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Orientation of plywood on top of joist, does it really matter?

I have heard that when putting plywood sheets on top of joists they should be perpendicular to them. Doe it really matter if, let's say, the playwood sheet has a square shape?

Or does the plywood sheet for example 2240x1220 has fibres oriented along the longest side (2440) and so they need to be perpendicular to joists to maximise strength?

Thank you

5 Answers from MyBuilder Carpenters & Joiners

Best Answer

Ply should always be laid with the grain running across the joists, not along the joist, to maximise the strength. Laying them length ways down a joist will cause them to crack along the grain which could end up in serious injury and or damage. The smaller the gap between joists the stronger the board will be. Minimum depth of ply should be no less than 12mm.


Answered 16th Jul 2019

It is usually correct to lay sheet material across the direction of the joists, this ensures even distribution of loading on joists, however, plywood is cross laminated so each layer is of different direction providing a strong floor covering. Either way the plywood can be layer providing the joists support the underside.


Answered 16th Jul 2019

Plywood sheets should be laid longest side (2440 mm) at 90 degrees to joists , also sheets should be staggered so that end of sheets don't all land on the same joist . This is generally accepted as the strongest way of fitting.


Answered 16th Jul 2019

Seems you have overthinked this. If the sheet is dimensionally mirrored, then no lay it any direction


Answered 13th Jul 2019

You're correct in that they should all be fitted across joists and not along. Stagger the joints and don't worry about if its square, just make sure ends are supported.


Answered 15th Jul 2019

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