There are many types of cement board available to use, however, American Clay recommends using HardieBacker® cement board. Durock® cement board can be used, although, the propensity for this board to crack over time may cause issues with the clay plaster after installation and/or will require further preparation prior to clay installation. American Clay strongly recommends against using Durock® cement board for these reasons.
Once the cement board has been installed onto the wall, follow the manufacturers guidelines (below) for taping and bedding the joints and seams:
HardieBacker®, JamesHardie Manufacturers Guidelines:
Fill all joints with mortar (Latex or acrylic modified thin-set or Type 1 mastic) and immediately embed 2 inch wide high-strength alkali-resistant fiberglass tape in the mortar and level.
Alternatively: American Clay also recommends using setting-type joint compound (also known as "hot mud") with fiberglass tape to tape and bed joints and seems. All purpose, pre-mixed joint compounds are not suitable. Examples of setting-type joint compounds are: "Durabond® 90", "Sheeetrock® Easy Sand 45 (or 90) Minute Setting-type Joint Compound", and “Rapid Set® OnePass® Wall Repair Material and Joint Compound”. Setting-type joint compounds come in a dry powder form and are mixed with water upon application.
Durock®, USG Manufacturers Guidelines:
*Please note: even with these guidelines, American Clay cannot guarantee that the Durock® cement board will not crack over time and show through the plaster coat. Warranty of the product will be voided.
Fill all joints with latex-fortified mortar or Type 1 organic adhesive and immediately embed 2 inch wide high-strength alkali-resistant fiberglass tape in the mortar and level. Let dry completely.
Additionally: Using the same mortar or adhesive, completely coat the entire board. Let dry completely.
After the above guidelines have been completed to prepare the cement board: prime the entire surface with an "Approved" paint primer mixed with American Clay Primer Sand additive. This will also help to even out any suction variations between the joint compound and the cement board.