Hardi Backer board warnings?


Old 01-27-06, 01:12 PM
Thread Starter
Join Date: Jan 2006
Location: Missouri
Posts: 28
Received 0 Votes on 0 Posts
Hardi Backer board warnings?

Ok - firstly this is the first time I've worked with silica products (at least since I've been seeing warnings on even silica sand as of the past several years).

I drilled some holes into it becuase that's what the book I had said to do with cement board. It kicked up dust and I'm just wondering how harmful of an issue this is as a one time thing. Is this just like smoking is bad for you but people do it for years (usually) before the effects compound? Do I need to get a cleaning service in my garage for 12 holes drilled in (ps - I was stupid and wasn't wearning mask but I have since I read it)

Secondly - I failed to leave the 1/4 inch around the walls. Is this going to end up being a problem and whY?

Lastly do you HAVE to mortar the board down? Someone at HD told me they did theirs without. Someone else said just use glue. The book and the board both say thinset motar.

Thanks for your advice.
Sponsored Links
Old 01-27-06, 07:48 PM
Tileguybob's Avatar
Join Date: Dec 2004
Location: Ocean Grove, NJ
Posts: 931
Received 0 Votes on 0 Posts
Yes there is a lung disease brought on by over exposure to silica. I cant pronounce it much less spell it so I wont embarrass myself here. Vacuum up as much as you can, then use a damp old towel to get the left behind stuff. Open the doors and windoes on a windy day and let nature take care of it.

By not leaving a 1/4" gap at the room perimeter you run the risk of the walls expanding with seasonal weather changes. If the expansion is strong enough it will push on the tiles and cause some in the middle of the room to fly up off the floor. Its not a given that this will happen but if it does and someone is standing there it will be scary at best and dangerous at worst with the flying pieces of tile. This condition is called "tenting".

Thinset is absolutely needed under the cement boards. It is NOT there for bonding like the glue guy thinks it is (too much sniffing on his part) the nails or screws adhere the board to the floor. The thinset is there to fill voids between the plywood and the cement board that would otherwise compress under point load pressure causing the tile to crack.
Old 01-28-06, 12:56 AM
Thread Starter
Join Date: Jan 2006
Location: Missouri
Posts: 28
Received 0 Votes on 0 Posts
Thank you again Tileguybob

I appreciate the advice. So I probably didn't get lung cancer from the one exposure I hope

I will recut them a bit to allow for the gap.

And I'll use the motar. I was mainly worried about whomever redoes the room again down the road and the trouble they will have getting all of that up.

Oh well. I guess that's just the way it is
Old 02-07-06, 10:19 PM
Join Date: Jan 2003
Location: USA
Posts: 96
Received 0 Votes on 0 Posts
I have a similar question regarding this "gap" when installing Hardibacker.

I am using 1/2" 3' X 5' for steam room wall. Does it mean I have to give 1/4 gap between each board? After installed, do I need something to fill up the gaps?

Also, I have couple of 1/2" pipes (such as shower head, body jet, etc.) go though the backer board, what material I have to use to seal the gap between the hole of the backer board and the brass pipes?

Thanks in advance for your advice.
Old 02-08-06, 03:27 PM
Bud Cline's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Nov 2001
Location: Central Nebraska
Posts: 1,300
Received 0 Votes on 0 Posts

HardiBacker is manufactured by the James Hardi Company and on their websit you can find the (Required by Law) MSDS (Medical Safety Data Sheet) for the HardiBacker product. The MSDS contains information about the hazards of cutting and sawing HardiBacker. You should take a look at this information before you continue to work with this product.


The 1/4" gap referred to applies to the perimeter of a room and not each board however each board should be gapped slightly then each gap covered with alkali-resistant mesh joint tape and then filled and bedded with thinset mortar.

Did you also know that it is recommended that steam rooms have the walls (and ceiling) waterproofed on the surface of the wallboard before any tile is installed?
Old 02-09-06, 10:01 PM
Join Date: Jan 2003
Location: USA
Posts: 96
Received 0 Votes on 0 Posts
Thanks, Bud Cline. I found the info about mesh tape and thinset to connect two boards on www.hardibacker.com.

Would you mind helping me to understand how to fill the gap in the corner?
Do I use the same thinset, or silicone/rubber seal material?
Do I use the thinset for the connection with the old wall which I didn't replaced?
Old 10-20-09, 06:53 AM
Join Date: Oct 2009
Location: Texas
Posts: 1
Received 0 Votes on 0 Posts
I am putting in a granite tile counter top in my kitchen and just finished putting the hardi board down over the plywood. What i forgot to do was put thinset down between the plywood and hardi board. I know it was mentioned that the reason for the thinset is as a gap filler to prevent tile cracking, which makes sense on a floor, but i am wondering how critical it is for a counter top. Do i need to unscrew the hardi board, put thiset down and screw it back down? thanks.
Old 10-20-09, 07:21 AM
Join Date: Jan 2006
Location: USA
Posts: 3,505
Received 4 Votes on 4 Posts

These instructions are right from the James Hardie website. Yes you need a bed of thinset under the hardie.

Countertop Installation
1. Ensure cabinets are level and secure
• Use minimum 1/2” exterior grade plywood or equivalent, positioned across the wood
cabinet. Space between plywood supports is not to exceed 16” on center.
2. Determine layout of HardieBacker cement board
• Do not align HardieBacker cement board with plywood joints.
• Score and snap boards to required sizes and make necessary cutouts.
• We recommend an 1/8” gap from board edges.
3. Attach HardieBacker cement board to countertop
• Apply a supporting bed of dry-set mortar or modified thinset to plywood with a 1/4”
square-notched trowel.
Mastic can also be used with a 5/32” V-notched trowel.
• Use the fastener pattern as a guide. Fasten HardieBacker cement board with specified
nails or screws (as listed in “Materials Required”) every 8” over the entire surface.
Keep fasteners 3/8” from board edges and 2” in from board corners.

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: