Depending on the slope(the steeper, the deeper) you need to backfill the wall with gravel and install a drainpipe with holes in it and route the water away from the patio.
French drains and rear wall drainage A French drain is a drain with no pipe. The water collects in a gravel or stone filled channel that starts from the surface of just below it. The advantage of a French drain is that is easy to construct and with the use of modern ground fabrics very efficiant. Stage One
The photos we show in this article show a French drain being constructed behind a low retaining wall. First you need to dig a trench behind the wall about 15cm wide and as deep as required with a fall off 1 in 100 so that the water flows. Next the trench should be linned with ground sheeting which is avaiable at most builders merchants. This sheeting allows water to pass through but not the soli particles such as silt and clay that often block drains after a number of years. Stage Two
Fill the trench inside the sheeting with a clean stone, 20mm is ideal or clean broken bricks. The trench can be filled to the surface or as shown in our photo it has been filled to about 10cm below the surface and the sheeting wrapped around it. Then backfilled with topsoil over the top. Stage three
The drain will needed to end in a soak away. This is a large cube shaped hole linned with ground sheeting and filled with clean gravel or clean rubble. The idea is that when the drain is working the water runs into the soakaway and over a few hours soaks into the surounding ground soil. For this reason it is best to pick an area of land that drains freely ! | 
