Step 5 · Set and Secure the Bathtub
For a rock-solid installation that won't flex or creak, setting the tub in a mortar bed is essential for acrylic and fiberglass models. Mix a bag of mortar in a bucket with water until it reaches a thick, paste-like consistency, similar to chunky peanut butter. Spread a 1- to 2-inch thick layer of this mortar onto the subfloor in the area where the base of the tub will rest (refer to the footprint of the tub). With your helper, carefully lift the tub (with its drain assembly now attached) and lower it into the alcove. The drain tailpiece should slide into the hole in the subfloor. Press the tub down firmly, embedding it into the mortar bed. Use your level across the top rails to confirm it is perfectly level in both directions. Once level, secure the tub's integrated flange to the wall studs. Use 1-1/2 inch roofing nails or washer-head screws, driving them through the flange into the studs. It's best to pre-drill holes in the flange to prevent cracking. Do not overtighten the fasteners, as this can stress and crack the flange.