Decking round summerhouse

Planning on some decking round the front and both sides of our summerhouse. Only about 3 boards deep. The summerhouse sits on a 100mm timber base. Then the base of the summerhouse is 40mm tall and wrapped in a damp proof membrane. So, a couple of questions for anyone with decking experience:

1. How much gap should there be between the joists that run parallel to the summerhouse and the summerhouse?
2. Is there anything wrong with having the decking boards higher than the damp proof membrane?
3. Do i need to fix the decking joists to the summerhouse base? I’ll be using adjustable footing to level it all.

I always take my plans to a local timber merchant (not a wickes or whatever) the guys there help me figure out this stuff to get the order correct. I’m lucky as it’s at the top of the road.