6 or 9 mm ply on floorboards

I am a DIY’er who has done the Floorskills level 1 course.

i am planning to lay LVT in an upstairs landing and bedroom in my home. The subfloor is 120 year old floorboards.

Can someone please explain the perks of 9mm ply over 6mm and when would I use one instead of the other.

ie….How would I know I need 9mm ply ?

Thank you.

 

9mm especially when the floor is in rough shape

6mm when it's a pretty flat platform but still requires a boarding...

4mm....when run out of loo paper

 

Thanks Rugmunch - I take it its 9 mm when its worse than the +/- 3mm in 2 metres standard ?

 

Have you taken a reading of the floor boards? Being that old you could rot your floor covering it with vinyl

 

Pretty sure its bone dry up there, but will check.
What % does it have to be below ?

 

If the floor isn’t level the plywood won’t level it. You need to ply then use a fibre smoothing compound.
We use 9mm ply over old floorboards to flatten them out more.

 

Oh yeah it’s upstairs. Will be fine for moisture. Could have a 20mm sag in the middle or similar. I’d be surprised if a floor that old is anywhere near flat. Will likely need loads of ply or maybe a new subfloor

 

Thanks for the replies guys