Minimum ply thickness for bathroom floor?

[Nigel Watts]

My main bathroom, which I am trying to complete, is in a former bedroom on the 1st floor of my 1840 London semi. The current floor is mainly original rather narrow pine boards which are in poor shape and chipboard which has been laid over the major pipe runs to the cupboard housing the hot water tanks, pressure vessels etc.

We want to cover the floor with something like lino or rubber (to make it both fairly waterproof and comfortable for bare feet) but I dont want to have to build up the thickness too much. What is the minimum thickness of ply I should use to cover the boards first, and should I get the ply down before decorating? I do not intend to lay the floor myself and would rather not have to lay the ply either, which I assume the flooring contractor will do for me.

 

[Andrew Bodenham]

Hi, Nigel,

I'd be guided by the advice of your flooring supplier, or better still the technical spec from the flooring manufacturer.

I've done a couple of bathrooms where Karndean (heavy duty vinyl flooring) has been laid over exisitng boards. In these cases the installers laid 6mm ply over the boards first, stapled down. If your existing floor is poor then 9mm or even 12mm ply might be better, if you can accomodate the thickness.

Ideally you would do a first fix of the sanitaryware, then lay the ply, then do any cabinetwork, then lay the lino, then install the loo, basin etc. In this way you get continuous flooring under the loo, basin pedestal etc, much better than having to fit the flooring around these items. This assumes that you are using conventional sanitaryware of course. Just remember to allow for the thickness of the ply and flooring when doing the first fix.

As the quality of the final installation depends on the flatness and smoothness of the subfloor I'd be inclined to spend a bit of time leveling your boards - packing up any low points and taking off any high spots. A belt sander with a fierce grade of abrasive would be a good way to take down the high spots. In my experience the flooring fitters are usually in a hurry and don't worry too much about this part of the operation. If you can lay your own ply then you can choose a better quality material e.g. birch ply (also more eco-friendly than tropical timbers) and I'd be tempted to secure with ring-shank nails for long term security.

Worth considering the need to access you under-floor plumbing in future. Sod's Law says that as soon as your new lino is sealed down there'll be a weeping joint or pinhole leak that needs fixing....

 

[Nigel Watts]

Thanks Andrew, most helpful. Sounds like 6mm is the minimum and that I should lay the ply myself. The house has very thick brick outer walls but the interior structure is all wood and the floor sags significantly towards the interior and is a little bit flexible. This clearly cannot be compensated for but I take your point about making sure the old floor is reasonably smooth.

The shower tray, a large Matki, is already in, as is the cabinet on which the basin sits. Loo is wall-hung so not an issue and there is no bath. Fitted cupboards were there from before. The fitting could be a pain, but as you say a contractor will probably not pay enough attention to it.

I find that manhandling the 8x4 feet sheets on my own is virtually impossible. I am thinking of asking the supplier to cut them down into 4 x 4 first. Is this a good idea? Given the flexibility of the floor, wouldn't screws be better?

Final point. The room has a mantelpiece. As customary the hearth stone is flush with the floorboards. I dont intend to light fires in the room but I am not keen to carry the ply and flooring over the hearthstone either. Any ideas on how to deal with this?

 

[Andrew Bodenham]

Nigel,

With everything wall / cabinet mounted it should be quite an easy job to lay the ply.

I agree that 8x4 sheets are cumbersome - I recall one memorable occasion on a windy day when I was loading a sheet onto the roof of the van....... Cutting down into 8x2 sheets makes them more managable than 4x4 I find, though you'll need to consider the shape of your room too.
Screwing down the ply would work well.

Hearth - if you don't want to cover it up how about creating a timber kerb or fender around the edge of the hearth by fixing something like 75mm x 40mm oak, with appropriate mouldings to the top edges and mitred together at the corners. Then just run the ply and flooring up to the fender. If that's too much of trip hazard then maybe it would be best to floor over the top, unless you can raise and relay the hearth at a higher level. It would still need some sort of edging to achieve the interface between the two materials though - how about some thin brass strip, say 35x3mm screwed down on top of the lino to secure the edge.

 

[Nigel Watts]

Thanks Andrew. I was also thinking along the lines of a hardwood kerb/fender round the hearth, screwed to the floor - oak perhaps with brass or stainless steel screws in flush fitted countersunk washers, with a similarly detailed threshold to make the transition from the landing (original exposed boards and stair runner) to the bathroom. 8 x 2 rather than 4 x 4 is a great idea too and I am now convinced I should get all this in place before decorating and hanging my F&B wallpaper (I have visions of it getting ripped by cack-handed fitters, amongst whom I include myself).

 

[Penners]

I am now convinced I should get all this in place before decorating and hanging my F&B wallpaper (I have visions of it getting ripped by cack-handed fitters, amongst whom I include myself).

I absolutely agree - particularly if you're having to handle sheets of ply. It would only take one distracted swing-around of the corner of a board... Personally I would finish everything, and paper the walls last of all.

I agree entirely with all of Andrew's advice (would I dare to disagree, given his professional expertise?). The only small addition I would make is (and forgive me if this is blindingly obvious), do make sure that you countersink the screw heads. If you can face it, sinking them below the surface and then filling would be best, but a close second-best would to ensure that they're flush with the board surface. This will prevent the risk of, over time, footfalls pressing the lino into the countersinks and causing little dimples in the surface.

 

[Nigel Watts]

Following Andrew's advice, my 9mm birch ply delivered in 8x2 strips (from Moss & Co of Hammersmith, my favourite London timber merchant) is starting to go down. It looks so good it seems a shame to cover it with something.

 

[LadyArowana]

Following Andrew's advice, my 9mm birch ply delivered in 8x2 strips (from Moss & Co of Hammersmith, my favourite London timber merchant) is starting to go down. It looks so good it seems a shame to cover it with something.

Can you treat it and leave it au natural and have some mats where needed?

 

[Moo]

Can you treat it and leave it au natural?

That's exactly what we did in our first house. It was a tall, narrow, pooteresque semi; the carpet in the hall (also tall and narrow) had worn out, and we couldn't afford to replace it with anything that might last more than a few months. Unfortunately, the floorboards ran along the length of said hall, which made it look even more like a tunnel.
So we covered them with wide plywood planks running from side to side. Twelve years (and two children) later they still looked as good as new.

 

[Penners]

Following Andrew's advice, my 9mm birch ply delivered in 8x2 strips (from Moss & Co of Hammersmith, my favourite London timber merchant) is starting to go down. It looks so good it seems a shame to cover it with something.

Birch ply is my favourite joinery/cabinet material. It's easy to work, easy to finish, attractive in a bland sort of way (but can be stained to some very interesting effects) and it smells wonderful.

Its only downside - the cost!