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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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General: Advice on rafter spacings

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    • CommentAuthoroiseaux
    • CommentTimeOct 9th 2007
     
    I have just started to load the roof on my previously mentioned garden room, and I am now concerned as to whether I have got my rafter spacings correct. I am using green oak rafters, 2.6 metres long at 45 cm centres. The tiles are terracotta, with a weight of 75 kg. per sq metre. The first section I have installed shows a deflection of 3mm at the midway point. Is this acceptable or should I increase the number of rafters and to what centres. Thankyou for any advice.
    • CommentAuthortony
    • CommentTimeOct 9th 2007
     
    Carry on no problem. Hope that your battens are 25 x 38 or bigger?
    • CommentAuthorjon
    • CommentTimeOct 9th 2007
     
    oiseaux: How are you going to get this project through Building Control?
    • CommentAuthorchuckey
    • CommentTimeOct 9th 2007
     
    Thats the disadvantage of using green oak. Its soft when green, prop the roof straight using a temporary purlin and in 100 years time, the oak will take the load without so much deflection! Get hold of a piece of wood (an archery bow is ideal), bend it and see how much it bends before it breaks! An eighth of an inch deflection in a floor when walked on is a definite problem (furniture rattles and ornaments fall of) but with a static load such as a roof , not much of a problem. FWIW my house and barn roof has a deflection of 4-5" in its rafter centres CW the ridge to eave line.
    Frank
    • CommentAuthorjon
    • CommentTimeOct 10th 2007
     
    Agreed but can become a big problem for a roof if it hasn't passed ordinary British Standard guidance on deflection for the finish type specified because it may then become uninsurable: Particularly if the drying process will induce further 'creep' deformations.
    • CommentAuthortony
    • CommentTimeOct 10th 2007
     
    What are the dimensions of your rafters?
    • CommentAuthoroiseaux
    • CommentTimeOct 13th 2007
     
    Rafters are 8 cm x 8cm and building regs as we know them do not exist here in France. Thanks for your replies
    • CommentAuthortony
    • CommentTimeOct 13th 2007
     
    What is the roof pitch? -- they sound just OK if they are oak.
    • CommentAuthoroiseaux
    • CommentTimeOct 15th 2007
     
    approx 25 degrees
    • CommentAuthortony
    • CommentTimeOct 15th 2007
     
    Hmm may be you should have used slightly deeper bits of wood -- in practice it will be OK but dont walk on it carrying anything heavy.

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