Outdoor Plumbing

Rainwater Drainage System

unblocking a downpipe

In the UK, as in many other Northern European countries, an efficient and well-maintained rainwater collection and drainage system is essential. The average yearly rainfall in the UK adds up to around 750mm, and if your gutter, gullies and downpipes aren’t up the job, you will soon start to see damage being done to your house. It is important that you know your way around your rainwater drainage systems, so you can spot problems quickly and fix them before the damage is done.

Gutters and Downpipes

Gutters are the first line of defence against rain, catching the water as it runs off your roof and funneling it into downpipes. An average house should have gutters at least 100mm wide to be able to handle a heavy downpour. Houses with a bigger expanse of roof than average may need bigger gutters to deal with the increased amount of water. Your gutters feed the rainwater into the downpipes, which should be around 75mm wide (for guttering of 100-115mm).

Check your gutters and downpipes regularly, particularly when it is raining heavily. Check that water is not leaking over the edge (possibly caused by a blockage) or from the joint where two pieces of guttering connect. Blockages are particularly a problem if you have trees overhanging the house. If you think you have a blockage, get your ladders out as soon as possible and clear it to avoid seeing an ugly damp patch appear in your home.

Gutter Outlets

A gutter outlet is the part of the gutter which connects to the downpipe, feeding the water directly into it. These will either be straight or, if the eaves of the house overhang, have a double bend called a swan neck. If you have a roof where two slopes meet in a valley, you will usually have a fitting called a hopper at the bottom of the alley to catch the rainwater. The downpipe then runs from the hopper to ground level. In older houses, Hoppers are also sometimes used to collect water draining out of upstairs baths and basins.

Cast Iron, PVCu or Aluminium?

Most modern houses will have PVCu guttering, older houses may have cast iron gutters or they may have been replaced by PVCu. Both of these types are made up of similar components with only the material differing. The run of the gutter is split into sections, each fitting together tightly and all held onto the facia boards with u-shaped brackets. One of the sections needs to have an outlet so the gutter can be connected to the downpipe.

Cast Iron guttering tends to need more maintenance than its modern counterpart. Some modern houses may have aluminium gutters. These are usually formed in one long strip by passing a flat strip of aluminium through a forming machine. The gutter is then fitted with outlets, angles and stop ends to make a complete run. You can also buy cast aluminium guttering which comes in sections just like cast iron and PVCu.

When choosing guttering you will find that PVCu is the cheapest option, cast iron and cast aluminium is a mid range option, and formed aluminium the most expensive option. Cast iron and cast aluminium can both be painted whatever colour you wish, whilst PVCu is only available in a range of about 4 colours (black, white, grey and brown).


The next part of the rainwater drainage system is called a gully. This is the ground-level drain into which the downpipe directs the water. The gully will often have a u-bend beneath the ground to stop any smells from escaping and rising up the downpipe, particularly if the gully is also used to catch waste water from the house. Older houses will have gullies made of vitrified clay, whilst new houses will usually have PVCu gullies and drains.

Some gullies have a raised edge around them and will only catch water from the downpipe (and possibly a waste water pipe) and some are flush with the ground. Flush gullies, known as back-inlet gullies, are designed to allow surface water from your garden to drain away. On back-inlet gullies, the downpipe will usually pass through a plate and discharge beneath ground level.

You can read more about unblocking gullies (as well as soil stacks) in our Blocked Stack or Gully guide.

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