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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    I was going to post this request for advice a few days ago but when I came to the forum to do so I happened across a post that mentioned the Fischer Thermax product for stand-off installations so I immediately scurried off and did a bit more research.

    The problem: how best to fix one side of a deck (actually a low-level 800mm wide narrow walkway the other side of which is supported on low-rise concrete pillars) to an insulated concrete slab whilst minimising the cold-bridging effect as much as possible? The slab is the 'other side' of 200mm of XPS insulation protected by a 10mm concrete tile on the exterior surface.

    The slab itself is (coincidentally) also 200mm thick and is of strength RC40 with fibres and two layers of tied together A393 welded rebar mat - the relevance of this detail being related to the shear strength of the material being fixed into - IOW, it is more than up to the job. The question of shear strength was raised by Fischer UK Technical Support - of which more later.

    Having researched as much as I could on my own I thought it would be prudent to check in with Fischer to find out what the thermal conductivity of the Thermax system is and to get their opinion on my proposal to utilise Thermax 16 fixings (stainless steel variant for better thermal performance than the galvanised version) to support my narrow low-level walkway.

    I realised that having 210mm of stand-off is, in the words of Fischer technical Support "massive" but I fondly imagined in my naivety that my modest requirement to support a small domestic duty walkway would be eaten up by an M16 fixture without blinking. After all, the M16 Thermax variant is advertised as being able to handle a stand-off of 290mm when fixing into concrete, leaving me with plenty of headroom, or so I naively thought.

    As an electronics and computer engineer I've been happy never to have been too familiar with kNs down through the years and indeed last encountered them in anger over four decades ago whilst taking the compulsory Mechanics 101 course at university and this is another reason why I was keen to validate things with the very helpful Fischer technical advice department who promptly advised me that, in layman's terms, I could only support a load of 52kg per fixing at that distance of stand-off.

    This came as something of a shock as it means that I will need far more fixings than I had hoped to use in order to support my walkway and one thing these items are not is inexpensive! The other major concern being the multiplier on the heat loss implied by all the extra fixings as well. in fact, this latter reason is of more concern to me than the cost - blisteringly expensive though that would be.

    As I understand it I don't really have an option not to fix the walkway to the building via the slab and anyway even if I did I am most reluctant to go the 'free-standing' route as there is always movement over time that sees the two structures drift apart awkwardly.

    So before I put the dog, the cats, and all of the chickens up for sale to try and raise a bit of cash to pay for these super-premium fittings can anybody think of a better way of achieving this whilst retaining some semblance of a thermal break at the same time, because it would be a real shame to have gone to all the time and expense of arranging a PassivHaus slab only to leach out all the stored heat via the deck fixings?

    I'm thinking there must be loads of people out there with both insulated slabs and decks, so how did you do it... or are all of your slabs uninsulated direct on grade and the problem does not arise because your thermal barrier sits on top and so does not have to be bridged in this fashion?
    • CommentTimeJul 13th 2020
    Are you talking about an external walkway in the garden? Why does it need to connect to the slab? And especially why does it need to be supported by the slab? Is there a door opening onto this narrow walkway or something? (In which case I'd be aiming for a wider walkway if possible - 1 m absolute minimum, 1.2 m better minimum, 1.8 m comfortable)

    Why not support it underneath with low-rise concrete pillars, and then just tie it horizontally to the building if it needs to be constrained. PS Don't forget a drain slot between the door and the walkway.
    Excellent suggestion - thank you.

    Sometimes one just can't see the wood for the trees.

    Yes, it is an external walkway in the garden but it needs to run along the side of the building because space is constrained and some of the available space needs to be reserved for a line of special fruit trees (at least that is what 'da management' has informed me anyway) but also and mostly because the walkway will be the means of maintaining the building in terms of acting as a platform for step ladders and so forth in the future.

    I might be able to go to 1m and I will give it some more thought but I think I can make 800mm work as well if I need to. There won't be much traffic along this walkway anyway - just me and the occasional wheelbarrow on route to the compost heap to the rear really plus the even less occasional trip to maintain the rainwater recycling tanks which are also at the back there as well.

    Supporting with concrete and then just tying in will allow me to use much smaller (and much more reasonably priced!) fasteners for a considerably better thermal performance as well so even better.

    There are no door openings along the particular stretch of walkway that I am currently considering but the walkway will eventually be extended around almost the entire perimeter of the building and turn into a small deck at the front where there are two doors.

    Drainage has been accounted for but thanks for the reminder.

    My original thoughts on supporting it from the slab came about simply because that is the way that it is always (or at least usually anyway) done - especially in a simple domestic context and recent research (YouTube - so not exactly definitive, I know) showed everybody doing this but of course this is in the context of an uninsulated slab with an immediately available edge to fix into. Also, it just seemed a more 'elegant' solution somehow (not withstanding the killer thermal issues that revealed themselves upon deeper investigation).

    I do remain nervous though because it means that I will have to excavate in a number of locations all of them immediately adjacent to my insulation and go through my frost skirt (not a huge big deal) to get down to a depth where the concrete has a chance of staying put once poured. One sees adjacent structures, built at separate times, migrating away from each other quite regularly and I am very keen to avoid this. For sure, tying in will prevent horizontal movement, but it will not be successful in preventing vertical movement and there are generally both involved once something like that sets in.

    Yes, yes, I am probably worrying far too much about this and even over-thinking it but I am driven by the horror of the prospect of the damn thing not staying put exactly where it should do over geologic time.

    Better to spend time worrying about this stuff now than dive in all gung-ho and regret it for ever afterwards.
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