Straight cuts in cement backer board can be made using a circular saw.
I just remodeled one of our bathrooms, which included tiling the floor with 12”x 12” Turkish tumbled marble. It was a fun project to work on (it’s a small room) and the tiles look terrific, but the most important step of the entire job was prepping the subfloor.
After ripping up the old vinyl flooring, I discovered that the subfloor consisted of two layers of ½” plywood; the minimum recommended subfloor thickness for tile is 1¼” So, I had to cover the plywood with ¼” cement backer board, which was set in a bed of thinset mortar and then screwed to the subfloor.
Cement backer board can be cut with a carbide-tipped scoring tool, just score and snap the sheet, similar to cutting drywall. But since I had several cuts to make, I decided to use power tools.
Now the first thing you need to know when cutting backer board is that its dust contains crystalline silica, which can be extremely irritating to your eyes and lungs. So, be sure to wear eye goggles and a dual-cartridge respirator when cutting backer board. Also, always make the cuts outdoors and well away from open windows and doors.
A jigsaw works best for cutting curves.
For straight cuts, use a circular saw fitted with a carbide-tipped woodcutting blade, but here’s a little secret: use a blade with the fewest number of teeth you can find. Ordinarily a blade with more teeth is desirable because it produces a smoother cut, but in this case many teeth would bog down and create clouds of thick dust. A blade with fewer teeth (mine only had six) cuts quickly and produces less dust.
To make curved cuts or circular cutouts in backer board, you have a couple of options. You could drill a series of holes along the cut line and then tap out the waste with a hammer.
While that technique works, it creates quite a mess and leaves a ragged edge. I prefer another approach that is both cleaner and quicker.
When I had to cut a 7” diameter hole in backer board to fit around the toilet drain flange, I used a jigsaw fitted with a metal-cutting blade and made the cut slowly. To be honest, I wasn’t sure the blade would cut the dense, fibrous sheet, but it did so quite easily with a minimal amount of dust.
However, I should mention that by the end of the cut I noticed that all the teeth on the blade had been worn down to mere nubs, so if you have a lot of circular cuts to make, be sure to have plenty of metal-cutting jigsaw blades on hand.