Damp-proofing under an external stud wall?


Help Support UKworkshop.co.uk:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.


26 Jul 2020
Reaction score

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 have 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 the decking (with the DPM between them and the decking) and fix them to the sole plate of the stud 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.

Use a double sole plate and set out the bottom one as the layout for the size of the shed/room and put DPC on top before putting the walls on top.Will be fine to screw though the DPC to secure the walls in place. Rainwater you can catch in drums with a tap at the bottom to recycle it in the garden.
Hmmm...that's horrible.

Wood-on-wood with a piece of plastic between. I'm really not sure what that is supposed to achieve.

Don't screw through a DPC. Use galv MS straps to hold the plate down.