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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.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

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    • CommentAuthortony
    • CommentTimeAug 11th 2018
     
    Will we live to regret building suspended ground floor slabs.

    There are alternatives but engineers and specifies go for suspended to protect themselves.

    I have recently tried for a ground bearing slab and even though the ground investigation said ok to do ground bearing slab engineer and building inspector both wanted suspended.

    Problems with cold air under building, air infiltration under building, thermal bridging through substructure all of which are difficult to fix and I think will be regtrtted one day.
    • CommentAuthorowlman
    • CommentTimeAug 11th 2018
     
    1. Wanting and getting are two different things, Do they have the right to veto plans without giving reason? Possibly not, so push for an explanation, and then you can challenge it if you feel that's justified.
    • CommentAuthorgyrogear
    • CommentTimeAug 11th 2018
     
    Posted By: tonyWill we live to regret building suspended ground floor slabs.


    Yes, if there was another alternative !

    gg
    •  
      CommentAuthordjh
    • CommentTimeAug 11th 2018
     
    Posted By: tonyI have recently tried for a ground bearing slab and even though the ground investigation said ok to do ground bearing slab engineer and building inspector both wanted suspended.

    What owlman said. To some extent it's a choice of which engineer you use. My main engineer was very familiar with raft foundations, as he lives in an area with lots of old coal mines, but he wasn't familiar with passive slab design, so I had to use a specific engineer for that. I also priced up a suspended timber floor on piles (as a more PC 'eco' design) and that was yet another engineer. So find an engineer that's willing to design a ground-bearing slab in whatever style you want, and then the building inspector can argue with him, if he feels so inclined.
    • CommentAuthorPetlyn
    • CommentTimeAug 16th 2018
     
    Like you, we were looking for something similar but with the influence of tree roots, deep trench-fill was specified with suspended floors. We have used expanded glass with a similar U value to PU, EPS etc but indestructible and fire proof. The beads are easily poured or blown into place and would fill the sub-floor void to the underside of the floor. This should eliminate the problems you identify.

    We have unexpected availability and could offer you adequate quantities at a price considerably lower than the alternatives.
    • CommentAuthorgyrogear
    • CommentTimeAug 16th 2018
     
    Posted By: PetlynWe have unexpected availability and could offer you adequate quantities at a price considerably lower than the alternatives.


    Naughty, naughty - advertising ?

    gg
    • CommentAuthorcjard
    • CommentTimeAug 17th 2018
     
    Indeed, reading your comments Petlyn, one would think that you’ve discovered the cure for every ill, as any semi relevant recent thread sees a few quiet days and then you pushing your glass bead overstock

    I’ll give you credit for not floating 3 year old threads to try and pimp your oversight, but it’s getting a bit repetitive. If I start a thread asking for exfoliator recommendations, will I see you in it also?
    • CommentAuthorgoodevans
    • CommentTimeSep 4th 2018
     
    I was forced to go for a suspended floor because of clay heave concerns.

    Initially I was going to go for a cast insitu concrete slab with sacrificial, collapsible, polystyrene/corrugated cardboard form-work (which strangely does not seem to require ventilation in the void created).

    In the end I went for standard beam and block with void below but with a slight twist...

    The beam an block floor is placed on sleeper walls at 2.9m centres with the B&B beams running from gable end to gable end at 490mm centres and filled with aircrete blocks width wise (minimum cost/m2 for B&B). On top of this 225mm of EPS 70 insulation was laid and a 70mm reinforced concrete slab floated on top of that. The edge of the slab has embedded into it a strip of air tight membrane which will be bonded to the wet plaster.

    This combination reduces both air penetrations and thermal bridges and has the following advatages:

    The RC concrete slab is my floor air barrier - It has some discrete penetrations where services poke out/in but otherwise it is continuous.

    The combination of squidgy EPS and stiff RC concrete ensure that point loads and line loads on the slab are spread over at least 2 B&B beams below such that I am able to place partition walls above the slab in any location without consideration to the B&B layout below and those partition walls can be heavier than normal (900kg/m3 block density with a dense wet plaster both sides).
    • CommentAuthorgoodevans
    • CommentTimeSep 4th 2018 edited
     
    The EPS was laid in 3 No. 75mm layers. The bottom layer had cut into it my ground floor hot and cold plumbing (in conduit). This allows the cold supply to be kept away from the UFH and insulated from the void below.

