Hi, I need to build a soakaway for our extension/rebuild. I am unsure how big it needs to be. Our roof area at the front is approximately 41.5 sq mtr. I can't seem to quite understand the calculation. Do I just divide by 40 which would mean the soakaway needed is just over 1 cubic metre below the inlet pipe? Can the hole be filled with rubble? This would reduce the volume etc... I will buy some of those plastic crates (aquacell) and fill the hole with those to maintain the volume if that is what is necessary. But they are quite expensive Can anyone set me straight on this. It seems to me that the days of a little hole filled with rubble are gone, if you need to meet the building specs. Please advise. Thanks, Bobby P.S. - (I will need to do something similar for the back of the house, in fact a bit bigger as the rear roof area is about 55 sq mtr).