exposing cavities

Discussion in 'Builders' Talk' started by The South, Aug 4, 2008.

  1. The South

    The South Member

    when you build an extension, and a previously outside wall now becomes an inside wall, I was under the impression that you needed to fully expose (i.e. break/cut through) the cavity of the existing wall to ensure damp couldn't pass

    is this true or can you just make a cut a few inches deap and insert a vertical DPC?
     
  2. building control

    building control New Member

    Often its the easiest way to cut and insert a dpc
     
  3. ecm

    ecm New Member

    I'm not quite sure I know what you mean South. Is this the wall that you've created an opening in through to your new extension? So in effect you've now got an internal wall (which was external) and an open cavity where you've cut the opening?
     
  4. The South

    The South Member

    a new cavity wall is being built out from a corner of my existing house. it means one outside wall will now be an inside wall of a new room in my extension.

    I know when some other builders did the brickwork for my porch extension they cut through the cavity on the existing house meaning the outer skin of the existing house had a gap between what will still be the outside wall and what was then the inside wall of the new porch

    I'm just wondering if you need to fully expose the cavity, meaning there is no longer any physical contact between the outside wall and the inside wall, or if you can just cut a slot in, say, 2 inches deep, and insert a vertical DPC.

    guess I'm not explaining it very well, probably the beer ;)
     
  5. Rizzo

    Rizzo Member

    You dont need to remove the brickwork but you do need to cut right through the brick and insert a vertical dpc. Cutting in say 2" does nothing at all, damp can still travel through the brickwork.
     
  6. The South

    The South Member

    yep, thought so. ta.
     
  7. Major Mal Funxion

    Major Mal Funxion New Member

    South

    The way i'm reading this, you can cut in a vertical damp, but if that gives issues on dust created by the cutting of such a slot , to insert a break, on the now internal building.. you could just get a breaker on it, and create a cavity, as you 1st assumed.
     
  8. Astravan Man

    Astravan Man New Member

    South,

    you either open up the cavity as said by others.

    or you get a disc cutter and cut a slot all the way up, and all the way through the wall and slide a bit of 150mm wide dpc though it.

    This stops any moisture being transferd from the old external wall through to the new internal wall.

    If you cut a 2" slot like you said the damp can just travel behind the 2" slot and into the new room.

    So you MUST cut a slot through the whole wall, the FULL depth, this may mean needing a disc cutter with a 14" blde to get the depth.

    Hope that helps / makes sense.
     
  9. Mr GrimNasty

    Mr GrimNasty Active Member

    I know it's what you're supposed to do - but it must weaken the structure of the building to a degree?
     
  10. The Dormouse

    The Dormouse New Member

    I tried to think of an excuse to bring up 'mooning' under this heading but I gave up.
     
  11. gangman

    gangman New Member

    As colt says, cutting through the brickwork can weaken the wall. You're creating two stop ends in the wall, and you can bet there won't be the right amount of ties for a stop end. There should be a tie every 300mm vertically.

    So you have to rely on the bonding of the new walls, depending on the method, have you got the same strength you had before the wall was cut. You could always add remedial ties either side of the cut, or even not cut it at all.

    If you didn't cut it what would happen. You keep the strength in the original wall but you have the potential for damp to travel across the original outside skin to where the wall has now become an inside wall. But what if the original wall was 9in solid. You wouldn't cut all the way through that, so there is then a bridge between outside and inside.

    Sticking with a solid 9in wall if damp is not already crossing 9in to the inside of the original wall why would it travel a further distance, as in two 4in skins and a cavity.

    So is there a need to put a cut in, will damp definately show up on the inside of the wall.

    All I say is I personally put a cut and slip in some DPC, but coming from Wales I've seen how damp gets in.
     
  12. Major Mal Funxion

    Major Mal Funxion New Member

    Make that every 225 and gangman speaks the truth.
     
  13. building control

    building control New Member

    I know it's what you're supposed to do - but it must
    weaken the structure of the building to a degree?

    no more that a reveal.
     
  14. Mr GrimNasty

    Mr GrimNasty Active Member

    Yes - a lot more than a reveal - dear Gawd - no wonder the Daily Mail is full of collapsed houses (after the owners were poorly instructed by BCOs).

    (Yer slight dramatic exaggeration - but it has happened).
     

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