Garage conversion floor and 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 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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Thanks. I suppose I can put in some air bricks top and bottom and/or weep holes?

Which DPM arrangement is best?
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Being curious, would it be better as a cavity and vented or tight to brick skin without ventilation?
There is no need for a cavity. Line the wall with a DPM, build the frame tight to the wall, full fill insulation. No need for sheathing either, secure the frame to the roof/ceiling joists
Thanks. I suppose I can put in some air bricks top and bottom and/or weep holes?

Which DPM arrangement is best?
DPM across the floor and up the wall no cavity. Build the frame either off the floor or off the floor (if the floor is taken up to the existing wall) no two course of bricks required
Thanks Woody. That’s interesting that I don’t need a cavity… the first detail I ever drew was that of DPM across the floor and up the walls, then batten and hang insulated boards. Everyone I spoke to suggested that I needed a cavity because the walls do get damp (there is no render on the outside). Original thinking was like this detail from Kingspan FYI…


A cavity does mean I can get around the piers to create a flush wall but obviously it’s all becoming more costly.
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A DPM up the wall does the same thing as a DPM across the floor - keeps damp out. So it won't matter in the wall does get damp.

Internal piers are always problematic. Lining the pier with the thinnest PIR board (12mm) is normally enough to prevent pattern staining from the cold pier, but obviously that still leaves a section of the wall that is less insulated.
If you didn't want to stud it... would it be acceptable to build a second skin in lightweight block off the current slab, (no cavity) then celotex tight to this skin?

Was thinking you can make the wall flush to the piers, and not have any worry about 'wood'

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