My home was built in the 1950's and I'm in the middle of a full bathroom renovation. Basically I installed some door casing today, nailing it to the door box/frame. As expected, this left an ~ 0.33" gap between the casing and the cement board (since the cement board is screwed to studs which sit deeper than the casing). This will be fine as the wall will be tiled, and I can make up the difference with grout epoxy at the seams. However, the casing can "flex" i.e. if I push on the side of it furthest from the door frame / nails, it will bend toward the drywall until it makes contact. This bodes poorly for my tile / grout seams.
What's the best way to prevent the casing from flexing? I'm thinking about inserting some kind of shim in the gap every 6" or 12", but I don't really know how to make those. Maybe there's some kind of self-expanding solution I can use? Thanks.