Reducing floor thickness with UFH

[nell]

We are doing 2 upstairs bathrooms, the 1st has been completed with 15mm WBF ply on top of the floor boards (screwed at frequent intervals) then a layer of latex self leveling compound with the heating mats embedded and finally the flex adhesive and the tiles.
The only problem is that the floor has been raised by nearly 2 inches.

In order to reduce the increase in the floor height on the 2nd bathroom the Plummer suggested removing the floor boards and fitting 19mm “green” loftboard (with glue and frequent screws), then a waterproof membrane, and then either latex self leveling compound or flex tile adhesive to embed the heating mats.

Most of the posts I can find on the web suggest ply on top of floor boards, no mention of removing the floor boards.

I am concerned that even with the layer of level leveling compound that the floor may still flex and crack the tiles.

Our local tile store says it should be fine.

Any advice would be welcomed.

 

[grumpygrouter]

If you are going to the expense of removing the floorboards, you need to put down at least 22mm WBP ply. This is more stable than chipboard and stronger. You can then put the heat mat onto the ply and then put levelling compound over the top before tiling with flexy adhesive and grout with flexy grout. Not sure the waterproof membrane going down first will put up with the heat from the heatmat but I have no experience of this. What is the purpose of the membrane, just to protect what would have been chipboard from getting wet? If it is for full protection purposes then maybe putting a backerboard down instead of the membrane would be a better way to go.

See what some of the more experienced wetroom guys think when they respond.

 

[nell]

The floorboards have been almost totally removed in order to re-route pipes etc. I think the idea of the membrane was to stop the loft boards from blowing.

I agree that WBF would be better than the loft boards. I'm just concerned that normally you would have 22mm of floor boards with say at least 15mm of ply making 37mm.

I wonder if just 22mm of ply staight onto the joists would be sufficient to prevent the tiles cracking or is that taking a risk?

 

[David - Tradetiler]

I have done plenty by removing floor boards and using 18 mm ply straight on to joists.

Not an official recommendation but never had any comebacks. That way you only end up with the thickness of the tile as additional height.

I guess if you end up with a free standing bath (ball feet) in between joists it could be a problem but for normal foot traffic I have not seen it fail

22 mm ply is not easy to find in my experience

 

[DHTiling]

Also if going down the ply route , you will need to add noggins at 300mm intervals to remove as much flex as possible.....

 

[grumpygrouter]

22mm ply is much more rigid than chipboard. The reason floors are normally overboarded is to add strength to the floor. This is to compensate for any movement that occurs from the joists and to maybe account for the additional load of the tiling. A lot depends on the size of the floor you are looking to tile. Chipboard is not a very stable substance for tiling though it can and is quite often tiled directly onto with out problem as long as appropriate measures are taken, i.e correct adhesives etc.

Which way you go depends on the area of your floor really. if it is a large area, maybe 25mm ply would be the obvious answer, screwed to the joists at 300mm centres. You may need to add noggins between the joists for additional strength as well and screw the boards to this also.

it is the strength of the floor that is the issue. Chipboard is ok for a small area but as you have the floor up already, if it was me, I would go straight for the 22 or 25mm ply and forget the chipboard.

Ideally, and this depends entirely on the choice of floor adhesive, you will need to seal the back face and edges of the board before laying to reduce moisture sensitive movement. For instance if you are using BAL adhesives, you must seal the back and edges with BAL Bond SBR, or for Ardex-flex 7001, seal with Polyurethane Varnish.

You can then lay your 3mm of heating mat with virtually no step at all then.

 

[grumpygrouter]

Also if going down the ply route , you will need to add noggins at 300mm intervals to remove as much flex as possible.....

You must have written that as I was writing my long post.:mad2:

 

[David - Tradetiler]

Always look out for joists that have been butchered by plumbers. Once you have tiled over them you are accepting the floor as good enough to cover over and tile.

It is worth pointing out to you customer if a joists or two has been hacked too badly. I have seen some very bad ones

 

[nell]

Thanks all for the advice

 

[wetdec]

Hi Nell

I would sugest as you have the boards removed you do the following which will keep your height down :

Remove all existing boards.

Insulate between joists

Install an underfloor heating film (not a surface cable)

Overboard with 18/22mm Ply wood

Lay Dura-CI wetroom tanking / uncoupling membrane found HERE

Fix tiles and grout when thoroughly dry.



Tiler

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