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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
    • CommentTimeJun 14th 2018
     
    To size floor joists take the span in feet, divide by two and add an inch then use, 2” joists and double joists as trimmers. Always round any fraction in the answer to the next whole number.

    eg twelve foot span 12/2= 6 + 1 >> use 7x2 joists — ie 47x175 Most houses are built this way sometimes I have seen the mass market builders using half inch sizes to save a bit of Monet
    • CommentAuthoradam_w
    • CommentTimeJun 14th 2018
     
    Tony,

    Is this based on the assumption that joist spacing is 400mm?

    Thanks,

    Adam
    • CommentAuthorowlman
    • CommentTimeJun 14th 2018 edited
     
    "I have seen the mass market builders using half inch sizes to save a bit of Monet"

    That's in order to give the "IMPRESSION" of a better job, tony. :wink::bigsmile:
    • CommentAuthorskyewright
    • CommentTimeJun 14th 2018
     
    Posted By: owlmanThat's in order to give the "IMPRESSION" of a better job, tony.

    Could being more precise be considered needless "POINTILLISM"? :wink::bigsmile:
    • CommentAuthortony
    • CommentTimeJun 14th 2018
     
    Yes 400 centres
  1.  
    If you're using tile or stone, you need to use the John Bridge "Deflectolator" program to get the right sizing so as to not crack your tiles or stone:

    http://www.johnbridge.com/vbulletin/deflecto.pl

    Paul in Montreal.
    • CommentAuthorCWatters
    • CommentTimeJun 14th 2018
     
    TRADA and the NHBC publish joist, rafter and purlin span tables.
    • CommentAuthortony
    • CommentTimeJun 14th 2018
     
    +1
  2.  
    Posted By: tony
    eg twelve foot span 12/2= 6 + 1 >> use 7x2 joists — ie 47x175


    Maybe I misunderstood but for a twelve foot span, building regs table A1 requires 8x2 or 47*195 ?

    Edit: just been searching and can't find the reference anymore. NHBC's table allows 47*170 for up to 3.55m span which is very nearly 12 feet.
    •  
      CommentAuthordjh
    • CommentTimeJun 14th 2018
     
    I think the table you were looking for is in an old copy of the approved document: http://www.planningportal.gov.uk/uploads/br/BR_PDF_AD_A_1992.pdf
  3.  
    My (UK) tables, about 30 years old, but I don't suppose wood has changed much in that time, gives a whole bunch of options for a 12 foot span (metric 3.6m) depending upon joist spacing and loading capability. So any rule of thumb for joist size can not work due to the number of variables of spacing and load demands.

    But if you want 406 spacing and 3.51 span then 51x178 works at 48kg/m2 or 51x203 at 97kg/m2 and then again for 3.66m span a 51x178 for 24kg/m2 works.
  4.  
    Thanks Peter, that's dating back a while! from before we went 'metric' - joists now come in 47 or 45mm thickness.

    Thanks DJH, the table has been moved out of the old Approved Doc (free to download) which now refers you to TRADA (paywall) - grrr!

    Here's a handy 'on site' guide http://www.haringey.gov.uk/sites/haringeygovuk/files/note07_-_domestic_timber_floors.pdf
  5.  
    Posted By: WillInAberdeenThanks Peter, that's dating back a while! from before we went 'metric' - joists now come in 47 or 45mm thickness.

    Err - why is 47mm more metric than 51mm?
    • CommentAuthorjamesingram
    • CommentTimeJun 15th 2018 edited
     
    this is a handy little site I found a while back
    https://www.rightsurvey.co.uk/rafters-span-tables/

    also for those only able to think in new money the (span/2)+2 works in decimetres

    3m span =30 dm/2 = 15 +2 =17 =170mm =170/50 joist = 7"/2" old money
    :bigsmile:
  6.  
    Posted By: jamesingram3m span =30 dm/2 = 15 +2 =17 =170mm =170/50 joist = 5"/2" old money
    http:///newforum/extensions/Vanillacons/smilies/standard/bigsmile.gif" alt=":bigsmile:" title=":bigsmile:" >

    Except that 170mm is 6.7":shocked::wink:
  7.  
    And the span tables specify 145x47 for a 3m span...

    Peter, I dunno why the standard 47mm thickness is considered 'metric' either...! Makes no sense.
    Something to do with adopting the European standard thicknesses perhaps? EN336 standard.
    •  
      CommentAuthordjh
    • CommentTimeJun 17th 2018
     
    Posted By: Peter_in_HungaryErr - why is 47mm more metric than 51mm?

    51 mm is imperial, not metric (50.8 mm = 2").
    47 mm is 50 mm timber (i.e. metric) that has been regularised.
    • CommentAuthorjamesingram
    • CommentTimeJun 18th 2018 edited
     
    peter
    actually i should have put 7"/2" (175mm)
    edited:bigsmile:
    • CommentAuthortony
    • CommentTimeJun 19th 2018
     
    Belt and braces approach there, I reckon 3m = 10’ / 2= 5 +1 = 147 x 47
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