Drainage dilemma - Dpc too high!!

[jbuckler]

Need some help on working out how to avoid digging out my entire garden!
I am laying a patio (sandstone, 3m by 4m width, 150mm depth of construction, fall away from house) at the back of the house against the extension which is the full width of the garden. The extension has a suspended floor and folding doors that open nearly the full width. The intention is to step out of the extension onto the patio. The DPC is 155mm below the finished internal floor level but is low relative to the garden (almost same height).
I realise that there is a requirement to set the patio 150mm below the DPC but this is just not practical (would undermine neighbours gardens/side wall as well). I have looked over your DPC page and thought that the linear drain solution could work but if I position this 75mm below the DPC the step out of the extension to the patio would be 230mm - with 2 little ones running around this is a bit too much of a step. I want to avoid a gully as think this would be dangerous for the kids.
Can you position the linear drain at the DPC level and then treat the brickwork above to get around the splashback? Any other solutions?
Also, I was intending to connect the linear drain to the rainwater pipe coming from the extension roof and main roof which runs to a soakaway at end of the garden - is this OK? concern is that in heavy rain could overload and then water flows back into the linear drain and floods patio?
Any advice would be greatly appreciated.

[matt h]

seems you are between the devil and the dark place. if you dont dig out you will get water ingress and it will rot your suspended timber floor. i advise that you dig out the entire back and do the job properly. i had to underpin a neighbouring property when doin similar job for client as their extension had been built straight off the ground, and this was only found out after we started excavations in the end both parties benefited as the properties were corectly drained and supported with proper foundations . your soakaway may not be big enough to cope with the patio drainage as well. suggest you read main site reference sizing etc before proceeding, as you may have to fit new soakaway.

[Rich H]

Put in a step. Build the back of the riser away from the wall with plenty of clear air down to 75mm below the DPC. If the tread is deep it will enable you can use large slabs (90cm) to overhang the space at the back and set them slightly clear of the sill and with a fall away. Put in one brick on end underneath the overhang of each slab for safety. Before building the step, install a linear drain 75mm below and connect to the soakaway. This will take naff all water but will give you peace of mind.