Garden office in timber stud. Doubt on DPM membrane

[Jonny Vee]

All,

I`m planning to build a garden office on the back of my garden.

Idea is to pour a slab of 150mm and then build the office on top. The office will only cover half size of the slab and the rest will be slab paved.

Do I need to install a DPM membrane before the concrete slab?

How would you protect the stud wall from Dump? Do I need to lay over or 2 course of blockwork and then stud wall or can install the timber to the slab? I done a lot of DYI but never done one of this and need some guidance.

I`m also worried that timber on the slab will rotten as no protection from water on the slab.

Thanks

 

[mrrusty]

The office will only cover half size of the slab and the rest will be slab paved.

Don't do it like that - water will run in from on top of the slab in to the room. DPM under the slab. Slab same size as the outer dims of the stud walls. Frame on the edge of the slab separated with DPM, cladding down over the edge of the slab so run off is lower than the top of the slab. Or dwarf walls on a strip foundation.

 

[23vc]

I did a slab with a course of blue bricks on top. Then timber frame on top of the brick. Keeps the timber frame off the ground a bit, looks good and internal floating floor on insulation so no cold bridging.

 

[Deluks]

As above.
No dpm under slab, 2 brick course under the stud wall, roll of dpc in between.
Sit the dpm inside the bricks, run up against the wall to meet the dpc. You'll need a threshold for the door.

 

[mrrusty]

If you want a slab/raft foundation a dpm is helpful under the raft see here

Lots of different methods of making the wall/slab joint, but just standing a building on a slab which is exposed to weather is going to result in water passing under the wall across the top of the slab, so the design must take this in to account. My most recent raft build used a stepped raft as the linked illustration.