Maax 420005-L-501 B3square 59-7/8" X 31-7/8" Rectangular Shower Base - White

As part of the product team at MAAX, I've seen a lot of B3 acrylic alcove shower bases go in. The installs that last all have one thing in common: the base is properly leveled and fully supported from the start. Step 1 – Assess & Prepare for Shimming Begin by flipping the unit to assess the underside and identify any low points, using a tape measure to confirm shim placement distances from your reference wall and marking positions directly on the plywood subfloor. Place wood shims at the identified low spots — particularly along the rim and corners — before setting the base in its final position. Step 2 – Level the Base Once the base is set, start leveling by placing your level across the threshold from left to right, then check both sides front to back, adjusting shims as needed. As a final confirmation, place the level across the back from left to right — if the first two checks passed, this should read level. It's important that the level spans the full width of the base from back to front to get an accurate read. Step 3 – Fine-Tune & Secure the Shims With leveling confirmed, use a steel ruler or precise measuring tool to verify the exact shim thickness needed at each corner — measurements should be taken down to 16ths of an inch, so don't eyeball it. Tap shims snugly into position, trim any excess with a utility knife, and do a final check under the base to confirm there is no flex or movement. The drain rough-in should also be verified at this stage before the base is permanently secured. A base that doesn't rock, flex, or shift underfoot is one that won't crack, leak, or fail down the road — take the extra time here and the rest of the install will thank you for it.