Damp proofing a garden room

[DC1]

Hi,

I'm new on here and was hoping someone could give me some advice about how to damp proof a garden room (approx 3m x 4m) I'm looking to build on some newly installed timber decking. The decking is made from scaffold boards and they seem securely anchored to the ground. The room should be weatherproofed enough to make a comfortable office space in all seasons. I was thinking I'd to get professional roofers to do the roof.

I was wondering if anyone had any advice about damp proofing the lower structure and how I might go about dealing with rainwater at the base?

I'm intending to use 4x4 CLS tantalised stud work for walls, roof and floor. I had thought I would first construct the stud walls flat on the ground. I would then lay out a sheet of DPM on the timber deck and then sit my stud walls on this and bolt them together. After this I would then lay my flooring timbers directly on top of the DPM and fix them to the walls. But....

Firstly, I'm unsure whether this alone will be sufficient to keep out the damp/rain ingress from the decking?

Secondly, how will I fix the sole plate of the stud wall to the timber deck without piercing the DPM?

Thirdly, would rainwater sitting on the deck adjacent to the structure create any problems?

Any advice would be greatly appreciated.

Many thanks.

DC