Do I need drainage under a patio?
When a patio needs a channel drain or soakaway, how much fall it needs, and why water against the house wall is the thing to avoid.
Every patio needs somewhere for its water to go. Most of the time a correct fall is enough: the slabs are laid very slightly off level so rain runs away from the house and onto the lawn or a border.
How much fall?
A common rule of thumb is around 1 in 60 to 1 in 80 for natural stone, so roughly 15 to 20mm drop over a metre. You will not notice it when you stand on it, but the rain will.
When you need more than a fall
- The patio runs right up to the house and cannot fall away from it, so a channel drain along the wall is needed.
- The garden slopes towards the house.
- The patio would sit within 150mm of the damp-proof course.
- Heavy clay that holds water, which is common around Newton Abbot and the Teign valley.
The channel drain in the photo runs the full length of the house wall and takes the water to the existing drains, so none of it reaches the brickwork.
