Installing Rainwater Downpipes

The method is much the same for metal and plastic systems. The main difference is how the parts join together. The installer usually works from the top down:

1. Mark the position of the downpipe and use a plumb line or laser to draw a vertical line from the gutter outlet to the ground, clear of windows, lights and service connections.
2. Check that the gutter falls towards the outlet, usually by roughly 3 to 5 mm per metre, and set the outlet at the lowest point so it does not rest on the edge of a gutter bracket.
3. Get round the roof overhang with a pair of bends, known as an offset or swan neck, and cut the short connecting piece to length only after a dry trial fit.
4. Fix the brackets into solid masonry or concrete, usually no more than about 2 metres apart, and closer for plastic pipe.
5. Join the pipes so that the upper pipe always sits inside the socket of the lower one, and with plastic leave the expansion gap the manufacturer specifies.
6. Fit an access piece for cleaning roughly a metre above ground level, with a shoe or a connection to the underground drain below it.
7. Pour water into the gutter and check every joint for leaks.

A bracket belongs just below the socket of a joint, not in the middle of a pipe length. Tighten it firmly but not fully, because the pipe needs to move slightly as the temperature changes. A pipe clamped rigidly often cracks at a joint in frost.

The size of the downpipe does not come from what the neighbours have. It depends on the plan area of roof that the pipe drains. Manufacturers sell gutters and downpipes in matching pairs of sizes, and it makes sense to keep to those combinations. A narrow pipe under a wide gutter becomes a bottleneck in a heavy downpour, and water spills over the gutter edge. Where the roof area is large, a second downpipe in another position is usually a better answer than an ever larger pipe. For flat roofs and complex roof shapes, sizing is a job for a designer working to EN 12056-3.

• Galvanised steel: a simple metal option, but the zinc coating must not be scratched.
• Coated steel: a common compromise between durability and appearance, available in many colours.
• Plastic: light and rust free, but it expands more in heat and becomes more brittle in frost, so it suits lower buildings, garages and carports.
• Aluminium and copper: very long lasting materials that need an experienced installer.

A round profile is the standard and water flows through it with the least resistance. A square downpipe is mostly chosen for looks on modern houses. At the same nominal size it carries a little less water, and the range of fittings is often smaller.

The downpipe has to end somewhere, and this is where most problems start. Water pouring out right at the base of the house soaks the ground by the foundations for years and damages the plinth and the damp proofing below ground.

Discharge onto the ground works if the ground slopes away from the house, there is a paved area or gravel strip under the outlet and the water runs off at least roughly a metre from the foundations. Do not direct water onto a neighbour's property or onto a public footpath, where it turns to ice in winter.

Connection to a drain is reliable, but a gully with a silt trap belongs between the downpipe and the underground pipe, so that leaves and moss are caught before they block the drain. Ask your local water and sewerage company whether it accepts rainwater into the sewer and on what conditions.

A soakaway on your plot suits places with no sewer or where a connection is not possible. It has to be designed for how well the ground drains and placed at a safe distance from both the foundations and any well. Rainwater does not belong in a cesspit or a domestic treatment plant, and never send it into a drinking water well.

For a new drain connection, a soakaway or larger alterations, check with your local building authority what your situation requires under local building regulations. Do not assume in advance that a simple notification is enough. In a block of flats, agree any change to the downpipes with the building management or the owners' association.

The simplest case is a single storey house reachable from a ladder, a round downpipe in a common material and discharge onto the ground. These things make the job longer and more demanding:

• A taller house that needs scaffolding or a lifting platform
• An insulated facade, where brackets must be fixed through the insulation into the masonry with extended fixings
• Removing and disposing of the old downpipe
• Copper or aluminium, which need more precise working
• A downpipe concealed in the facade, or one running through a parapet on a flat roof, where every detail is more sensitive to leaks
• A connection to an underground pipe with a silt trap

When a house is being insulated, it pays to prepare the fixing points while the facade work is going on and to photograph where they are. Searching for solid masonry later through a thick layer of polystyrene is slow and damages the finished render.

Most failures are not caused by poor material but by an overlooked detail. In practice these mistakes come up again and again:

• Pipes overlapping the wrong way, with the lower pipe inside the upper one, so water runs out of the joint and down the wall every time it rains
• Brackets fixed only into render or polystyrene, which the weight of snow and ice tears out along with a piece of the facade
• Brackets spaced too far apart, so metal pipe bends and plastic pipe distorts
• A gutter that is level or falls the wrong way, so water stands in it, dirt settles and it freezes in winter
• No access piece, so the whole downpipe has to be taken apart at the first blockage
• An outlet at the plinth with no fall in the ground and no paved or gravel strip

If an existing downpipe keeps overflowing, the cause is often somewhere other than where the water shows. Typically the pipe is too small or the gutter has the wrong fall.

When you take over the work, ask for a rain test with a bucket or garden hose and go through these points:

• Water does not stand anywhere in the gutter and runs smoothly to the outlet
• The outlet has a leaf guard
• No joint leaks, especially at the bends under the roof
• Upper pipes sit inside the sockets of the lower ones
• Brackets are fixed into solid masonry, sit below the sockets and the pipe can still move slightly in them
• The downpipe is vertical and does not touch lights or service connections
• The access piece is fitted and its cover opens
• Water discharges far enough from the foundations or into a silt trap