I am tiling in my shower and am in the process of putting up the cement board. My problem is that between some of the cement boards are gaps due to the walls not being entirely straight. The gaps are at most 1 inch thick. What is my best option to fill those in before tiling? Thanks

You could mix up some thinset(versabond)and trowel it into the gaps. Smooth it out flat and let it dry completely for a couple of days. Next use fiberglass tape for cement board and versabond and tape all your joints with the thinset. Let that cure and you are ready to tile.

We should discuss this a bit further if your gaps are 1". Is that 1" on the studs going verticle or 1" horizontal? It is pretty hard to install a backer board with gaps like you say, how did it happen?

It is not an inch think, the first time I looked at it, it seemed much bigger. Its about 1/4 inch and its were the to walls meet in the corner. The wall was not straight up and down so when I put the bored flush with the ground the sides tapered off. I will use pattyg2's advice it seems very workable. Thanks you

If it's in the corners then you need to use silicone after taping and thinsetting and applying the tiles. Use silicone for all corners and bottom edges and you will be fine. Grout everything else.

I would shim out the walls to make them even first, then re-install your cement board. Man, that's going to be a lot of work trying to make up those corners with thinset.

I tile, so I can vouch forgetting those wall straight before putting on the cement board. Make sure you have plastic behind that cement board too.