Expert: Dave Davidson - 9/5/2007

Question
I recently tiled a kitchen that had a pine subfloor.  This is in an old house.  We took up the linoleum, put down a plastic vapor barrier then screwed in 3/8" hardy board.  Counter sunk the screws, filled them in with mortar.  Then tiled over the top and grouted.  A few days later, the grout started coming up in some places.  I took out all the grout and regrouted 2 days ago.  After 24 hours I looked and a little grout is coming up again.  What did I do wrong.  I have tiled many floors over concrete and never had this happen.  Thanks.

Answer
Hi Kathy. Thanks for asking.
The process goes something like this. If you encounter pine subflooring you have two options.
1. Install a mortar bed with wire reinforcing as the base for your tile.
2. Install an additional layer of plywood or OSB over the existing pine, fastening the underlayment through the pine and into the joists..
Since you went the route of cement backer board, I presume you have a feeling of comfort in that realm. Based on that I suspect number 2 would have been a good choice.
After installing the plywood/OSB, the CBU (cement backer board) should have been installed in a bed of thinset mortar and fastened to the underlayment every 6-8" throughout, without a cleavage membrane being installed (the plastic vapor barrier).
What you have now is excessive deflection that will not stop. The tile will need to be removed and re-installed with the above stated method to perform properly.
Sorry. Wish I had better news.
If you need more detailed information, I am here.