I have a vertical 7/8 inch deep flat depression along a brick bench. The space to fill is 1 foot high x 16 feet long and I'd like to fill the space with 1/2 inch backer board. Can I do that with just using thinset so I don't have to drill holes into the brick for screws (I doubt I could match up the holes) ? The backer board's bottom edge would rest on the floor. I'm tiling over the backer board. Thanks.