Sealing the foundation for a garden shed

Hello, I want to set up a wooden garden shed (3x3m, 44mm, insulated glass) for a few fitness machines and would like to use an old terrace bordered with edge stones and concrete slabs (40x40x4) for this. To protect the garden shed from rising moisture (e.g. through the joints), I thought about laying bitumen waterproofing membrane over the entire area. What do you think about that or how would you waterproof the substructure?

Regards Jochen

I would either make a clean break, i.e. substructure + floor. Or not seal anything at all. Where should any significant moisture come through the joints from?

The substructure and floor are of course included with the garden house. My concern is to protect the wood, especially the substructure that rests directly on the base, from rising moisture.

The substructure is usually made of KDI, which can withstand quite a lot. Additionally, my garden house company recommended underpinning between the substructure and foundation with pieces of roofing felt about 5mm thick. This way, there is no standing water at the edge. Nothing goes under the garden house anyway.

Are you proficient in English? If yes, there is a channel on Youtube of the English company Oakwood Garden Rooms, which does really great projects in the UK and shows from the very beginning how they make the foundations, waterproofing, roof constructions, vapor barrier + brake, install glass folding walls, etc. Totally cool!

The substructure is usually made of KDI, which already withstands quite a lot. Additionally, my garden house company recommended placing pieces of roofing felt about 5mm high between the substructure and the foundation. This way, there is no standing water at the edge. Nothing goes under the garden house anyway.

How have you secured the garden house against storms? We also have a slab foundation, and since the edge stones are only 6cm wide, I do not want to drill into them from above. Therefore, I want to screw the substructure to the edge stones with flat iron.