Thinset is fine, but screw it as well. Use tap cons which usually come with the proper size masonry bit. Set the board in place and then drill your pilot holes through the board and into the masonry. It will be easier if you hit the mortar joints.
Christopher Wright, CR - WrightWorks, LLC - Named to the 2010 REMODELING Big50
[URL="http://www.WrightWorks.net"]www.WrightWorks.net[/URL] [URL="http://wrightworks.net/Company-Info.htm"]Indianapolis Remodeling Contractor[/URL] [URL="http://www.facebook.com/WrightWorks"]Facebook[/URL] [URL="http://twitter.com/ChrWright"]Twitter[/URL] [URL="http://www.formspring.me/ChrWright"]Formspring.me[/URL]
[B]"When we build, let us think that we build forever."[/B] --John Ruskin