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jonathan03 07-27-2010 11:24 PM

Hardie Board
I have two questions about Hardie board for two different projects.

1) I'm using the 1/4" variety on a bathroom floor. There is 3/4" plywood under it. I understand it needs thinset between the wood and hardie board. Can I use modified thinset? I know its more expensive but its a small area and I can use the rest to stick on the tiles. Also, do I have to wait for the thinset between plywood and hardie board to dry before applying the tiles?

2) I have a kitchen floor with 3/4" quarter sawn pine tongue and goove flooring. Can I put 1/4" hardie board over it and tile or do I need to pull up the tongue and groove floor and put down plywood? What if I just screw the hardie board into the joists with 2 1/4" screws?


PWilcox 07-28-2010 01:32 AM

1)The modified thinset will only make the bond stronger. And yes, you want to wait for it to be dry before walking or working over it.

2)As long as the T&G is solid and acts as one piece than you would be fine to attach your Hardie Board directly to it. I would definitely catch the joists where possible. I would also use a thinset between the two.

In your quest to reduce cracking it's always a good idea to use an isolation membrane, especially if it is over a large area.

General Contractor
Wilcox Construction
Orange County, CA

Bud Cline 07-28-2010 10:22 AM

These are EXACTLY the types of posts that gets me into trouble around here all too often.

The OP poses legitimate questions and along comes a respondent that is posting information in direct opposition to most of the rules of the day.


FlooringDude 07-28-2010 11:33 AM

1) Unmodified thinset is super cheap so just suck it up and buy a bag to be safe. You should wait for the thinset to dry but I normally put all my full tiles down on the same day I backerboard. I know people out there advise against this and it goes against TCNA rules but it makes no sense to turn a five hundred dollar job into a three day thing waiting for backer board, tile, and then grout, to dry.

2)This is not something I think anyone can assess without seeing it and feeling it. I have seen it done in the past but I would opt for replacing the subfloor if you are inexperienced with tile and flooring in general.

Proby 07-28-2010 11:49 AM

What's the deal with modified and unmodified thinset?

FlooringDude 07-28-2010 11:52 AM

I'm not positive so maybe someone smarter can chime in but...unmodified has no additives like latex or acrylic to help bond it better. You really only use it under backerboard to fill voids or as a thinset for tile on ditra. It's not used in your case for adhesive purposes.

oh'mike 07-28-2010 04:27 PM

Well howdy---First --The floor with 3/4" ply---I must assume the joist spacing is 16" and joist are sized for the span---do check that---

As to the thinset used --modified will be fine--once screwed down it is safe to set the tile immediately.

Next floor--Durrock or Hardy can not be safely installed over pine boards--ever--the pine moves to much over the course of a year.

Most common cure is to nail or screw 1/2 b/c exterior glued plywood--then your thinset--then the backer.---Mike---

Proby--unmodified thinset is a cement based thinset--when setting tiles it is used only over concrete--seldom used any more --

Modified thinset contains latex will stick to just about any surface that is common to tile installations.. Most popular--

Liquid latex additive---Used by many pros---added to the unmodified powder instead of water---remarkable holding strength--shorter pot life
---rather expensive---used when you need the extra holding power.

Proby 07-28-2010 04:57 PM


Originally Posted by oh'mike (Post 476869)
Liquid latex additive---Used by many pros---added to the unmodified powder instead of water--

I assume that's the Milk looking stuff I see the floor guys using in commercial work?

oh'mike 07-28-2010 05:21 PM

Exactly--add some milk--I swear you can stick steel to glass with that.--You do not want to put additive into the modified powder---Just doesn't mix right---

Bud Cline 07-28-2010 09:08 PM


I swear you can stick steel to glass with that.
H-m-m-m-m-m! OK, show me the spec's of any manufacturer that says that. I'd be curious to see and learn. So point me to those recommendations written on somebody's website:)

JazMan 07-29-2010 12:50 PM

Hey Mike,

Be careful what you write, some people are likely to believe everything that is written.

I'm sure you were kiddin about bonds to steel & glass, but it's not that unreasonable to believe it.


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