Expert: Bruce E. Johnson - 2/21/2011

Question
I am finishing a surround for a gas fireplace. I had to install hardibacker over the surround to meet the noncombustible requirements. What is the best way to adhere a natural stone to the hardibacker?

Answer
Hi Lance, All I ever used is mortar for attaching stone to a wall surface.  Depending on the stone you may also be able to use thin set mortar, the sanded type as used with tile.  My dad was into natural stone during his lifetime and he mixed his own mortar using cement, sand, and lime but you can buy premixed mortar now that has all the ingredients together and all you add is water.  For a little more adhesion you can paint the hardiboard with a latex additive and also use the additive in your mortar mix substituting some of the water for the liquid.  This additive can be purchased at most lumber or big box retailers such as Lowes or Home Depot.  It is found in the cement section and comes in a bottle.  It is either blue or white like milk but will dry clear and shouldn't affect the mortar color.  Follow all manufacturer's recommendations for any materials used.  Another factor with Hardibacker or any cementious substrate is moisture.  A dry backing material will suck the moisture out of the mortar too quickly in some instances leaving you with a poorer than average bond.  Hence the latex additive.  However, you can also dampen the wall board prior to attaching the stone, this prewetting process is important for a long lasting result.  Don't overdo it but have a big sponge and a bucket of water handy to wipe the wall ahead of your work to keep it moist but not wringing wet. I hope this information helps, please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com