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Green Building Bible, Fourth Edition
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    • CommentAuthoradam_w
    • CommentTimeMay 2nd 2018
     
    Good morning All,

    I'm aiming to have my slab poured Friday afternoon and I'm still undecided on a couple of things which I'm hoping you may be able to help me with. This is for my integrated garage/gym floor so I will be spending quite a bit of time in there either fixing my car or breaking my body, one of the two.

    My first job it to level the base which I believe is to be done with sand, which sand would you recommend?

    The next is insulation, building control have recommended PIR board but I know this forum is keen on EPS or XPS, is there a downside to pleasing the BCO?

    Next is my large thick polythene sheet then 100mm of concrete poured over reinforced A152 mesh 50mm from the top surface, does this sound right to you guys?

    Any help or advice you could give would be greatly appreciated.

    Thanks again,

    Adam
    • CommentAuthorjfb
    • CommentTimeMay 2nd 2018
     
    Any sand should do but softer building sand better (its just trying to reduce chance of holes in polythene sheet).
    I presume there is compacted hardcore below the sand.

    I think most on this forum would be having 200 or 300mm of EPS below slabs for better insulation. Off hand I am not sure if 100mm of EPS would even be enough for building regs. Presuming that you don't have the option of going more than 100mm of insulation you might be best off going with PIR. No gaps between sheets (foam insulation if there are gaps).

    Mesh in middle of slab sounds fine. Are you doing the pour yourself? If you are and it is a first time you want to make every effort (and more) to get the whole thing level especially at edges and corners. A line around the edge on the polythene to go to is a good start (assuming the sheet is well tucked down at the edges). A laser level/site level is very useful to be able to find any high/low spots. The better you make it now the easier the floor laying. I'll say that again….the flatter the pour the easier the tiling or whatever floor you are putting down!

    Also - assuming it is being poured straight from a lorry - make your life easier by making sure the mix isn't too dry. If you can't rake the concrete around easily you will have a much harder time levelling it. Get them to tip a little to start with to see how much slump it has.

    Good luck.
    • CommentAuthoradam_w
    • CommentTimeMay 2nd 2018
     
    Hi John,

    Thanks for the reply! Just to clarify, as the floor is for a garage, it doesn't have to comply to building regulations, I'm purely adding the insulation to increase the comfort levels based on a recommendation of another forum member some weeks ago.

    Also I only have enough room for 50mm of insulation, I know its not a lot but I thought some was better than none in this instance and also from a conduction point of view PIR is better performing than EPS.

    This isn't my first concrete pour but it is my first slab pour, I've got someone coming to help me who has much more experience in this field but he recommended sharp sand where as everyone else has said to use building hence my need for clarification.

    The concrete lorry is mix on site so I'll make sure I pass on your tips when the operator arrives. Is there a specific strength I should be requesting, ST2 perhaps?

    The final thing I'd like clarification on is the position on the DPM, does that go over or under the insulation?

    Thanks again,

    Adam
    • CommentAuthorMarkyP
    • CommentTimeMay 2nd 2018
     
    standard building sand, spread it about and use a 4 x 2 to rule it and tamp it down to compact.
    DPM under the insulation.

    personally I wonder if insulation under slab in a garage will help much. I'm assuming the space wont be regularly heated so you will have a big lump of cold concrete under foot. Any space heating you run while in there will not warm the slab for ages so not sure the insulation will result in much benefit felt under foot.
    • CommentAuthorgyrogear
    • CommentTimeMay 2nd 2018
     
    Posted By: adam_wI'm purely adding the insulation to increase the comfort levels based on a recommendation of another forum member


    In which case, lay the insulation over the slab, then floor it with, say, 9mm OSB.

    I added 30 mm XPS + OSB, and it made a great difference (so then I put 40 mm XPS on the walls, and 50 mm XPS on the garage roof (becos my lounge is above...).

    gg
    • CommentAuthoradam_w
    • CommentTimeMay 2nd 2018
     
    Posted By: MarkyPstandard building sand, spread it about and use a 4 x 2 to rule it and tamp it down to compact.
    DPM under the insulation.

    personally I wonder if insulation under slab in a garage will help much. I'm assuming the space wont be regularly heated so you will have a big lump of cold concrete under foot. Any space heating you run while in there will not warm the slab for ages so not sure the insulation will result in much benefit felt under foot.


    Thanks Mark but I thought direct contact between the insulant and the concrete would cause degradation of the foil?

    As I say, it was recommended to me by a fellow member who fortunately has just commented on this thread, GG!

    Posted By: gyrogear
    Posted By: adam_wI'm purely adding the insulation to increase the comfort levels based on a recommendation of another forum member


    In which case, lay the insulation over the slab, then floor it with, say, 9mm OSB.

