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Damp Proof Course - Slab / Raft Foundation

Discussion in 'Building' started by UKG, 27 Aug 2015.

  1. UKG


    14 Aug 2015
    Thanks Received:
    United Kingdom
    Hi all,

    I'm currently designing an outbuilding - 6.3 m x 7.1 m.

    I've made the decision to use a slab (sometimes called raft) foundation.

    I'm having difficulty deciding on the best way to DPC the slab. Can anyone offer any advice?

    I'll be using 4 inches of aggregate throughout the slab and 6 inches of concrete.

    Some designs seem to have the DPC ontop of the aggregate and others have a floor screed ontop with a DPC under the floor scree (so much higher). Can anyone offer advice on protecting the edges of the slab? And how to connect the DPC in the black work to the DCP under the slab?

    I've attached a cople of images as examples of the two ways that Ive found.

    UKG Damp course 2.jpg DCP under screed floor.jpg
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  3. noseall


    2 Feb 2006
    Thanks Received:
    United Kingdom
    The height of the DPC is dictated by floor and ground levels. Your design must make sense of this and be applicable to your own scenario.
    It's no good looking at various designs hoping one may suit, rather the position of the DPC should be factored into your own design.
    A raft usually sits below ground with the masonry taking the building up to ground level. The oversite is then constructed internally along with polythene DPM, insulation and floor screed etc. The DPM is tied into the internal leaf of masonry.

    You must design out the (damp/thermal) vulnerabilities associated with raft type foundations as they can be a headache. This is why first choice is always strip footings. It's no good wearing your designer hat if you can't design. I'd try something simpler than a raft if you are struggling.
  4. theprinceofdarkness


    25 Feb 2005
    Thanks Received:
    United Kingdom
    I don't know where you got the first arrangement from, it seems to have loads of problems. Have you ever tried to drill a wall plate so it sits flat over reinforcing rods sticking out of concrete?. Have you ever tried to bend reinforcing rod to a tight radius? The front edge of the "dry" screed is exposed to rain.
    The second arrangement is OK , but the screed DPC should be folded up against the wall.
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