Garage conversion floor / wall detail

10 Jul 2022
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United Kingdom
I am designing my detached garage conversion, it’s a single skin brick building on a solid raft slab.

I’ve had a look on multiple previous posts and cannot find a specific answer. I’ve post here and in the ‘building’ forum, not sure which is best.

I am need of advice at the wall/floor junction to ensure I do not get damp issues. My design is largely finalised, a timber stud wall offset from the brick by 50mm, with PIR between and plaster boards over the top over the vapour barrier. My confusion is the arrangement of DPM/DPC and the breather membrane.

I understand that I am supposed to lap the breather membrane behind the DPM and/or form a cavity tray.

The detail below is the closet to my proposed arrangement though my sole plate will be on one course of bricks and not two but it doesn’t change the question at all.

The ground level is c. 150mm below the top of the concrete slab.

I have two potential solutions in mind, which is best?


1) DPM laid under floor insulation and up over sole plate bricks (shown pink), meaning I can lap the breather membrane (shown blue) on the proper side. Then vapour barrier (shown green) to run on inside face of stud and above insulation on floor. Red is an existing DPC.


2) DPM run across entire floor and lapped up the wall (again shown pink), this was originally preferred as in heavy rain there can sometimes be moisture in the brick/concrete connection. My challenge (I think) is that the breather membrane (blue) is now going to land any collected moisture on top of the DPM with no way to escape? For good measure I was going to throw DPC under the timber sole plate.

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