    I added 30 mm XPS + OSB, and it made a great difference (so then I put 40 mm XPS on the walls, and 50 mm XPS on the garage roof (becos my lounge is above...).

    gg


    Thanks for your reply GG, this is all in reference to reply of your on another thread of mine however due to the fact its primary function will be a garage and I like tinkering with cars, I'd have concerns over the impact resistance of the 9mm OSB to something like a trolley jack or staining from stray oil, may my concerns are ill founded?

    Thanks again,

    Adam
    •  
      CommentAuthordjh
    • CommentTimeMay 2nd 2018
     
    Posted By: adam_wits primary function will be a garage and I like tinkering with cars, I'd have concerns over the impact resistance of the 9mm OSB to something like a trolley jack or staining from stray oil, may my concerns are ill founded?

    No, I don't think they are. I wouldn't use a wooden floor in a garage because of weight loading. Is it even legal because of fire regs?

    DPM position seems to be a matter of debate. Underneath keeps ground moisture out of the insulation but collects any spills or condensation from above. On top lets the insulation get wet sometimes, but at least it can drain dry. The answer probably depends on your ground conditions and other factors. Maybe ask your BCO?
    • CommentAuthorMarkyP
    • CommentTimeMay 2nd 2018 edited
     
    I'd go with insulation over slab preferred to under in this situation. if you need to beef the floor up go for 18mm OSB or even quality T&G flooring chipboard, prime, apply a good quality heavy duty smoothing compound over that and then apply a couple of coats of quality floor paint, or better some epoxy paint. Nice warm feet, but a durable floor.

    sorry, yes you said it might be PIR. In that case a further layer over.
    • CommentAuthorgyrogear
    • CommentTimeMay 2nd 2018 edited
     
    Posted By: adam_wI'd have concerns over the impact resistance of the 9mm OSB to something like a trolley jack


    Could you not clad those work areas with chequer-plate (steel or aluminium ) ?

    gg
    • CommentAuthorjfb
    • CommentTimeMay 2nd 2018
     
    I wouldn't be happy with an osb floor if you want to be driving cars onto it and working on them.
  1.  
    Either way, definitely not 9mm. 18mm min. Point-loads (as previously mentioned - jacks and so on) could put a serious 'dinge' in 9mm OSB and insulation.
    • CommentAuthorgyrogear
    • CommentTimeMay 3rd 2018 edited
     
    Posted By: Nick ParsonsPoint-loads ((...) could put a serious 'dinge' in 9mm OSB and insulation.


    Point taken !

    I singularly failed to mention that my garage has been converted to a "DIY workshop" and will not be receiving any vehicles, save maybe a motorbike in some distant future. Whence my admittedly thinnish floating floor.

    gg
    • CommentAuthoradam_w
    • CommentTimeMay 3rd 2018
     
    Good morning All,

    thanks for your numerous replies but with all things considered I still think my best solution is the slab on top of the insulation however I do appreciate that I will be losing warmth in the winter months.

    I guess this then begs the question, is 50mm of under slab PIR really worth the expense? I have a deficit of 170mm which could all be filled with concrete once levelled off, to then provide a super strong base and works out about ÂŁ100 cheaper overall, decisions, decisions!

    With regards to the position of the DPM, I'll get in contact with my BCO today and see what he prefers.

    Thanks again,

    Adam
    • CommentAuthorMikC
    • CommentTimeMay 3rd 2018
     
    I would omit the insulation altogether if it's purely an unheated garage. If the garage is heated at all then it's justifiable. If you can get e really nice smooth finish this is way better for rolling around Jack's and cleaning up. I powerfloated my workshop floor and covered it in epoxy floor paint. You could play marbles on it if you wanted, so moving heavy things around is much easier. Sweeping up is easy too.
    • CommentAuthorCWatters
    • CommentTimeMay 3rd 2018
     
    Are there two separate rooms (garage and gym)? You could put down say 100mm PIR under the gym area and nothing under the garage area, them pour your concrete. You end up with 170mm of concrete in the garage which is plenty to support a car and 70mm in the gym which is fine for that. May need to stake down the PIR to stop it floating and possible put some mesh across the join.
    • CommentAuthoradam_w
    • CommentTimeMay 3rd 2018
     
    Posted By: MikCI would omit the insulation altogether if it's purely an unheated garage. If the garage is heated at all then it's justifiable. If you can get e really nice smooth finish this is way better for rolling around Jack's and cleaning up. I powerfloated my workshop floor and covered it in epoxy floor paint. You could play marbles on it if you wanted, so moving heavy things around is much easier. Sweeping up is easy too.


    I think you're right, solid slab it is! Plus as its a garage I need to be 100mm under damp course anyway should I want to retrofit any insulation as a later date.


    Posted By: CWattersAre there two separate rooms (garage and gym)?


    Its just one room, I've only got one window at the back and sperating the two would block of light not just from the car portion but the adjoining utility room as well.

    Thanks again for all your input guys, wish me luck with the pour tomorrow!

    Adam
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