Reducing floor thickness with UFH - TilersForums.co.uk | Tile Forums

nell · 03-08-2008

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** · 03-08-2008

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 · 03-08-2008

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** · 03-08-2008

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

**Dave** · 03-08-2008

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

**grumpygrouter** · 03-08-2008

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** · 03-08-2008

Quote:

Originally Posted by Dave

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.

**David - Tradetiler** · 03-08-2008

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 · 03-08-2008

Thanks all for the advice

**wetdec** · 04-08-2008

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

..

**Dave** · 04-08-2008

Quote:

Originally Posted by 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

..

Hi tony..

Can't say i have heard of this system.....is it similar to the under laminate one..?

and i thought you have to leave ventilation under timber floors...? when tiling and UFH..

A bit more info on this system might help the member ...cheeeeeeeeers..

**wetdec** · 04-08-2008

A complete electric underfloor heating solution for new build - Suspended Timber Floors.

A Unique Under Floor Heating System that is installed under the floor, between the timber joists to provide either primary heat or supplementary heat in new build projects. To achieve the best results, insulation must be installed below the heating element.

The result is a warm, dry which is maintenance free and has a reduced installation height

Quick Installation

The film element for this type of wooden floor application should have a maximum heating effect of 90W/m . The maximum floor temperature should be controlled by an effective floor temperature sensor to a temperature recommended by the flooring manufacturer, usually 27 C. Particle chipboard flooring is recommended for installation over Ecofilm elements. Care should be taken when Pine T&G boarding flooring is used as some stability problems may occur. An un-coupling membrane should be used when tiling onto any wooden sub floor to prevent cracking caused by latteral movement.

Available in 3 widths; 300, 400 and 600mm to match joist centres. The product is perforated along the 50mm clear edge. This allows the border to be bent to a right angle (90 deg) to allow attachment to the joist when installing, Maintain a 40mm space between the perforation and the top edge of the joist. The element should be attached with staples to the wooden joists.

Hows that ............

..