Floor joist: How to calculate spacings/centres

Floor joistAlthough it is fairly easy to calculate floor joist centres, it is just as easy to mess it up! I know I still do.

Start by measuring the full length of your deck (outside of bearer to outside of bearer) and then subtract the thickness of one of the floor joists.  Then divide this measurement by the maximum floor joist spacing that is mentioned on your drawings. Remember to complete all calculations using millimetres.

The formula looks like this:

(Overall length of deck – Floor joist thickness)
“Maximum floor joist spacing/centre”

Now your final answer is almost never a round number. No matter how small the decimal amount, you should always round up to the next whole number.  By rounding up you add an extra floor joist to your deck, which decreases the size of your floor joist spacings.  This in turn ensures your deck complies with your approved deck plans (To read some example disasters where DIYers have not complied, download your free copy of the “15 Common Deck Construction Mistakes“). The final rounded up number you are left with is the amount of floor joist spaces and if you add one to this number you have the number of floor joists required (this is of course, if you don’t have any double floor joists. If you do, we will cover what needs to be done in another blog).

The next step required is to calculate the formula for the actual floor joist spacing/centre measurement that ensures all your floor joists are equally spaced apart. Remember to complete all calculations using millimetres.

(Overall length of deck – Floor joist thickness)
Number of floor joist spaces

Once you have the results of both these calculations, you are ready to mark out and install your floor joists. For more information please contact us with your questions.

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