only really effective tools for this job are a set of rods.

You could try plunging with a mop or poking with a bamboo stick, but because the outlets and inlets will be hidden by effluent, you really need something more flexible.

1. Lay out your rod attachments before assembling the first rods.

2. Try the plunger head first, working it vigorously

3. After the blockage has cleared hose the chamber thoroughly

4. Chambers vary in size, but all demand the same procedure

Drain rods can of course be hired, but this is inconvenient. If your drains block regularly or you are worried that they could, it really is worth buying a set of ‘unbreakable’ polypropylene rods and attachments — they are now very cheap. Among the attachments you can get are a plunger (for gullies), a worm screw (for retrieving debris), a scraper (this unfolds when you withdraw the rods), a flexible wire leader (for acute bends) and a small wheel (this screws onto the front rod to stop it catching on joints).

When using rods, always \ turn them in a clockwise direction to stop the joints un-screwing.

Screw two rods together and lower them into the chamber. Feel for the inlet or outlet where you think the blockage is. And try to slide the rods into it.

When von feel them go in, screw on another rod and keep pushing. Continue in this way until you feel an obstruction.

Now turn the rods clockwise and at the same time keep manoeuvring them back and forth until you feel the blockage ‘give’. You may have to do this for several minutes — and quite vigorously — before it has any effect.

After you have cleared the blockage, always flush the chamber thoroughly with water to remove the last traces of debris.


Gullies, particularly the old open sort, are very common sources of blockages. If the blockage is in the trap, you can usually clear it from the gully itself. But if all else fails, rod in a plunging motion up the inlet in the inspection chamber that leads to the gully.

Small gullies can be baled out by hand. Lift the grid then, wearing rubber gloves, scoop out the debris into a bucket. Check that the outlet is clear with your finger: if it isn’t try poking some coathanger wire up it.

Gullies receiving sink waste get clogged with grease. This can usually be shifted with boiling water or scraped out by hand.

If this doesn’t work try pouring down a solution of caustic soda.

But take care and follow the instructions to the letter because caustic soda is dangerous.

Deep gullies and blockages on the other side of the trap can be shifted by plunging — you can get an attachment like a sink plunger that screws onto the end of an ordinary drain rod. A household mop makes an effective standby, as do old rags wrapped in a plastic bag and tied to a broom handle, but only if there is enough bulk to fill the gully neck completely.

You could try threading a garden hose around the bend in the gully trap to flush the blockage with water. In this case stop up the gully itself with polyethylene bags rammed tightly in place — the extra pressure this creates will help the water do its work.

Note that in some drain pipes (gully pipes in particular) rodding eyes are provided at places where blockages might occur.


These are rare in single stack systems and simple two pipe arrangements, but common in older, complicated drainage layouts.

Fortunately there are often clearing eves at junctions and direction changes.

1. Soak eye covers (or their bolts) in penetrating oil before attempting to remove them.

2. You can then either insert a rod with a head to match the stack diameter, or else feed in a screw clearing wire. Use the same procedure as for an inspection chamber.

3. Clear hopper stacks from above with a worm screw rod.


These can work their way through a joint in a drain pipe, in which case no amount of plung-ing will shift them. You may be able to clear them partially with a powered screw augur similar to the type used by professional drain clearers — they are available at some hire stores. But a more satisfactory solution is to excavate the drain at the site of the problem.

Find out where this is by making note of how many rods it takes to reach the obstruction. Then lay the same rods out at ground level in the same direction and see where they end.

Dig down to the pipe, hack away and pull out the offending roots, then make good the damage to the joint with a strong mortar mix of one part cement and two of sharp sand.

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