Substrate for new bathroom floor to tile on

[cazzerroofer]

Hi guys,

I have just done a new loft conversion and I'm about to tackle the bathroom floor with underfloor heating. Right now I have got floor joists down and I was wondering what's the next step. Is the following correct?

1. Start with 18mm ply screwed down into the joists.

2. 6mm insulation board for underfloor heating

3. Underfloor heating electric matting set down in cement

4. Then the tiles.

In addition, should I screw 6mm Aquapanel or Hardyback boards on top of the ply? Or are steps 1-4 enough?

Thanks in advance

 

[Richard C]

1. Start with 18mm ply screwed down into the joists.

You must ensure there is no flex in the floor; 18mm WBP is OK in many instances but it will depends on joist size/pitch/span & if the floor area is large. Sometimes it’s necessary to use 22mm or even 25mm. If the floor flexes unduly the tiles will fail, even with a quality flexible tile adhesive.

2. 6mm insulation board for underfloor heating

Something such as Marmox?

3. Underfloor heating electric matting set down in cement

Don’t set the mat in cement, it will fail; use a quality flexible powder adhesive such as BAL, Mapei, Webber, Ardex etc.

4. Then the tiles.

Use only quality trade flexible adhesive & grout of the correct type for your times (as above) & follow the manufacturers instructions for use.

In addition, should I screw 6mm Aquapanel or Hardyback boards on top of the ply? Or are steps 1-4 enough?

Rather depends on the floor construction as explained above, your need to ensure floor rigidity. The problem is that you don’t want too many separate layers under the tiles. I would go up on the WBP ply floor thickness or beef up the joists rather than lay an additional backer board as well as the insulation board but if you’re in any doubt about floor rigidity, maybe advisable; lay it on a thin set adhesive bed as well as screwing down.

I assume your loft conversion has been completed in accordance with Building Regs. & undergone the necessary inspections ? If so, you will be aware that all electrical work in a bathroom is subject to Building Notice (not worth the hassle IMO) unless you use a Part P registered electrician who can install, test & sign it off.

 

[cazzerroofer]

thanks for that i will go with 25 mm ply and go from there.

also tiling all walls which are currently just stud work,what's the best to put on them?

thanks in advance again.

 

[Richard C]

This is for the bathroom part of your lofty only ! You must use WBP ply not conventional ply. For walls, in wet areas use a minimum 12.5mm waterproof tile backer board but if you use plaster board, tank it if you want it to last; if you factor that in, the additional cost is negligible. In all other areas use 12.5mm Moisture Resistant plaster board but it must be primed in areas where you want to plaster. Don’t plaster where your going to tile as it will reduce maximum permissible tile weight from 32 - 20 kg/sqm (including addy & grout); this can be a real problem if you’re going for a heavy/large format tile. If the tiles are over, around, 200 x 200mm, use a powder cement adhesive not tub mix; in this case Gypsum plaster or plasterboard must be primed with an acrylic primer to avoid a possible reaction. Use only quality trade tiling products suitable for the tile base & the tiles you are laying, in your case it will at least be flexible addy/grout. Don’t use the cheapo tiling products sold by all the major DIY sheds; it looks like a bargain but you only get what you pay for with this stuff, it’s strictly DIY & generally crap. Follow the manufacturer’s instructions regarding primer (as above) but never use conventional PVA.

Read the Tiling forum sticky & browse through the forum archive posts, it could prevent you making expensive mistakes; lastly, feel free to come back with questions as & when you need to.

 

[cazzerroofer]

thanks

i will use wbp ply for the floor and 12.5 water proof tile backer board for the walls

thanks again