Hi All, Any advice on calculating this correctly would be of great help. I have a space of land that is 31.4ft(l) by 9 ft (w). I want to patio this with slabs and have been trying (and failing) to figure out the correct fall needed from the top to the bottom for drainage. I have read that the ratio is 1:80, with my calculation coming out at 11 inches from top to bottom. I am using the following patio slabs: http://www.stonemarket.co.uk/product-bourton-riven-garden-paving Any help calculating the fall is really appreciated!