Retrospective installation of stepped and raised dpm for flush patio

Discussion in 'Building' started by r1cho, 8 May 2021.

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  1. r1cho

    r1cho

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    Hi there, hoping someone can assist with this particular headache!

    We had an extension built (now 2 years ago) and are currently in process of adding a patio. The key design feature of the extension was to have a flush through floor in and out of the large patio sliding doors. Now we are half way through the patio, my (new) builder has pointed out that the DPM level on the extension means that whilst the patio level will obviously be above the DPM in front of the doors (we intend to use brick slot drains in front of all windows) the DPM in the bricks either side of the windows will therefore be lower than the patio surface since the DPM has been installed with only a minor (cm or two) step up from the under-window DPM level. To be fair, I can't see how the original builder could have done this differently, as if he had stepped the internal and external leaf DPMs up around the windows then the internal DPMs would be significantly above finished floor level, which is also a no-no I believe?

    As a result of this, builder (number 2!) has suggested that we leave a gap (I'm thinking match the brick slot) and fill with gravel, presumably to a level below the DPM (although filling the hole level would look much better IMHO, but would bridge the DPM with gravel and therefore I assume a bad idea?). I will of course make sure he puts in some formal drainage into this channel too so that all water gets whisked away rapidly. The patio will of course fall away from the brick slot, with the small infill section in front of the windows falling towards to brick slot.

    I have seen a number of references on the web to people using such a solution, and flush in-out patios are all over the home magazines/sites, so this must be possible. I just feel uneasy and worry about the effect when selling the house eventually.

    So, what is the best practice here with a through floor arrangement - I don't want to introduce damp to our new extension of course. Is it possible/advisable to neatly install a stepped DPM retrospectively to the outer leaf to raise the level (Debeers method I read somewhere?). It is only for Id say a max of 3m of brickwork on the three pillars that separate the patio doors and adjoin the patio surface? Does the resultant difference in inner and outer leaf DPM level (I would also leave the original in situ) cause an issue? Is there a better way to achieving this effect? Grateful for any steers on this...
     
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  3. r1cho

    r1cho

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    Thinking a bit more about this, would it be acceptable to insert an aluminium shroud between the wall and the gravel in the French drain extending up to one/two bricks above the DPM? If it had some spacers attached to the back there would be zero risk of bridging the DPM and also the first two bricks above DPM would be protected from any risk of splashing. There would be ventilation of the very small gap from shroud to brick and it would all be removable for cleaning, inspection etc. If in a RAL powdercoat then could also match the windows. Any thoughts on this as a potentially easier solution to installing a higher DPM? Thanks.
     
  4. ^woody^

    ^woody^

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    Capillary action may have something to say about that idea.

    The other proposed solutions are for different problems - drainage.
    Your problem appears to be rain splashing above the DPC, but it's not clear from that mass of text. Perhaps a single photo would explain things more succinctly?
     
  5. r1cho

    r1cho

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    Thanks Woody - sorry, I’m always a bit too verbose! Here are some photos, hopefully self explanatory -grateful for any ideas/thoughts!
     
  6. r1cho

    r1cho

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    Was hoping we could do a detail similar to this...
     
  7. ^woody^

    ^woody^

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    The surface should be kept at least flush with the DPC - the top of the course not the joint.

    There is no need for a gravel or drainage channel unless the patio is sloping back to the house

    The problem is one of splashing above the DPC, and whilst a cavity wall should prevent dampness reaching the internal leaf, if the cavity is already breached for any reason it is a risk. To deal with this, the wall would be coated with a silane-based clear water resistant coating for about 300mm above the DPC.

    The best thing to do with those vents is to raise them and fit telescopic "periscope" liners - they may well be already fitted and will just need extending. Its a bit of a crazy idea to try and vent them under a patio.

    You don't have a level access cill on those doors, so unless you can carefully detail something, you need to be careful not to create a route for damp across the threshold.
     
  8. r1cho

    r1cho

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    Thanks Woody,

    - level will be approx the same as shown in photo and patio slopes away from house..
    - coating sounds like a plan. I may still do a trap as it looks quite nice and having tiles that close to DPM makes me nervous! I can drop the gravel a little to ensure no contact with DPM.
    - yes will need a good detail above/around cills- if anyone has any ideas grateful received!

    Many thanks,

    Rich
     
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