tile cement board over old cement board?

[Mark P. Miller]

Way off topic, but thought I'd ask.

Is it OK to put 1/4" cement board over an existing 1/2" layer of cement board? I just ripped out some old tiles and will be extending a different tiled area to join up with the one that I just demoed. To keep heights consistent, I either need to

1) rip out the old 1/2" cement board, install 1/2" plywood, and then install a 1/4" backerboard before tiling OR

2) just add a second layer of 1/4" backerboard to the existing one and go from there.

Is one of these two approaches preferred? There is a small amount of residual mortar on the 1/2" stuff in place, if it makes a difference.

Thanks if you can help!

[Tim Boger]

I can't see an issue, use a 1/4" X 1/4" notched trowel to apply a thinset mortar over the existing cement board then flat coat 1/16" thick the mating surface of the cement board that you'll be adhering to the first .... once they're stuck together screw through both layers to the studs behind.

Can't go wrong.

Tim