The oversite must be free draining or of a gradient of 1 in 40 to allow water "run off". If the area you have chosen is grass you can either remove the turf or cover it with a weed barrier, as long as its edges are tucked deep into the soil using an edging spade. If your deck is to be at ground level then the removal of 100mm of soil is recommended. Replace the topsoil with 100mm of compacted gravel or hardcore. This provides a very solid but free draining oversite on to which you can build the sub-frame. On any over site it is essential that you lay a weed barrier. It is advisable to cover the top of the weed barrier with a light stopping layer of pea shingle. You could use strategically positioned patio slabs instead of compacted gravel or hardcore, but we do not recommended this as they tend to settle/subside causing problems with your deck later.