Downpipe disconnection and retrofit rain planters at home

Downpipe disconnection rain planter work is the smallest piece of drainage engineering you will ever do and one of the few that a householder can carry out without consent. Taking a single downpipe off the sewer and feeding it into a planter, a butt or a shallow rain garden removes roof water from a system that was never sized for today's storms, and on a combined sewer it reduces the volume that turns up at the works. Most houses have three or four downpipes. Disconnecting one is a weekend job.

The constraint is not permission. It is where the water goes next, and whether the ground and the space you have can take it.

Why taking a downpipe off the sewer helps

A typical pitched roof sheds a surprising volume very quickly, and in older towns that water goes into a combined sewer along with foul flow. During heavy rain the combined system surcharges, which is what produces street flooding and discharges to watercourses.

Keeping roof water on the plot does three useful things. It cuts the peak flow leaving the property, it puts water back into the soil where it supports planting, and it gives you a store you can use in dry weather. Some water companies also offer a reduction in the surface water element of the bill where a property no longer drains to the public sewer, so ask yours what evidence it needs.

The same logic drives the full scale version of this exercise, which is the hierarchy in a Drainage Strategy for a new development: infiltrate where you can, discharge to a watercourse if you cannot, and use the sewer last.

Where the water can go, and where it must not

Four destinations work at domestic scale. A soakaway, a rain garden or shallow planted depression, a rainwater planter against the wall, or a butt with a controlled overflow into one of the first three.

Two destinations do not work. The first is your neighbour's land: directing water onto it is a civil wrong as well as a bad idea, and it is the quickest way to turn a modest improvement into a dispute. The second is anywhere within a few metres of a building foundation or a basement wall. Approved Document H guidance keeps soakaways away from buildings, generally at least five metres, and the same caution applies to a planter that will leak into the ground.

Ground conditions decide what is realistic. Sand and gravel will accept water readily. Clay will not, and on clay the answer is storage with a slow release rather than infiltration, which means a larger planter, a butt, or a rain garden with an underdrain that eventually returns to the existing gully.

Dig a test hole before you commit. Fill it with water, see how long it takes to drain, and repeat it after a wet week rather than in a dry August. A hole that empties in an hour in September and holds water for two days in February tells you what you need to know.

Planters, butts and rain gardens compared

A butt on its own is not a drainage measure. A full butt in October has no capacity, so the overflow does all the work and the water goes wherever it went before. Butts count only when the overflow is connected to a planter, a garden or a soakaway.

Sizing it for your roof

The arithmetic is simple enough to do on paper, and the method is public, so you can check a contractor's figures.

Take the plan area of the roof that drains to the downpipe, in square metres. Multiply by the rainfall depth you want to capture, in millimetres, and divide by one thousand to get cubic metres. One cubic metre is one thousand litres.

An illustrative example. A rear roof slope of 24 m² drains to one downpipe. Capturing a 15 mm rainfall depth needs 24 multiplied by 0.015, which is 0.36 m³, or 360 litres. A planter 2 metres long by 0.6 metres wide with 300 millimetres of free draining growing medium above a gravel reservoir holds roughly that volume in its pore space, so one planter handles that roof for the events that matter most. A larger storm will overtop it, which is why the overflow design matters more than the storage figure.

Treat that as illustrative. Design rainfall depths vary by region and by the return period you are designing for, so if the exercise is part of a planning application rather than a garden improvement, the figures should come from the proper rainfall data for the site.

Overflow design, which is the part people skip

Every store needs a route for the water it cannot hold, and that route has to be deliberate.

Set the overflow at a level below the inlet, pipe or channel it to a point at least a couple of metres from the building, and make sure the ground there falls away from the house rather than towards it. Where the original gully is still in place, the simplest and most reliable overflow is back into that gully, with the planter taking the first flush and the sewer taking only the excess.

Protect the structure. Keep the planter's waterproof liner intact against the wall, leave an air gap or a drainage void behind it, and do not raise damp retaining material above the damp proof course. A planter that solves a drainage problem and creates a damp one is not progress.

Check for a basement or cellar first. On a property with habitable rooms below ground, infiltration close to the wall is the wrong answer altogether, and there is a fuller discussion in the note on basements, flood risk and sleeping accommodation.

Permissions, sewers and boundaries

For a normal house, a planter, a butt, a rain garden or a soakaway in the garden needs no planning permission. Three situations need a check.

Listed buildings: altering historic rainwater goods, cutting a downpipe or fixing a planter to the facade can need listed building consent, and cast iron downpipes are often part of what makes the building special. Flats: the external walls and the drainage are usually not yours to alter, so the lease and the freeholder come first. And any work that changes a shared drain, a connection used by a neighbour or a public sewer within your boundary needs the sewerage undertaker's agreement before you touch it.

Work close to a boundary wall may also engage the Party Wall etc. Act 1996, particularly excavation for a soakaway near a neighbouring foundation. Serve notice if in doubt, because it costs far less than the alternative.

If the water would end up in a ditch or stream rather than the ground, that is a different consent again, covered in the note on land drainage consent for works to an ordinary watercourse.

Maintenance over the first two growing seasons

The first autumn is the test. Leaves block inlets, so fit a leaf guard at the gutter and clear it twice between October and December.

In the first year the growing medium settles, so expect to top it up once and to replace a plant or two that drowned or dried. Check the overflow after the first heavy storm, not in the spring, because that is when you will see whether the water went where you intended. After two seasons the planting roots in, the infiltration rate usually improves, and the maintenance drops to clearing the inlet and cutting back.

If your street already floods, read the published surface water mapping first, and read it carefully: the note on reading the surface water flood risk map explains what it does and does not show.

This is general guidance, not legal advice; check the position with your local planning authority, your sewerage undertaker or a qualified professional before relying on it.