    The middle layer had the feed and return for the ASHP cut into it.

    Each layer of EPS had its joints staggered to assist in load spreading.

    The slab has UFH within it.

    one day, long after I'm dead, it will be possible for someone to gut the ground floor completely, without disturbing the external structural walls - it would be messy but the whole ground floor space (walls and floor) can be replaced with new right down to the B&B layer (and even under that should drainage needs replacement).
    •  
      CommentAuthordjh
    • CommentTimeSep 4th 2018
     
    Posted By: goodevansI was forced to go for a suspended floor because of clay heave concerns.

    I'm interested in how these decisions get made. We went for a passive slab because of clay heave concerns. Can you share the features of your site that led you to your decision, perhaps offline, i.e. expansivity of your clay, nearness of trees or water or whatever other factors etc. How deep are your sleeper walls?
    • CommentAuthorgoodevans
    • CommentTimeSep 4th 2018
     
    I can comment here (if Tony does not mind me hijacking this thread).

    We have highly expansive clay on this site - and my site had fruit trees at one end, a semi mature Horse Chestnut tree and mature hawthorn trees down the edge of one of the buildings. Several houses in the village are suffering settlement including the house next door but one to our site). The hawthorn trees alone (high water demand) would have forced a limited warranty house build (LABC or NHBC) to be piled.

    If I had have known that planning would have taken a year I could have first arranged to remove all the trees from the site and give the clay a year to hydrate and stabilise. With that done a raft could be justified however the clay here takes years to finish it's movement so it may not have been a good long term decision.

    The engineer's first thought was to go for a raft foundation, however as one house is 16m long and the trees were not evenly distributed on the site he was concerned that the raft may suffer differential settlement and end up on a tilt or with significant hollows underneath (I have seen a garage nearby with a hollow underneath it - not nice). If you can't raft then its going to be piles. If it had been a treeless, homogeneous site I would have been tempted to go for a shallow raft - but even here the clay under the middle of the house can end up drying out or getting wetter at a different rate than the clay at the edge.

    To complicate things further the site slopes a little - over the length of the 16m long building there is a height difference of around 900mm so again a raft could 'experience' different conditions simply because one end would have either had more fill or more excavation.

    To be honest I've seen how the clay gradually shrinks over years around here (and it always seems to shrink not heave) so I did not fight the pile decision. In the end I went for 300mm open bored cast piles (overall - 33 piles on the two buildings with an average pile depth of 8m including a 3m slip tube at the top of each pile) the finished piles came in at £12,300 all inclusive they just needed cropping with a cutter and a sds drill. The house won't move but I will need to ensure that I have rocker segments on the underground drainage where the pipes come out of the side of the house.

    Between the piles are 450 wide by 600mm deep RC beams (with clay heave protection under and on the inside edge of the external beams. The top of those beams reached the top of the 'pile mat' (the beams were excavated after the piles were pored).

    On top of these beams my sleeper walls are 375mm deep on one house and 225mm deep on the other. I think the building guidance intended to have a 150mm air void plus a 150 allowance for heave - but if you read the guidance literally it appears that the heave allowance is only required for timber suspended floors. Either way the BCO was happy with the arrangement - I just ensured that extra room was given under the drain pipes that were suspended below the B&B floor in case the clay swelled. The beam and block floor sat on top of the sleeper walls and the single skin external wall with the filler blocks also embedded into the external wall so the wall and floor act structurally as one.
    •  
      CommentAuthordjh
    • CommentTimeSep 5th 2018
     
    Thanks for that description. It makes a lot more sense to me with the piles underpinning it all. It seems like your clay is bit more expansive than ours, and more trees, closer than ours. Enough anyway to tilt the balance between rafted slab and piles.

    This summer, we can quite clearly see the difference between the part of our patio that is sat on clay and the part that is sat on the hardcore underneath our house. There's a quite noticable crack opened up along the edge of the hardcore - a couple of mm between slabs. I'll probably put some fine sand in it to make it less noticeable.
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