I moved into a property about a year ago. It's late Edwardian and there are suspended floors and air bricks. The patio installed by the previous owner was set above the air brick level, although recesses were provided in the patio so that the air bricks are not totally covered. However, when it rains, water flows into the recesses and then into the air bricks. I also believe the patio is set well above the DPC although I have not been able to find it yet, because the patio level is set so high. The patio does not slope away from the house and when it rains water also sits against the outside wall of the house. Some damp was detected on the internal wall during the pre-purchase survey.
Long term, the plain is to build an extension. I don't want to spend a lot of money now reducing the patio level to 15cm below the DPC if the patio will need to be dug up again when the extension is built. I am therefore looking for an interim solution. The surveyor suggested a French drain.
Bearing in mind that the water on the patio is surface water, would a trench drain be enough positioned against the house wall to collect the rain water or will this still cause problems if it is above the DPC?
If I instead go for a French drain, is it sufficient to make a trench to 15cm below the damp proof course or does it need to be deeper? Can I make the gravel level at the same height as the rest of the patio to avoid people tripping into the trench? Do I need to put any waterproof membrane against the house wall to prevent penetrating damp and how high up should this go? The soil is a clay type. I would put in a pipe with holes at the bottom of the French drain to help take any water away.
To overcome the air bricks being below the current patio level, I was thinking of installing periscopic air bricks. What do people thing of this an interim solution or are there any other ideas?