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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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  1.  
    I have a 4" concrete slab I want to build up on to form a load bearing wall

    The span is only 1.5m and will be attached to a concrete wall one end (timber and joist hangers). And rest on the new proposed wall....

    The timbers for the span will be easy to size from a span tables but I need to work out the 'wall'. Are standard 4x2 for the wall sufficient to take the load of a floor?

    I guess there's some method I could find this out? Like a Walter Segal calculator or something?

    Any suggestions?
  2.  
    I would be worried about the 4" slab. Do you know how thick the 4" slab actually is and how good is the concrete mix that makes it up? When I recently did a similar thing my friendly neighbourhood SE suggested I dig out the 4" slab (which turned out to be a bit less than 4" and probably about a 12:1 mix for the bottom 3".) and replace this with a 6" thick strip 2 ft wide with a bit of rebar thrown in for good measure.

    Are standard 4x2 for the wall sufficient to take the load of a floor ??? I'm not sure, it depends on spacing and the number of noggins. The danger is bowing under the weight. 4 x 4s should be OK or 2 x (4x2s) screwed and glued together if they are more easily available.

    The proper job would be to get an SE to calculate but it's probably cheaper and easier to over build !
    • CommentAuthortony
    • CommentTimeSep 9th 2021
     
    Basically if it is load bearing it will need foundations to get past building control
    • CommentAuthorphiledge
    • CommentTimeSep 10th 2021
     
    Our internal structural stud walls were specc'd as 38×89 C16 at 600 centres with 9.5 mm ply fixed to one side for racking resistance.
    • CommentAuthorrevor
    • CommentTimeSep 10th 2021 edited
     
    I had a very similar situation resulting from the architect not including the foundation on the plans so never got dug. The 4" slab is on 6" compacted hardcore
    Our span is 2m with easi-joists at 400 centres. The room is a walk in wardrobe 3x2 m. Went back to structural engineer who did the original calculation he stated that a 147x47 stud wall at 400 centres with ply lining would suffice. At the time the floor insulation and screed had not been laid so to avoid the timber being buried we built a concrete block base to sit the frame on. This effectively I suppose is a substitute foundation.
    •  
      CommentAuthorfostertom
    • CommentTimeSep 10th 2021
     
    No, the danger is that a load that bears on ordinary slab, esp only 4", may crack the slab, and hardcore 'gives' under it. A found goes down to undisturbed ground of prob higher loadbearing capacity, leaving the slab unloaded.
  3.  
    Any reason the posts (say 4x4) couldn't rest on breeze blocks? A bit like garden decking?
  4.  
    Posted By: VictorianecoAny reason the posts (say 4x4) couldn't rest on breeze blocks? A bit like garden decking?

    Do you mean 4" blocks on edge. If so IMO no reason but what for? You would be better off with a 2 x 4 laid flat as it is easier to fix (nail) the uprights to it.
    If the plan is to lay the blocks flat then as far as I know the blocks aren't load bearing like that if laid 1 row thick. If you built a dwarf wall with flat blocks then that doesn't solve the foundation issue because you have just added more weight to an unknown base.
  5.  
    How about a 6x2 sole plate, 6x2 verticals at 400 centres?
  6.  
    Posted By: VictorianecoHow about a 6x2 sole plate, 6x2 verticals at 400 centres?

    OK but what about what the sloe plate sits on? adding size or weight to bad or non-existing foundations won't help anything. If the slab is good enough then 4 x 2 sole plate would do the job.
    As I said above unless you know the slab will take the load
    Posted By: Peter_in_Hungaryreplace this with a 6" thick strip 2 ft wide with a bit of rebar thrown in for good measure.
    • CommentAuthorSimonD
    • CommentTime7 days ago
     
    I'm intrigued. What is the wall actually going to be holding up? I.e. what is the complete structure you're planning?
    • CommentAuthorrevor
    • CommentTime7 days ago
     
    Posted By: SimonDI'm intrigued. What is the wall actually going to be holding up? I.e. what is the complete structure you're planning?

    Would be useful information

    In my example it was okayed by the SE and BCO was happy with it but depends what you are loading it with. If you are really concerned you could dig a foundation for it or even a hole at either end and put a UB across.
  7.  
    Just light storage for my business, tools, small bits of plant and materials etc

    Then below would be an office space
  8.  
    Posted By: VictorianecoJust light storage for my business, tools, small bits of plant and materials etc

    Then below would be an office space

    The trouble is that light storage is today, later or the next person changes the usage then somehting fals down. Best to design it to minimium building standards.
    • CommentAuthorJonti
    • CommentTime1 day ago
     
    Posted By: Peter_in_Hungary
    Posted By: VictorianecoJust light storage for my business, tools, small bits of plant and materials etc

    Then below would be an office space

    The trouble is that light storage is today, later or the next person changes the usage then somehting fals down. Best to design it to minimium building standards.


    Minimum building standards are dependant on what the use is going to be. If you are worried about the next person then you can take it down when you go. Does the OP actually own the building?
    • CommentAuthorsquowse
    • CommentTime6 hours ago edited
     
    Do you know what the constuction of the slab is? If it's got rebar in it may be strong enough but typically any mesh may not go right to the edge where it's needed. If in doubt you could underpin the edge.

    ie dig underneath the slab edge and construct a concrete edge beam down to good ground.
    • CommentAuthorsquowse
    • CommentTime6 hours ago
     
    to answer your original question if it's just supporting 1 wall or roof then I would say 4" studs should be sufficient.
    Is it a 2 storey building though - so the wall will take a floor and a roof above?
    Building regs would probably require 6".
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