15mm ply on bathroom floor

[DavidJ]

I've discovered the existing bathroom floor is only 15mm ply. I believe this is thinner than regulation requires. The adjoining chip board on the landing appears to be slightly thicker. The previous owner had laid cheap tiles directly on the ply which, to their credit, hadn't cracked in 4 years so it 'seems' solid.

I have some no more ply to go over it and the mega strength adhesive. Should I bite the bullet and change the floor?

 

[Flintstone]

If the floor is butchered and a miss match of thicknesses, change it !

 

[DavidJ]

It's in reasonable nick and luckily there was so little tile adhesive on it that it cleaned up easily. The different level is only with the landing so that's not the issue.

Just not sure if it's solid enough for the NMP and tiles

 

[Flintstone]

Only you can tell that!

 

[Old Mod]

15mm ply is minimum thickness for an overlaid floor by BS standards.
So it appears you’re back to Original ply floor, yes?
Easiest and quickest way to check is to place a brim full glass of water in centre of the floor and then walk purposefully and heavily around the glass.
If it spills, way too much movement and will need to be addressed. If not, an overlay of your nmp should be enough to improve its condition.
Never used nmp myself but I believe it to be a cement type board, you can improve its effectiveness by adding screws to the nmp, however, make sure they’re only in the ply, do not go through to the joist beneath, this will negate the purpose of the overlay.

Ok so I just read install instructions, and screws are standard fitting anyway 🙂

 

[DavidJ]

Hi 3_fall. Thank you for that. The floor passes the test with the glass of water.

Thank you also for the tip about avoiding the joists with the screws.

Do you know what the limit is for flatness of the floor for cement boards? It is flat apart from the last joist towards the door where it drops by approximately 3mm over 300mm (1%). I can't find any technical information online that stipulates what the limit is. I just tried bending a bit of the NMP board by hand and it seemed to have enough spring in it to take up this small deviation.

 

[Dougs Third Go]

rather than use the mega strength adhesive I use flexible tile adhesive (and screws), gives more of a solid bed imo, plus by angling the trowel slightly you should be able to make up the 3mm difference.

 

[GoneGuy]

I use hardiebacker board which is also a cement board, I put a bed of flexible tile adhesive which helps fill any voids and then screw the board down.

 

[GoneGuy]

@HardieBacker

 

[HardieBacker]

As @Adey1980 states above our boards would be ideal. We recommend a thin layer of tile adhesive underneath, screw down with 12 x screws and tape the joints with an alkaline resitant tape.

I have attached our brand new installation guide here which may be of use.