Where (and if) to place dpc

[barryhince]

Good evening to you all. I am new here and as wondering if any of you could settle an argument for me.

The pic shows a very basic monolithic raft foundation. (Steel and thickness's have been omitted from diagram). The concrete is poured on top of DPM that laps up the sides to the upper level of the concrete. When building off this slab, where would you place the DPC? I am not convinced one is required - given that the concrete is lapped in dpm. My old man believes it should go between courses one and two, my son reckons it should go between the slab and the first course.

(4" concrete blocks, single skin detached store room, rendered outside)

 

[NS215]

It most certainly needs one, If I was doing the job then I'd run 2 course of brick round first then put the dpc on that, a dpc straight on top of the concrete is a total waste of time externally but normal practice internally, you could put the dpc on top of the first block but that would mean putting the bell bead at that height as any lower the dpc would be bridged and a waste of time again

 

[tony1851]

As above really.
It does need to be at least 6" above ground level to avoid splashing; and if you're setting the wall in a little from the edge of the slab, then you'll get damp at the edge of the floor whatever.

 

[barryhince]

Thanks chaps -

The wall will be set in slightly from the edge of the raft. Would a few coats of synthaprufe applied to the first course lapped to the edge of the slab prevent ingress of water at the slab/wall junction? (It wont be seen)

 

[NS215]

It certainly wouldn't do it any harm and maybe a little flaunch to help any moisture run away to the edge of the concrete

 

[Blagard]

If it not too late, the design can be improved substantially by incorporating a step down in the raft edge where the walls go. This provides much better protection to the slab from ground water seeping directly through than a level slab. Not to mention it looks neater externally. DPC would then simply be 150mm above ground level

 

[NS215]

^^^^^^^
agreed