I am putting in a new bathroom with a tub. I plan to cement backerboard, but have a question concerning it's installation. The tub I purchased has a 1" lip around the three wall sides. When installing the backerboard do I overlap that lip or just set the backerboard on the top edge and fill in the remaining gap with something?

I have been told to overlap the lip with the backerboard.

I ran into this just a couple weeks ago and ended up at my own judgement overlapping the lip. Problem then was when it came to tiling the walls were not perfectly straight. The thickness of the lip pushes the backerboard out and out of plumb. This was more evident in the corners while cutting tile.

I'm not an expert but if I was to do it again, I would not overlap the lip and place tin flashing over the joint to ensure no water leaked through.

I am sure the lip is meant to go behind backer board. Could you cut a recess in the back of the board for the lip?

Thanks everyone, I'm sure you right about placing the backer over the lip, but as Chadamataz says it pushes the board forward out of plumb.

The tiles around my tubs are actually set up to accommodate 2 layers of dry wall/green board/whatever behind them... the bullnose tiles have a large lip on them allowing for this.

You could always trim out a section of your cement board with an angle grinder and diamond blade to make it fit the "lip"... I have thought about doing this before.

Good luck