blueboard as a substrate under floor tiling

[nt900]

Morning all.

Any reason I would not use blueboard as a substrate under floor tiling? Instead of cement sheeting? It is for a bathroom with suspended baltic pine flooring.

The reason I ask is because I already have half the required quantity of blueboard, which is in my way at the moment, and maybe it would give a better result?

[journeyman Mick]

If by "blueboard" you mean the 9mm?, recessed edge, fibre cement sheeting, then yes, it's probably okay. But the recessed edge will be a pain and should probably be cut off rather than filled. You say you have half the required amount, you may find that buying the other half will cost the same as, or more than, getting the whole lot in the proper underlay sheets and will mean less mucking around with cutting.

Mick

[nt900]

Thanks Mick. I really want to get the blueboard used that is in the yard. We are only talking one sheet in the yard and one to buy. But you are right about no cost advantage.

I assume blue side up if there was any additional water resistance benefits.

Would anyone know if it is preferable to make sure the floor tiles overlap the joins in the cement sheeting?

[journeyman Mick]

Anthony,
if you assume the worst (always wise I reckon ;) ) and say that there will be movement at the join of the sheets then it's best to treat the sheet join as a control joint. Lay your tiles so that the grout line coincides with this joint, that way, if any movement does occur it will only crack the grout rather than the tiles. Believe me, tiles will crack if layed over joints with movement in them - it's not pretty! :( If you want to be really sure of a trouble free floor then don't grout this joint, rather caulk it in a polyurethane sealant in a colour to match your grout.

Mick the cautious

[sailingamerican]

Use the cement board. Tape with nylon mesh tape and cement the joint like you would drywall. Lay the board in the opposite direction of the floor boards. (run it accross) They use the cement board because it bonds to the mastic and tile the best. Using anything else will cause failure. May reasons other materials are to soft and give. We either float the floor with lightweight cement and wire mesh or use cement board. If you are on the second floor a rubber sheeting under it is a must! Overlap and glue it according to the instructions. The same thing they use for shower pans or under roofs where ice damning can happen. I had to do the bath at the condo in Coffs over because some one did not put the rubber sheet under it. The building codes fall short in Australia in many ways. I would be out of business if I did it here. I would be in court all the time. A friend of mine is an architect in Sydney. They come here to see how we build stripp malls. Why they want to build them beats me.
Remember code is a minimum standard.
Richard in CA architect and general contractor.

[nt900]

Hmmm. Now I like both options, and prefer the control joint method. But, I probably will want to lay the tiles diagonal to the room. Therefore I don't have a choice but to run over the joint.

The cement sheeting is runs opposite to the floor boards, so that is good. So I guess I have to make the joint a strong and hopefully stable as possible. The only joint I have to worry about is less than a metre long.

Would the recess in the cement sheeting be an advantage? Could I use something better and stronger than nylon in the joint seeing I could have a recess available?

[Sturdee]

Would the recess in the cement sheeting be an advantage? Could I use something better and stronger than nylon in the joint seeing I could have a recess available?


The proper cement sheet type floor tiling underlay as sold by my tiling shop does not have recessed edges. They are only but joined and nailed down. For convenience the nailing spots are marked on the sheets.


Peter.