Flatten a Wall for Tile With a Laser Plane and Depth-Gauged Shims

The Problem

Large-format tile shows every hump in the wall behind it — lippage between tiles is usually a framing problem, not a tiling problem. Studs bow, plates wander, and a backer board screwed straight to them copies every flaw. Flat tile needs a flat substrate, and "flat" has to be measured against something truer than the framing.

The Technique

1. Mark the stud faces at whatever spacing your backer board manufacturer requires for fastening — every 8 inches, in the demo's case.
2. Set up a laser to throw a vertical plane a fixed distance off the wall — say 4 inches — and square it to the adjoining wall. This laser plane is your reference; the framing no longer is.
3. Find the farthest-out point on the wall — the proudest stud — by measuring from the laser plane, and zero a depth gauge there. Everything else on the wall is now some measurable amount behind that point.
4. Work through every marked point, reading its distance against the zero. Each reading tells you the shim thickness that spot needs to come out flush with the proudest point.
5. Apply shims of the measured thickness at each point — adhesive-backed foam shim systems apply straight from a caulk gun, or use conventional shim stock — and check with a straightedge or level as you go.
6. Hang the backer board on the shimmed plane. The wall is now flat, plumb, and square before a single tile goes up.