i am building a deck out over sloping garden. want to build storage room under deck and thought i would put in retaining wall and 4metres another and run block and beam over top. ive googled and people talk about building normal joists on top, however we want to make storage as high as possible so how to minimise depth,could i build up 50mm wide 30mm concrete joists on the screed over blocks and screw boards down through into block?or any solutions?