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Retrofitting A Whirlpool Tub, continued

Plumb and Level

If the wall studs of the tub alcove aren't straight and plumb, I correct them with shims. I also check the floor for level. A fiberglass tub, unlike a cast-iron tub, can't be leveled with a ledger board under the back rim. The tub's weight will be carried by the base, which should be uniformly supported. Otherwise, the tub can be stressed and might crack. If the subfloor isn't level, there are three options: 1) shim the base of the tub carefully, following the manufacturer's instructions; 2) install a new, level subfloor; 3) install the tub on a mortar base. I don't recommend using aerosol foam to help support a tub, since it's extremely difficult to judge how much the foam will expand. Too little foam gives inadequate support, and too much can actually move a tub. I often install Aker tubs, which come with a fiberglass base that requires uniform support and cannot be easily shimmed. In a remodeling situation, where the existing floor is typically not level, I usually find it easier to remove the floor under the tub area down to the joists. This allows me to shim the joists level and then install a new plywood subfloor under the tub. Once the tub is installed, I screw through the flange on the base into the subfloor. (To avoid cracking the flange, I predrill before screwing.) Some manufacturers, including Aquatic, recommend that their whirlpool tubs be installed in a mortar base. A mortar base holds a tub securely and reduces vibration, giving it a solid feel. For mortar, I use a standard bagged mortar mix like Quikrete or Sakrete. I'm careful not to use too much — the tub shouldn't be floating on the mortar. Under the tub base, I drop dollops of mortar, like putting cookies on a sheet, so there's room for the mortar to squish together as the tub settles in. There should be no activity in the tub for 24 hours after installing a mortar base. Once the tub is secured, I insulate it with fiberglass batts to reduce noise and vibration when the tub is running.

Providing Power

A whirlpool pump comes with a cord and plug and requires a GFCI-protected duplex receptacle. The electrician should wait until after the tub is in, to be sure the receptacle isn't installed in an awkward location that blocks access to important components. Since a GFCI receptacle can't be used in an inaccessible location, the whirlpool receptacle should be either on a circuit with a GFCI breaker or downstream from an accessible GFCI receptacle. Once the power and plumbing have been hooked up, I once again test the tub for function and leaks to ensure that nothing was damaged during installation.

Backerboard and Tile

For a tile base, I use Hardibacker (James Hardie Building Products, 888/542-7343;, which I prefer to other cementitious backerboards. I set the bottom of the backerboard approximately 3/16 inch above the top of the tiling flange, so it won't interfere with tub movement. If the backerboard overlaps the flange, the backerboard and tile will flare out at the bottom. This flaring can be especially noticeable at the inside corners. I use temporary shims to maintain the gap between the flange and the backerboard while the backerboard is being fastened. When it comes time to install the finished wall surface, I extend the tile or tub surround material beyond the bottom of the backerboard, overlapping the tub flange but stopping about 3/16 inch short of the tub edge. If a customer wants to install an island tub (a tub without a tiling flange) in an alcove, I notch the studs at the level of the tub rim, so that the beveled part of the rim is recessed behind the backerboard. The backerboard and tile are then brought down to within 3/16 inch of the tub. Whether or not the tub has a tiling flange, I install silicone caulk, not grout, between the bottom edge of the tile and the tub, because caulk allows the tub to move independently of the tile.

Building the Skirt

There are two types of whirlpool skirts: factory skirts and site-built skirts. Some manufacturers offer tubs with a removable skirt that provides access to the plumbing and wiring while giving the exposed side of the tub a finished look. On many jobs, I make a site-built skirt out of 2x2 framing followed by fiberglass sound insulation batts, backerboard, and tile. A site-built skirt is usually installed so that the finish surface is slightly recessed from the front rim of the tub. But in some cases, I will build a skirt that includes a horizontal shelf in front of the tub, designed to be finished with tile (Figure 4). As long as the shelf is located on the open side of the alcove and cementitious backerboard is used as a substrate, I install it without a waterproof membrane under the tile. I have had no water leakage problems with such shelves.


Figure 4. A site-built skirt can include a tiled shelf just below the tub lip.

To avoid the sizable gap caused by butting square-edged tile against the beveled rim of the tub, I install the horizontal tile under the rim. I carefully check that the tub rim, even when flexed, puts little or no stress on the tile face. People are likely to sit on the rim of the tub when they get in and out, and it’s critical not to stress the horizontal tile.

In an island installation, a whirlpool tub will usually have a skirt on all four sides, with a horizontal shelf all around, giving the tub a drop-in look. Such a skirt is built in exactly the same way as the site-built skirt in an alcove installation (Figure 5).



Figure 5. Self-rimming tubs can be installed in either an island or an alcove but are not intended to be combined with a shower.