Hardiebacker over MDF boxing?

Discussion in 'Tilers' Talk' started by Wumi, May 16, 2020.

  1. Wumi

    Wumi New Member

    I have MDF boxing around my waste pipes in the downstairs toilet (similar to image below). I was originally going to just paint it, but have now decided to tile the top with mosaic tiles and paint the sides.

    I know I can't tile directly on top of MDF, so was thinking of getting some 6mm Hardiebacker, attaching to the MDF and tiling over that.

    1. Is this a stupid idea? If so, what would be the alternative?
    2. How would I attach Hardiebacker to MDF?
    3. What to do with the exposed Hardiebacker edge?

    Note - One part of the boxing has a removable top panel for access to toilet / bidet spray isolating valve.

    Thanks in advance 47e329947c9eb161190d8e49b6cb7b0a--attic-shower-slanted-ceiling.jpg
  2. DIYDave.

    DIYDave. Screwfix Select

    Ideally, would have been better to box in using plywood, rather than MDF

    Although unlikely to have masses of splashing water, there’s the chance that some water will end up on the floor

    Those cloakroom basins are small and hand washing can lead to water on the floor plus every time you mop the floor (won’t even think about the ‘bum gun’) :eek:

    If water is allowed to make contact with MDF it will soak it up like a sponge and swell up and possibly loosen tiles

    Anyway..... it’s all boxed in now so -

    I would Silicon all around base of MDF where it meets tiles and really force the Silicon under any gaps to make a good defence against contact with water

    Then seal the MDF with 2 coats of diluted SBR - wickes sell a 1lt whilst most places sell in 5lt

    Could also use a tile specific brand of sealer, Ardex / Bal / as an example
    but SBR will do a great job and cheaper as well

    Now ready to tile

    Ok, again, MDF isn’t the best to tile onto and many people will suggest cladding the MDF with cement board / tile backer board

    But this is just pipe boxing in a fairly dry area so will be fine

    As to raw edges, simply use a tile trim to cover. Loads of types available, plastic, stainless steel, ali, curved, square, flat, coloured, etc

    You could also edge the removable panel to neaten and then secure panel in place using chrome mirror screws with flat heads

    Finally, silicone around boxing/floor
    junction as a 2nd line of defence against water meeting the MDF (and to neaten)

    Good luck
    Wumi likes this.
  3. Wayners

    Wayners Screwfix Select

    I have similar and filled all joints with dry flex resin and painted. Can't see joins.
    You could stick tiles on with polymer mastic or stick Hardie board with polymer mastic and tile.


    Make template up and get perspex made to cover with edging beads

    Just some ideas
    Wumi likes this.
  4. Wumi

    Wumi New Member

    Thanks all.

    @DIYDave. - The boxing has pine skirting around the entire room should I add silicone between floor and skirting?

    I’ve got some 2mm plywood sheets to hand, is it worth cladding the side of the mdf boxing and butting up against corner trim so no edges are exposed?

  5. DIYDave.

    DIYDave. Screwfix Select

    Ok, as skirting is fitted, you would usually then silicone around skirt where it meets tiles. There’s bound to be small gaps between skirt and tile (?) so the silicone does 2 things here;

    Cosmetically, leaves a neat finish, silicone available in a wide range of colours so don’t automatically think white !

    And importantly, stops water from seeping underneath (as I’ve mentioned above)

    So once finished and painted, last job would be to silicone around skirting

    As for using thin ply and exposed edges ? I’m not really with you here

    Are you not tiling the boxing completely ? So the only exposes edges will be tiles edges and again, mentioned about tile trims to neaten this up

    Or have I missed your point ?
  6. Wumi

    Wumi New Member

    6E913B40-AE0D-4172-A04E-2AECEB05D427.jpeg 200C9CB6-A65D-4FE6-AE08-9F3BA6D06B86.jpeg
    Thanks Dave,

    I’ve sketched up some (very) rough drawings so you see what I mean. The way the boxing was constructed left a raw MDF edge on the side.

    I’m only tiling the top of the boxing. The sides will be painted the same colour as the walls and skirting.

    Hope that makes sense!
  7. DIYDave.

    DIYDave. Screwfix Select

    Ok yes, get it now thanks !

    The sketches are perfect, really, that’s how I work - no fancy design software unfortunately - scrap of paper and pencil, does the job :)

    I would still use a tile trim on edges on top of box otherwise you get that raw tile edge on show

    Square edge stainless steel or aluminium looks good and modern as well but loads available

    As to the MDF edge, a frequent question on the forum when it comes to painting

    Basically, the raw edges go ‘fluffy’ when you paint them, especially when using water based paints - causes the ‘wood’ fibres to swell

    Seal with primer, will dry mega rough. Sand down, prime again, slightly less rough, light sand down, prime again ...... light sand ..... should now be lovely and smooth. Now ready for Undercoat and Topcoats

    Smooth face of MDF is perfect for painting and has non of these problems

    Can use various sealers for the MDF edges to prevent this problem but I’ve only used Leyland Acrylic Water Based Primer, dries in an hour between coats so just get on with something else whilst it’s drying

    It’s worth putting in the effort to get a great finish as all the details are equally important - good luck with the project
  8. Astramax

    Astramax Super Member

    Zinsser recommend Bullseye 123 for sealing MDF, maybe to do with it being mould and mildew resistant, my money would have been 100% on BIN which can also be used for MDF!
  9. Wumi

    Wumi New Member

    Thank you so much! I’m going to trying tackle over the next week.
  10. Wumi

    Wumi New Member

    Thanks, i do actually have Zinsser Bullseye 123, I bought it to prime the MDF and skirting boards. Wasn’t sure how it would handle MDF edging.
  11. Astramax

    Astramax Super Member

    It will be fine to use on the edging.

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