Ask the pros-April 2010

By Reggie Bullard



  What is a durock system?

Can I apply stucco mortar over durock ?
mortar falls off durock Mortar falls off durock when 
durock is lifted up. This is a sample board made by the  competition.
The durock system was a synthetic stucco system made by United States Gypsum in the 1990's and was discontinued by USG due to water infiltration.

An acronym for this method is DEFS, of Direct Applied Finish System.

I have heard there are people still using durock with a synthetic basecoat and a synthetic finish coat. This method isn't recommended by USG  and should be stopped. USG is the manufacturer of durock.

Cement mortar or conventional stucco basecoats won't bond to durock (cement board, also wonderboard, hardie backer, etc.) A synthetic basecoat, or EIFS basecoat will bond, because synthetic basecoat is an adhesive. Only a synthetic finish will bond to a synthetic basecoat. This is the same synthetic finish that traps water against the wall on EIFS

I spoke to a USG representative about durock synthetic stucco shortly after it was discontinued. The main reason was water infiltration and trapping water against the wall. Also, the rep told me, it was specified by USG to use felt paper over wood studs or sheathing prior to installing the durock board. This practice was rarely used.

Durock is a good crack resisting substrate for stucco, providing metal lath is applied over the durock, attaching to the wood through the durock with nails or screws.

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