Photo: joist undertie batten, used to tie joist together, loadsharing batten 1 1/2 x 4 screwed fixed to joist, helps stop deflection of single joists when live load is applied, spreads the load across several joist, gives a more solid feel, less movement.
Joist spacing is 400mm and a 100kg load applied to centre of joist at midway of 2800mm span, the maximum deflection area, is 4mm, this is reduced to 2mm with the joist undertie, acceptable design if you want to use ceramic tiles, if applied tiles with a 2 pak glue the deflection will be less, the tiles give a stronger laminated floor, I will be sanding the ply floor
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