I am installing 1/2" hardibacker cement board (Hardiebacker 500) around bath tub. I have read somewhere on other forums about leaving 1/8" gap between sheets to compensate for an expansion. However, install instructions posted on James Hardie's website only recommends leaving 1/8" gap between sheet edges for countertop and subfloor installs, and instructs not to force edges together. Nothing about leaving gaps for wall installs.

I wonder if the 1/8" gap between boards is really necessary. The cement board appears to be structurally solid. Please share your experiences.