[Oddjober]

I am building a 8x12 shed and would like to add a redwood or trex deck to the top. I was planning to just have oversized roof joist and then add plywood with some type of water proofing and attach decking material to the roof joists.
Can someone point me to the proper method to waterproof this and what would be a way to make sure that all the decking fasteners through roof into joists don't cause leaks?

 

[joecaption]

I'm going to wait until a few more chime in, because I can not think of a single reason why any of this is a good idea for many reasons?
Why not make it a two story shed and cantilever a deck off of it and if done right would give you even more storage under the deck?

 

[Nealtw]

You are going about it backwards, desigh your deck with a water collection system and then plan to close that up for a shed below.

 

[3onthetree]

You do not want to fasten the deck through the roof membrane. You would build a platform (or 'checkerboard' of little platforms) to rest on top of it, and depending on the type of membrane and platform material the roof may need pads or standoffs. The roof needs sloped with water drainage in mind. Any guardrail posts would need those penetrations sealed though (since usually a platform does not provide enough meat to attach them to), so how you do these might help dictate how your structure is designed and what type of roof you use.

Is this shed cut into a hill or against a house or something?