Calculating Joists
Calculating Joists

So in this video. I want to look at calculating the number of joists that we need For our floor and once again whether it’s a platform floor or cut in floor doesn’t matter we can use the same method so we’re looking at choice this time so we’ll get around get a plan view there’s our bearers that we worked out in the last video and this time we’ll look at the joists, so The formula is similar, oh, before we get to the formula remember that we need to make take off our brickwork and our cavity on either side on a Brick veneer house, so we actually get the dimension of the floor not the dimension of the Brickwork so commonly 150mm on either side or 300 on both sides taken off the Brickwork dimension And down here where we have our internal corner it works out that we don’t need to take anything off these internal dimensions or the Dimensions of the walls that make up the internal corner because whatever you take off here you just end up adding back on here so you don’t end up taking anything off so they stay the same as they were in the bearer video as I was saying before the formula for joist is similar, but not exactly the same this time we’re going to take the length of the building the length of the area and divide that by the Joist Spacing so if we’re going to use this area here as we did with the bearers We’ll take the length this way rather than the width so the length of the area and again we have to add one at the end of it, and then we also add an extra joist for every external wall and any load-bearing walls that are in that area as well, so formula in this case would be all the actual figures 4.570 divided by 0.600 which was our joists spacing plus one plus two because you’re gonna have an external wall at this end an external wall at this end and there’s no load bearing Walls in this building So we worked that out and it ends up with 7.617 + 1+ 2 We round the 7.617up to 8 Because we can’t have p0.6 of a joist So 8+1+2 equals 11 so we’re gonna Have 11 joists in this area and There they are there so two doubles at the end doubles at this end and then 600 centres in between for our second area over here again, we’re going to take the length dimension or the dimension the length of the bearer basically if you’d like so Area 2=2070 divided by 600 and This time we have to add the 1 because we don’t have any extra joists over here unlike what we had with the bearers So we have to add the 1 and we have to add 2 because again external wall here External wall here, and no load-bearing walls in the middle So that comes out to 3.45 plus 1 plus 2 so that gets rounded up to4 5, 6 so 7 joists for Area 2 and Total 11 and 7 obviously gives us 18 joists. So there’s our 18 joist positioned on the actual frame For the timber order we’re going to need 11 joists at 2.4 again, you could go 2.100 but it’s pretty close you’re only playing with 30mm depending on you know if you knew the manufacturer and you knew they’re gonna be Spot-on 2.1 or a bit over then you could go 2.1’s, I played it safe so when I rounded up to 2.4’s this area here and oh sorry also on this one you have to allow for some lap on this bearer here, so you’re probably better off going for 2.4’s and with these ones here 2.5 so 2.5get rounded up to 2.7 So 7 / 2.7 for that area So there we go. That’s the number of joists that we need for this floor frame

9 thoughts on “Calculating Joists”

  1. Buildsum says:

    Hi Mylene, Glad you like them, thanks for taking time to comment.

  2. nikai123456789 says:

    why is your bottom joist spaced slightly more than the others? Also, do you measure from the inside or the outside or centre of the (Double) floor joist.
    Great videos by the way.. Thanks a lot

  3. Godffroi Djadide says:

    thank you for this video, why you +1 and +1 per ext, sorry i don't understand that, can you explain me please ?

  4. Godffroi Djadide says:

    yes thank you but at class we made an exercice with exactly the same plan and the teacher divide 4.570 /0.600 +1=19 si it's for that i didn't understand you add 2

  5. Simaa Hadi says:

    hi.. please how you calculator the space 0.600?

  6. Alex P says:

    Does the bearer and joist spacing always EQ?

  7. Alex P says:


    All of your is actually more clearer than our teacher in University.

    Wish to know the size of the timber you use (of course with compliant with AS1684) and dimensions from each other.
    It will very helpful

    thank you

  8. CVD_Cameron says:

    Hey mate, is there a formula for calculating the blocking/noggin as well? Cheers.

  9. Baya N says:

    Pls add subtitles your video has good details

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