How important is it to leave 1/16" to 1/8" gap between backer boards so that thinset can get in between when we tape? If the backer boards are flush up against each other, what issues will this cause in the future?
Also, in the hardibacker installation guide, it says screws should have be 3/8" from edges and 2" from corners. How important is this guideline? If we screw right at the edge, is there a chance the backer board could separate from the wall?
I don't worry to much about the gap size---I do not use Hardibacker, but with Durrock, it makes no difference.
You should be painting on a water proofer before tiling--that Hydroban or Redguard will fill the small gaps.
That stuff is brittle and can crack off the corners if screwed to close to the edge---
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I'm not a pro, and have only done maybe a half-dozen bathrooms, all with Durock (tried Hardi once for a tile floor and didn't care for the stuff). I have never left any gap between sheets. Not have I ever had a problem with any of the installations. I would not put screws right at the edge as the backer would probably crack. Good advice above to use something like Redgard. It's a bit expensive (maybe $45 a gallon at Home Depot) but it doesn't take a lot.