Hardwood decking joist spacing

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  • #68
    Mark Mark


    I’m in the middle of decking one side of my garden, 10m x 3.5m between my house and next doors garage. I have laid a 4×2 frame with 600mm joist centres and supports to the ground every 1m with noggins to suit.

    The decking I am now looking to use is Yellow Balau decking boards 21mm x 145mm in 3.5 metre lengths so no butted joints, the boards will run away from the house in the 3.5m lengths cut to suit.

    Everywhere I have read has said joist spacing would be best at 400-500mm centres, but I have now finished the framing.

    Would you say that 600mm spacing is too much and I risk warping the boards? Is my only option to take apart the framing and rebuild it with 400mm centres? Or are there any other solutions to this problem?

    I’ve looked into 40mm thick cedar boards but they don’t suit the look I am going for.

    Any help gratefully recieved!

    Update: As this is a home project I’ve been advised that 21mm boards will be fine on the joist spacing. I am very tempted to lay it with stainless torx screws with the possibility of taking it back up if the framing isn’t up to the job. I have added pics of the frame.


    Hi Mark

    I could write for hours on the complexities of decking structure design, have a look here 


    There are post, beam, joist and decking span calcs that should be adhered to.

    The main reason is if you don’t, whilst your completed project may look ok, in 6 months it will simply twist, buckle, snap screws and become unsafe, and you would have wasted your money.

    In short 400 mm joist centres are only a small part of decking design. These numbers are there for a reason, 400 centres with pairs of countersunk holes for bugle head stainless screws are essential to prevent the timber from twisting, at 600 it will twist alright, snap the screw heads off too. The numbers are also integral to the loading the decking can withstand safely, 1.5kn is what is required and can only be achieved at 400 centres.

    There are several pages on this site with advice for decking construction, if you are in need of advice, ask your questions and I am sure they will be answered…

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