If you’re thinking of installing your own underground drainage, be prepared for some hard work! You might want to hire some digging machinery or get a professional in to dig the trench, but if you decide to do the whole thing yourself, here’s a rough guide to getting it right.
Plot your route
The route of the pipe should be as direct as possible if draining a particular point or can meander widely if a general area needs to be drained.
The route of the pipe should be as straight and direct as possible if it’s draining one particular point. If it’s draining an area, it can “wander” quite freely to access as much of the ground as possible. The main thing to remember about underground drainage is that the pipe must start at a point higher up than the end point – a gradient of one foot per hundred feet is recommended.
Choose your pipe
For spot draining you need a closed pipe – the water enters at the top end and exits at the bottom. You can buy this as flexible rolls, several feet long, which also come with all the fittings to join them together as you wish. For overall drainage you need open pipes – with rows of holes – and these come in shorter sections.
Dig your trench
A drainage pipe needs to lie in a trench, so dig your trench along the route you’ve planned – it should be 8-10 inches wide and 18 inches deep. Once you start digging, save the turf you cut up so you can cover the trench up when you’ve filled it in again. Make sure the trench bottom is firm and that it slopes slightly down all the way along.
Line the trench with gravel
Line the bottom of your trench with gravel – to a depth of two or three inches – and then level it off. If you’re laying open pipes, you need to line the trench with filter fabric over the gravel.
Install your pipes
If you’re using closed pipe, unroll it into your trench, making sure it’s connected to the drain at the top end.
If you’re laying open pipes, lay them on the bottom of the trench. The pipe’s holes need to face downwards and your filter fabric should be wrapped around the pipes as you lay them. This fabric prevents the pipe from being blocked by larger pieces of debris getting washed into it.
Re-fill your trench
Once all the pipes have been laid and you’re satisfied that there’s a gentle downward slope from top end to the eventual outlet (remember you should aim for one foot of downward incline for every hundred feet of pipe length), you are good to re-fill the trench. Do this carefully, making sure you don’t disturb the filter fabric.
Replace the turf
In order to have a neat and pleasing finish to the job, place back the turf you cut when you dug out your trench. If it’s hot, make sure you keep the sections of turf watered while they wait.