Designing Kitchens and Bathrooms - installing hardibacker for bathroom floor tile

In preparation for tiling our bathroom floors I pulled up the old linoleum and the particleboard underlayment to expose the plywood subfloor. I'll be putting down 1/4" hardibacker before installing 12x12 ceramic tiles. When installing the hardibacker should it be screwed down every 6" just around the perimeter of the sheets, or every 6" over the entire area? Also, should the screws be installed just after laying down the CBU, while the adhesive is till wet and before or after the seams are sealed?

I am planning to use the mapei polymer-modified thinset to install the tiles and their sanded grout for 1/4" grout lines- are these good choices?

Thanks!

Use a 1/4" notched trowel to bed the hardibacker in thinset. Screw the hardi to the plywood immediately. 1/4" hardi has a screw pattern on the board. Follow that screw pattern. Generally every 8" in the field and every 6" at the edges. Use the square drive hardi screws as they are easier to set the heads flush with the hardi. Use alkalai resistant mesh tape and the same thinset youll use to set the tile for taping the joints. Its best to tape the joints as you set the tile so you dont have any dried humps to deal with. Ready hardi's installation instructions carefully and follow them.

I dont use mapei products, but havent heard anything bad about them. Yes on the sanded grout for 1/4" joints.

What is the thickness of your subfloor? If it's 1/2" plywood it's probably not rigid enough for tile.

Thanks for more good instructions HJ.

The hardibacker is now down (tiling will happen this weekend). We used the green hardi screws, however, some don't sit flush. Really you can only tell most of the raised ones are proud by running a hand over it (not a measurable height), but a few in a difficult corner under the vanity went in slightly crooked and the raised side of those worries me. It's under an 1/8 but I'm wondering how much trouble this will cause when laying the tile because it does cause some bit of rocking when a tile is placed over this area.

Will it be ok when these screws get embedded in the thinset to lay the tile or do I need to do something now? I've heard about pounding them in, trying to drive them in more (will this affect the integrity of the now-set mortar under the hardi?), or we can shave down the raised edge with a dremel.

Suggestions, advice?

PS- the hardi is down nicely, very firm floor over the 3/4" plywood subfloor.

If the thinset under the tiles will cover the crooked screws, you'll be OK. Otherwise, grind them off a bit with a dremel tool or a hand grinder. Don't beat on them, that will only break the bond between the hardibacker and the floor loose - BAD BEWS!

Sounds like youve done a fine job so far.

Thanks formula. The tiles are 12x12, so with the recommended 1/4 x 1/4 x 3/8 notched trowel the offenders should be well embedded in the thinset and not contact the tile. I was afraid that banging on them would compromise the thinset under the hardi, so thanks for advising me away from that. We'll grind down what looks like trouble and hopefully it will be a smooth work day this weekend.

If the thinset is compressed enough so that the screw head is even a little proud of the mortar, the screw will act as a stress point and the tile could crack. Try removing the screw and driving a new one, or using a roofing nail or drywall screw in that spot, grinding the screw head or just leaving the offending screw out.