Good drainage is essential for successful gardening. Poorly drained soils are sour and acid, requiring frequent applications of lime. Also, wet soil is a cold soil, retarding the growth of shrubs and plants.
Raising the level of flowerbeds above the adjacent lawn or paving and adding peat, sand or compost to “sticky” soils is often sufficient.
In extreme cases, the heave and shrinkage of clay sub soils as they absorb and lose water can cause cracking in the fabric of a house. Less damaging, but still annoying, poor drainage will also cause doors and windows to jam whenever heavy rains are experienced.
In temperate climates, where the rainfall is moderate (about 500mm to 750mm per annum) and well spread throughout the year, many drainage problems can be solved by gardening, rather than constructional methods. Raising the level of flowerbeds above the adjacent lawn or paving and adding peat, sand or compost to “sticky” soils is often sufficient.
If pools of water form on an old lawn, it may merely be that the soil has compacted from years of traffic. Lifting the lawn, double-digging the soil underneath, adding new soil if necessary, and then re-sowing or returfing can cure this very easily.
But in areas where topsoil contains a high proportion of clay, or where the rainfall is exceptionally heavy, more drastic measures are needed. In all cases, you should aim to have the water table (level of water permanently in the ground) at least as low as the bottom of your house foundations – unless, that is, you have a below-the-ground basement, when you will have to settle for something less.
To establish the depth of the water table, simply dig a narrow hole the same depth as the foundations and then cover it and leave it for two or three non-rainy days. If it starts to fill with water, your water table is too high.
Drainage basics
There are three basic methods of laying garden drains, whether they are to be below ground or behind a retaining wall.
The cheapest, but the one with the shortest life, is to dig a trench, about 600mm deep by one spade’s width, and half fill it with bundles of brushwood.
The next best method is to fill a similar trench with about 255mm of rubble – use bits of bricks, broken concrete or large stones, but do not use old gypsum plaster, which will only disintegrate and clog the bottom of the trench. If you have no rubble to use, you can always source some from a demolition site.
The third, and best method, however, is to fill the trench with a 50mm layer of coarse gravel, then lay a row of pipes with a diameter of 75mm to 100mm, and covered to a depth of approximately 100mm with rubble.
Pipes come in a wide range of materials and can be either perforated or unperforated. Both types of pipes are available in plastic and concrete. If you use perforated pipes, be sure to lay them with the perforations towards the bottom of the trench, so that the water seeping into them takes a minimum of sediment with them. If you use unperforated pipes, leave a space of about 10mm between each pair.
Whichever type of drain you use, there are four main pointers that you need to be aware of, namely:
- The slope must be consistent. If part of the drain is steeper than the rest, sediment will lodge where the drain “flattens out” or stops altogether.
- Only a gentle slope – anything from 1:60 to 1:100 – is needed. If you lay a step slope, the water will rush along it, leaving a rim of sediment to cause problems later. Where a sharp fall is unavoidable – for example to enter a stream or sewer – lay the bulk of your pipes in a gentle slope and finish with a vertical drop, so that silt cannot accumulate.
- Always start laying at the lowest point and work your way uphill. This is the only way of getting a consistent fall.
- Above the brushwood or rubble, you must install a layer of finer material to filter out the soil that will otherwise clog the drainage system. Use 150mm or so of cinders, coarse sand or very fine gravel. Then you can fill the rest of the trench with topsoil.
Planters and retainer walls
The most effective method of draining a small garden is to divide it into naturally self-draining areas. On a sloping plot, you could make a series of terraces between 1m and 1,5m high, depending on the slope.
On flat ground you might divide the area into planter boxes with paving or lawn strips between. Either way, you will improve the appearance of the garden, as well as its drainage.
Field drains
In large flat gardens, converting the garden into a series of terraces or planters is a back-breaking job, involving moving tons of soil. Unless you have access to cheap second-hand bricks, the materials can also add up to a large sum of money.
A simpler method is to install a field drain system under sections of the garden. Essentially, the system comprises a large central drain, with various branches or arms leading from it. In slightly uneven sloping ground, the subsidiary drains can follow the natural “dips” in the ground. In flat ground, a herringbone pattern is effective.
Outlets
The simplest method of disposing of surplus water is to run the drain into a nearby ditch or stream.
If the pipe is much higher than the stream, it is advisable to pour a concrete channel, starting below the outfall pipe and sloping down to the stream for the water to splash on and prevent scouring.
Another method is to dig a soakaway, where the water will permeate the surrounding ground. Soakaways work well in soils containing a high proportion of gravel, sand, chalk, limestone and some types of crumbling clay, but less well in tightly packed, small-grained clays, rocks and similar ground.
If your property has a clay subsoil or rock half a metre down, try breaking through it as you might find more permeable material below. In all cases, you should site a soakaway as far from the house as practically and economically possible.
Thirdly, you can connect the outfall pipe to the domestic sewage or storm water drainage system. Alternatively, you can run the pipes to a water tank, and use the water run-off for garden irrigation at a later stage.
Driveways and paths
If you have a large driveway or concreted/paved area sloping towards to the house, it is advisable to install a small drain across the drive at its lowest point to divert the water sideways and away from the house.
At its simplest, the drain could be a merely a shallow depression about 75mm deep by 450mm wide, which can be driven across to enter the garage. At its most elaborate, it can be a 150mm by 100mm square sided channel with a row of iron gratings inset across the top.
Small paths running beside the house should be sloped sideways to carry water away from the house. Alternatively, you can slope the path towards a gulley into which the roof drainage empties.
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