I have an enclosed screen porch under the orginial house roof that I have just permanently enclosed. I have already insulated the underside of the existing deck floor and am now ready to install 1/4" Hardibacker over the floor. My question is: "Should I lay modified thinset to the decking prior to the Hardibacker installation or will it suffice to screw the Hardibacker directly to the decking without first applying thinset?" My first inclination is not to apply the thinset because of the cracks between the decking boards. Most cracks are very narrow although a few are roughly 1/4". What do you think?