Large corner gaps between installed cement board

Fantastic forum and very helpful!

I removed my bathroom walls (around tub - 3 wall alcove) to stud then insulated/ vapour barrier and screwed cement board to the studs. In the process of cutting the cb and installing to walls the gaps in the corners (ceiling to wall corner; wall to wall corner) are anywhere from 1/8 to almost 1 inch wide (thanks to my rookie cutting abilities-groan). What should I use to fill these large gaps or will this even be a problem when it comes to adhering the tile? Probably will go with 8 or 12 inch tile.

Thank you for your help.

With a large format tile, it isn't as big a problem. I'd mush a bunch of thinset in there so you don't have a cavity - try to coat the edge of the cbu so it will stick. Note, you should use the reinforcement tape on the seams and corners. This is a mesh tape, but make sure you get the stuff designed for cement work - it looks like the same stuff you use on drywall, but isn't. You won't be walking on the walls, so 100% support isn't as critical.

While some people grout corners, the industry standard is to caulk all changes of plane. You can buy color matched caulk in both sanded and unsanded consistencies.

Next time, measure twice, cut once!

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