LAWN DRAINAGE

Most drainage systems are based on agricultural systems, which can be very effective. Follow this guide and sort out that soggy lawn.

The key to designing and installing a drainage system in a lawn is to remember that the water must be able to travel from the surface to the drain, which is installed under the lawn.

Firstly you must consider that all drainage systems work because water will always flow to the lowest point, so you need to design the system so that the drainage water will drain to the lowest point in your garden - into a ditch or a soak away. To achieve this, all of the drainage pipes that you have must have a fall on them no less than 1 in 200 in the direction of which you want the water to flow (ideally 10mm fall for every 1m of pipe laid).

To lay the pipes you must first mark out the area from the plans you will need to draw up. If you need any help doing this please feel free to contact me for any advice. Once you have a basic plan you can start to install the drainage system by following these basic steps :

1. Mark out the drain runs on the grass with some sand and cut the turf out in 125mm strips wide and place by the side of the drain run.
2. Next dig out the run. This is best done with a trenching spade. The depth shoul start about 400mm and slope in the intended flow of water, ideally at 6mm fall per 1m of trench. To test the fall, run some water from a hosepipe to the higher end of the trench and see that it runs smoothly down the trench.
3. Once you have dug the first trench, you should fill it with 20mm of 10-12mm clean stone, then place a 75mm perforated drainage pipe on top of it. This can be purchased from a builders merchants and normally comes in 25 metre rolls.
4. Next, back fill the trench on top of the drainage pipe with clean stone to approximately 75mm from the top of the trench. Next fill the remaining space with the soil that you dug out. Firm it down and replace the turf. It is important that when you are filling with stone that you keep it off the grass as this will damage your mower.
5. The next stage is to make a series of slit trenches at right angles from the drainage pipes across the lawn, with a spade at 400mm centres, about 75mm deep and fill them with a washed sand. It is important that the slits connect with the washed stone over the main pipes - you should be able to feel the stone with your spade as you make the slits.
6. The final stage needs to be carried out over a number of months. It involves top dressing the lawn with a washed (sharp) sand over a number of months to build up a carpet of sand at the base of the grass plants to allow the rain water to move freely on the surface and down the sand slits to the drain.

See my other guides for more information on top dressing.

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