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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorPord
    • CommentTimeOct 22nd 2018
    Afternoon all

    We're about to DIY build a strawbale house in Scotland and have one final matter to agree with Building Control - treatment of the solum - before we obtain Building Warrant.

    Straw Works have been pushing for a free draining solum of compacted hardcore under the suspended insulated timber floor. However, BC want either a dpm treated with bitumen on 100mm hardcore, or 50mm concrete on top of a dpm, or 100mm concrete.

    We're trying to keep the build as uncomplicated, affordable and sustainable as possible, so are still leaning towards the option of treated dpm on 100mm hardcore. Has anyone any experience of this? How technical/affordable a process is it?

    Muchos ta in advance for any input.
    • CommentTimeOct 22nd 2018
    Is this a Scottish Building regs matter? Page 10G appears to match exactly what BC are asking for, so you may have a hard job persuading them of anything different. What reasons do Straw Works give for variation and in particular, what evidence do they have in Scotland that what they suggest is a reasonable alternative?
    • CommentAuthorPord
    • CommentTimeOct 22nd 2018
    Yes, it is a Scottish Building Regs matter in as much as it's the Building Control officer who's asking for it. The thinking behind the harcdore alone is to make the solum free draining. SW have apparently successfully used this approach on numerous similar builds in England.

    Where would I find page 10G?
    • CommentTimeOct 22nd 2018 edited
    I found it at <http://www.south-ayrshire.gov.uk/buildingstandards/standards/part g.pdf>

    I believe from some of the things said in that document that the Scots believe conditions in Scotland are different to those in England* and so the regs are different. Hence why I asked about evidence in Scotland.

    * and I suspect they're right! :bigsmile:

    PS note that the forum software has mangled the URL. It does have a space in it.

    edit: Also, what are your foundations?
    • CommentAuthorPord
    • CommentTimeOct 22nd 2018
    Ah, ok. I'm familiar with those guidelines. My question is specifically how best to treat the dpm in example 3.5 Suspended timber floors (the one with the dpm on top of 100mm hardcore). Our BCO says it has to have some sort of bitumen finish to comply with section 3 CP102: 1973.

    The foundations are rammed (pea shingle) tyres.
    • CommentTimeOct 23rd 2018
    Sorry, I have no idea what CP102: 1973 says. It seems crazy to me that we're required to pay to see rules we're required to obey. :angry:
    • CommentAuthorskyewright
    • CommentTimeOct 23rd 2018
    Posted By: djh
    I believe from some of the things said in that document that the Scots believe conditions in Scotland are different to those in England* and so the regs are different. Hence why I asked about evidence in Scotland.

    * and I suspect they're right!

    I recall reports of housing stock surveys that listed lots of cases damp solums, some even had water running across the surface!
    • CommentAuthorRousay2
    • CommentTimeDec 16th 2021
    Hi Pord.

    Did you resolve this issue?
    We are in a similar situation with our build in Orkney.
    • CommentTimeDec 16th 2021
    One point to bear in mind is that I think DPMs are sometimes as much a part of radon defence as moisture defence, so an open solution is unlikely to be satisfactory in such places.
    • CommentAuthorGreenPaddy
    • CommentTimeDec 17th 2021
    DJH is quite right, re. radon or other ground gases. There are radon maps for the UK (noted in the Scot Buiding Regs 3.2.1), and that will dictate whether or not you need to uprate the DPM to a radon barrier. The BSO will have stated that in his/her response to your warrant application. It's not much more costly, so I always just install it anyway. There are different levels of radon intensity, so some times radon sumps and extraction fans are required. Not had that situation myself, thank goodness.

    As to damp in solums, the warrant requirements are quite clear, section 3.4.4, and the BSO has to follow those. If you wish to deviate, you'll need to provide proven/tested evidence for the performace of your alternative.

    The 50mm concrete over the DPM could be a fairly week concrete, with recycled materials, as it's only to hold the DPM in place, and probably stop rodents burrowing through (still will if determined enough). There's no actual spec for the concrete.
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