A slow-draining sink or a gurgling shower is usually the first sign of a blockage building up in your waste pipes. The good news is that most household drain blockages can be cleared quickly without calling a plumber. This guide walks through how to unblock a drain — indoors and outside — and how to keep your drains clear and odour-free.

What causes a blocked drain?

Most blockages are caused by a build-up of fat, grease, food waste, hair, soap scum and other organic matter that sticks to the inside of the pipe. Over time this narrows the pipe until water can no longer flow freely. Outside drains and gullies can also block with leaves, silt and garden debris.

How to unblock a sink or shower drain

Start with the simplest methods and work up:

• Hot water: for a minor grease blockage, pouring a kettle of hot water down the drain can soften and shift it (let it cool slightly on plastic pipes).
• Plunger: a cup plunger creates suction that can dislodge a nearby blockage. Cover the overflow first for a better seal.
• Clean the trap: under-sink U-bends often trap debris. Place a bucket underneath, unscrew the trap and clear it out.
• Chemical drain unblocker: when the blockage is further down the pipe or will not shift, a caustic (sodium hydroxide) drain unblocker dissolves the build-up chemically.

Using a caustic drain unblocker

A sodium hydroxide (caustic soda) based product such as Bond-It Drain Blaster is the fastest way to clear stubborn fat, grease and hair blockages. It sinks through standing water to reach the blockage, dissolves it in around 15 minutes, and kills the germs that cause bad smells. Pour in the recommended amount, leave it to work (overnight for tough blockages), then flush through with hot water. Always wear gloves and eye protection, and never use a caustic unblocker in a toilet, waste disposal unit or septic tank.

How to unblock an outside drain

For a blocked outside drain or gully, lift the grid and clear any visible debris by hand (wear gloves). If the blockage is deeper, a set of drain rods pushed through the pipe will usually break it up. For grease and organic build-up in the pipe run, a caustic drain unblocker can be poured directly into the chamber. If the water still will not drain and the blockage is on the main sewer side, it may be a shared or public sewer — contact your water company.

How to stop drains smelling

Drain odours are caused by trapped food, grease and bacteria decomposing in the pipe. The most reliable fix is to remove the build-up: a weekly maintenance dose (around 100 ml) of drain unblocker keeps pipes clear and eliminates the source of the smell. Make sure the trap always holds water, too — a dry trap lets sewer gas back into the room.

When to call a plumber

If you have tried plunging, cleaning the trap and a chemical unblocker and the water still will not drain — or if several fittings back up at once — the blockage may be in the main soil pipe and is best left to a professional.

Keep drains clear

Prevention is easier than cure: avoid pouring fat or oil down the sink, use a plughole strainer to catch hair and food, and run a maintenance dose of drain unblocker through your sinks once a week. For plumbing and heating system cleaning, the PC400 flushing chemical and PC300 system cleaner keep pipework and radiators running efficiently.

Always read the product label and follow the safety instructions before using any drain-cleaning chemical.