nonstandard floor hardibacker and porcelain tile

jep · 10-05-2012, 05:43 PM

Hi. I have a nonstandard floor/ceiling construction. The top floor is in a newly remodeled kitchen. THe room below is a bedroom. THe "rafters" in the bedroom are 4x12 exposed beams on 2' centers. Resting on the rafters are 1x3 tongue in groove planks which form the bedroom ceiling. Directly on the bedroom ceiling is 1/2" OSB which I consider the kitchen "subfloor." Thus the distance from the top of the OSB to the visible ceiling in the bedroom is about 1 1/4 - 1 1/2".
I am going to put down 1 foot by 2 foot porcelain tile for the kitchen floor on the OSB. I am wondering whether I can get by with 1/4" hardibacker or if I need something a little stouter like 1/2" durrock? ALso, I would prefer to just screw the hardibacker into the OSB and not use thinset. But I could be chastised into setting the hardibacker in thinset and screwing if that is best.

joecaption · 10-05-2012, 05:57 PM

Backer board adds 0 strenght to the floor, plywood will do that.
I'd add at least 3/8. 1/2 would be better A/C subfloor rated plywood, thin set then 1/4 backer board.
Or this can be used instead over the plywood.
http://www.schluter.com/8_1_kerdi.aspx

JazMan · 10-05-2012, 10:30 PM

Jep & Joe,

Jep, from what I can gather it sounds like your framing should be strong enough for ceramic tiles, even though you didn't mention the span of those timbers. 1x3 planks should be good for 24" o.c.

The 1x3's are your subfloor, the OSB is the underlayment. I don't like 1/2" OSB cuz it's usually a cheapo grade, very absorbent material used mainly as sheathing or utility such as shelving. I don't trust bonding to it.

So, If everything is installed correctly and in good condition you can keep the OSB and install 1/4" backer as directed into fresh thinset, taped etc. Otherwise add 3/8" or thicker B/C underlayment ply then either 1/4" backer or Ditra. http://www.schluter.com/6_1_ditra.aspx Ditra is what you want for floors, not Kerdi as Joe thought. Save the Kerdi for your next shower installation.

You said;

Quote:

I would prefer to just screw the hardibacker into the OSB and not use thinset.

Where did you get that idea? That would be a very bad thing to do. I guarantee at minimum grout cracks within a few seasons.

Jaz

jep · 10-06-2012, 11:13 AM

I'm not sure if I understand the question about the span of the timbers. Those beams run from the front of the house through the house to the back and out the back to support a deck. I think they're 35' long or something like that.

I didn't give enough thought to the floor when I did the kitchen. Initially we were going to install linoleum but the boss later decided she wanted tile. THis was after the OSB was down and after I put the cabinets in. Increasing the height of the floor very much would cause problems with the dishwasher so I think I will go with the 1/4" hardibacker set in thinset. I do have a backup plan for the dishwasher: unscrew the butcherblock countertop above it when the dishwasher needs repair/replacement. I prefer not to take a route that would force me to use the backup plan but that flexibility does exist.

What is an easy way to cut a curved line in the hardbacker? Right now the transition will be from porcelain tile to carpet along a curved line. I guess the curve is a 1/4 circle with radius of roughly 18" . I do have a jig saw but I'd rather not use it on hardibacker.

pgc555 · 10-06-2012, 02:22 PM

1/4" Hardi cuts easy with the utility knife. Score it a few times and you can snap it. It would also cut with your jig saw - But will create dust. I'm with Jaz on this one. Thin set it down - you want to do this once,right?

jep · 10-06-2012, 02:29 PM

I'm definitely setting the hardibacker in thinset. I did a bathroom a few years ago and just screwed it down and it is ok so far. But this is a much bigger area.

Can you snap hardibacker on a curved line? I've snapped it on straight runs. Never had a curved line before. I could use the jigsaw outdoors. I was more worried about damaging the saw with the dust than spreading the dust around. I don't think it should be big deal if I do it outside.

pgc555 · 10-06-2012, 02:42 PM

It's not hard enough to damage your saw. Your talking one time cut - not a day to day use. If you score the board enough you will go right through it without needing to "snap"it

jep · 10-06-2012, 05:38 PM

5 hours later: The peninsula is 1/2" higher than it used to be. The dishwasher is in its cave sitting on hardibacker and tile (with no thinset yet) and has maybe a 1/2" of clearance. I think I am going to declare victory. I'll probably have to pay an extra day's rent on the fancy tile saw.

JazMan · 10-06-2012, 07:42 PM

Hardie is cut by scoring several times using a carbide tipped laminate cutter or similar. But when you snap, you have to snap up, not down. Or use a jig saw outdoors and have a fan blowing from behind away from you. Silica is not nice.

Jaz