I am installing tile floors and am ready to install the hardi backer board. Two questions:

1. What is the purpose of the thin set between the hardi backer and the subfloor? Having cement under your cement board seems to be redundant if you are installing cement board over your subfloor and attaching the board with nails or screws. Just wondering. I am sure there is a good reason.

2. When attaching the hardi backer board to the subfloor is it o.k. to attach with 2" nails using a nail gun. I saw the use of a nail gun on a diy show on television. Just wondering if this is an acceptable practice and why or why not?

Thanks in advance for any and all input.

You did not say what your sub is made of and that is pretty important, no dimensional lumber here. The cement does a couple of things and is needed. It is not only an adhesive it also level/smooths the surfaces so the finished floor will be flatter. Glued and screwed is the way to go, do not nail and use a special Hardiebacker screw made for this installation.

The sub floor is 1/2" plywood installed over top of 1 x 12 dimensional lumber. Thanks, I will use the thin set between the sub and the hardi backer.

And how was the ply attached to the dimensional lumber. If it was not screwed and glued too I would tell you to take it all up and go with 3/4" or 5/8" ply glued and screwed to the joists. Dimensional lumber moves and movement is a killer for tile floors.

It was nailed. Im going to have to take my chances with that one. Im talking almost 600 sf with a center island and cabinets that would have to be removed to pull all of the sub floor up. Not to mention the cost. The house is 14 years old and the sub floors are in perfect alignment... so like it said... I will have to take my chances there. Thanks for the tip though.

If you have to keep the ply attached to the dimensional lumber then screw the heck out of it before screwing and glueing your backer. Nails have a tendency over the years to back out . Allow movement and cause squeaks. I am talking a 2 1/2 " screw every 4-6"