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Extension DPC, how to join to existing

Discussion in 'Building' started by schiz, 12 Mar 2008.

  1. schiz

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    Hi all,

    Just wanted to get some ideas about how best to tackle DPC placement for my extension.

    Here is what it looks like now. The wall on the right, and about 20cm of the one on the left will be demolished (it's a single-storey), and the internal opening will be closed off.
    [​IMG]

    And here is the plan for the extension. The extension has a step-in of about 20cm, and closes off the internal opening, with a short wall extending to take the lintel for the new outside door.
    [​IMG]

    I'm a bit unsure as to the best order of doing things, but decided to block up the interior opening first just so the place is secure before I demolish the wall on the right and start on the extension foundations. Seems I should feed in the start of the DPC part-way under the opening I'm bricking up since it's ending up as an external wall from POV of BCO.

    The problem is that the existing DPC is about 10mm below the pavement outside, so I have to decide whether to somehow bring it up for the extension or lower the ground outside it. The opening to be bricked up has a *very* solid piece of concrete that looks a) Hard to break up, and b) an excellent base for the blockwork, I've dug down in the room to the left about 70cm and it's all bricks and concrete underneath, so I wondered if really should break it all up just for the sake of inserting DPC at the same level as the exterior wall.

    Any advice on what I can get away with would be great. I get the feeling if I ask BCO they will tell me I have to do this the hardest way possible. I prefer to take a bit of a risk not because I'm lazy, but because I don't want to weaken the foundations unecessarily just for the sake of compliance on some minor issue. Sorry I've rambled on a bit.

    thanks for any help.
     
  2. ^woody^

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    A DPC should be at floor level and also about 150mm above external ground level.

    The idea is that rain does not splash above the DPC. But if there is a cavity, even if rain does splash above it, then it should not cross to the inner skin.

    But, you wont convince a BCO this.

    If the DPC is near the ground, then you have to form a tray in the wall so that the internal skin has a DPC at floor level and then a second one higher up which angles down to the external wall - a cavity tray

    [​IMG]

    This sort of arrangement - but remember the need for the 'normal' DPC at floor level on the inside skin

    This is formed in a wide roll of DPC, you don't have to buy a purpose made one
     
  3. schiz

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    Thanks for the response woody, but informative though it is it doesn't have much relevance to my problem. If I was building in isolation, I wouldn't have a problem. I know the regulations, so I would just follow them - 150mm is fine - either I create a trough, along the side of the house, or I raise the floor a bit.

    My problem (perhaps badly expressed), is that I am bricking up an opening, and I've got a step-in for the new extension, the step-in will expose the 'end' of the exterior wall. BCO cannot complain about my old DPC, but this is not just a question of butting up against the old DPC, but butting up against a previously interior wall. My suspicion is that the slate runs under the interior doorstep anyhow, because the existing extension to be demolished (on the right) is only a single storey, and more like a lean-to than part of the house, so I think I won't have a problem with damp. Obviously I am going to have to put a DPC on the 12 cm or so of new wall left of the exterior door, but it will be hard for BCO to confirm that this meets with the slate underground (if there is any). They will not be able to easily see any join exists because of the step-in.

    I'm also confused about what constitues part of the new build and the old.

    An example: If I was to remove a porch, thus making an interior door an exterior door, would I then be forced to insert a DPC under the interior door if there wasn't already one?
     
  4. Joe Malone

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    I've read through your problem twice and I have to be honest and say it's very difficult to understand. Reading what response you've had also makes me think other contributors are finding the problem difficult to understand. It would really help if you identified the heights of internal finished floor level in relation to external ground levels. Also, what are you doing with the floor in the extension? Is it a concrete slab? Are you grubbing it up for replacement? Are any internal floor slabs original or are they retrofit? Is there an existing DPM?
    How old is the existing building? Is it cavity or solid construction?
    Is there a difference in height between the two internal finished floor levels?
    If you can answer these questions it might give me a better idea of what you're trying to do and what you need to do.
     
  5. schiz

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    First - thank you for taking the time to read my post twice!

    The property is victorian, internal floor level is aprixmately external ground level. Existing DPC would not therefore pass BCO today. In many places it's been bridged. There is a small existing single-floor 'extension' to be pulled down, floor of which appears to be tiles (rest of house is wood floor), no idea what is beneath tiles, but i plan to pull tiles up for the new extension so I can put DPM at the appropriate level (it will have a concrete floor). There is no existing DPM as far as I can tell. The slab I was referring to is only one - it's the internal 'doorstep'. All internal floor levels are currently (and if poss will stay) the same. Solid/Cavity walls are as indicated on the diagram, so basically all old walls are solid, all new will be cavity with the exception of a short length of wall forming a pier for the external door lintel.

    If you have any patience left for me I can try to draw an isometric view or take some pics. Sorry for my explanations, however it is a bit complicated, and that's why I'm asking the forum. If it was simple I'd probably figure out what to do myself :(.
     
  6. ^woody^

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    You only read it twice , Joe? :eek:
    Its actually etched on my brain from over-reading and I see it as flash-backs like some horrible past nightmare event.

    From the new info (and I don't know if I have read correctly) it seems that the OP is concerned about making existing wall/floor conform as part of the new work?

    If that is so, then the answer is that the BCO only has the authority to check the new work, and not any existing construction. The BCO can't insist on 'upgrading' the existing
     
  7. Joe Malone

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    OK, the difficulty you'll have is in joining the new dpm to the dpc in the existing dividing wall. Personally I'd install a timber floating floor in the new extension to match what you have in the rest of the property; this is the only way to ensure you don't have damp problems in the existing dividing wall. The BRE have produced guidance on replacing suspended timber floors with concrete and this will detail how you should go about lapping in the new dpm to the existing dpc. I know that technically you're not replacing a timber floor but the same problem exists because you need to form that dpm/dpc connection in the dividing wall. Perfect lapping or linkage of the dpm/dpc is really only possible in new construction.
    You already know that you're external ground levels are far too high so you need to lower external ground levels or form a french drain round the property perimeter that is at least 150mm deeper than the dpc level.
    My guess is that the ventilation for the existing timber floor has already been compromised due to raised ground levels. If this is the case then lowering external ground levels should expose the airbricks. If this isn't possible then consider installing ducted air vents to protect the timbers in the existing build. Are there any air bricks on the right hand side wall of the single story extension? If so these may well be piped to the base of the dividing wall to provide additional ventilation to the sub floor of the left hand room in your diagram. I've seen this many times and people tend to dispose of or smash the pipework when installing retrofit concrete floors. Bear this in mind and don't remove subfloor ventilation during works, it's very important that it stays.
    Woodys right about the BCO's responsibilities but you should be aware of the potential problems that I've mentioned.
     
  8. schiz

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    I would love to install a timber floor in the new extension, however it's a garage, so presumably that's out. The existing property doesn't seem to suffer from damp. I think this might be because most air bricks are exposed. the DPC is just bridged in many places away from the air bricks. I was quite surprised.

    Regarding BRE, do you mean this: http://www.brebookshop.com/details.jsp?id=961
    £9 for 4 pages? Good work, if you can get it :).
     
  9. Joe Malone

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    Actually I'm not sure that's the one. Make sure your e-mail address is made visible on the site and I'll dig out an electronic copy and send you it. I know I've got it, just not sure where at the moment.
     

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