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Low Screed Depth over Underfloor Vents at Cavity Wall Gap

Discussion in 'Building' started by BLT2016, 10 Apr 2016.

  1. BLT2016


    10 Apr 2016
    Thanks Received:
    United Kingdom
    I have an opening between 2 internal rooms. One is now extension so old external cavity wall is now internal wall. I want to get this area ready so I can screed the extension room and finish at same level as floorboards in old part of house.

    I have 2 problems:

    1. I have underfloor ventilators protruding up from concrete slab and I need to extend the ducting across this cavity area to the floorboard side and at same time have enough screed depth over vents and ducting so no weak spots (i.e. if I step on vent or an extension sleeve it does not break). I can get about 18-23mm screed depth over my first vent (LHS of room), and about 25-30mm over 2nd one (RHS). These u/floor vents have a lip on the back of them hence the difference.

    2. I have 2 plastic pipes from heating 1st fix taking these radiator pipes from extension to other side of cavity. How do I lay the DPM around the pipes at the cavity?

    A very similar post in 2011 (RichDarbey) and answered by Noseall did not have a screed depth over duct issue- his duct pipe looked lower in his photos so that's the difference with my thread.

    What has been done: course of bricks taken out to level area ready for DPM so can get good depth of 3:1 cement/sand mix over the cavity. Cavity filled. Going by past advice, plan to screw tanalised 4x2 to the old joist to cover void under f/boards and it will sit on internal cavity skin with DPC placed under it unless someone suggests otherwise.

    Could use timloc horizontal extension sleeve from vent to joists but as its thin plastic with thin layer of screed over it, you won't be able to stand on it. The vent itself is not much stronger than their sleeve so problem is screeding depth over that also. Maybe use their vent duct adaptor (part 1205) into the vent, fit 110mm pipe which would take weight and take this over cavity, then try to get as much screed over this levelling with f/boards??

    Any solutions/suggestions most appreciated. Many thanks.

    Attached Files:

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  3. noseall


    2 Feb 2006
    Thanks Received:
    United Kingdom
    The existing floor void should have been vented lower down and at an earlier stage before over-site installation.
    We knock a hole through and into the existing floor void, run a 110mm pipe from the existing floor void through to the new outside wall, then telescopically vent the new duct out through the new wall.

    One option is to extend the timber part of the floor so that it crosses over the thickness of the cavity wall and finishes flush with what was the old outside wall. This will mean several short 'joists' (timber battens) across the old cavity wall ensuring timber floor joists will be covering the vent duct rather than thin screed.

    The other option is to remove more of the masonry beneath the opening and drop the duct down, re-polythene, insulate, concrete and screed up to the existing joist/floorboard etc. Looks the more sensible option as those vents are doing nowt in their current position.
    Last edited: 12 Apr 2016
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