Bath Tubs 2
This new section in TUBS will cover the application of backerboard as a substrate for tile. Remember, the backerboard takes the place of the mortar bed in the original section. It will be a good idea for the reader to familiarize with the original section for more details especially during the actual installation of the chosen tile.
This project involves a tub surround in “new construction” meaning that this is a new home and not a remodel. Therefore, it is necessary for the installer to determine that the previous trades did their work satisfactorily. The installer must determine that the tub is set and anchored properly; the studs are set plumb, on-plane, and the correctly spaced, and the plumbing is the correct distance from the stud cavity to allow room for membrane, backerboard, thinset, and tile.
The tub must be set according to the manufacturers recommendations relating to necessary framing, ledger, and anchors. Also, it must be set level across the back and sides. This will aid in the smooth running of the project and tile.
In terms of the studs, they must be spaced at no greater than 16” on-center, plumb, and on-plane. Remember, backerboard will follow whatever the studs are doing. To check the studs, the installer simply measures the distance between the studs for proper placement.
Next the installer simply uses a level to check them for plumb. Using the same level or straightedge, the installer checks the studs for being on-plane one to another.
As far as the plumbing is concerned, the installer uses a level or straightedge to measure from the valve face to the stud face. Each shower or tub valve assembly will indicate a “finish flush with wall” or similar maximum spacing which must be met in order for the valve trim and escutcheons to operate properly. If the valve assembly is not in the correct position, it must be moved prior to the backerboard installation.
Since the installer is very cautious and concerned about the final product, he protects the tub surface against damage during the installation by masking the tub thoroughly.
On most tubs, a flange is present that normally measures ¼” in thickness that will prevent an on-plane installation of backerboard and tile. This means that a system is necessary to “fur out” the backerboard from the stud face. An acceptable method is to use ¼” furring strips the length of the studs from the top of the tub flange to the top of the intended tile installation. These furring strips are simply held in place with two or three small nails. Remember that furring strips will be securely fastened when the backerboard is installed later.
The next step is the installation of the vapor barrier. Most backerboards in use today require this step, not all do. It is best to check with the manufacturer of the chosen board for recommendations. In this case 15lb roofing felt is used for the vapor barrier and is held in place with sufficient staples to the furring strips and studs.
The roofing felt is applied “roofing style” so that each upper layer covers the lower a minimum of 4”. The corners at each 90° should be tightly folded and tucked into the corners to avoid complications with the installation of the backerboard panels.
Next the backerboard panels are cut and fastened to the wall studs. The panels should be held ¼” above the top of the tub shelf and should partially cover the flange discussed earlier. There must be a minimum of 1/8” between the panels for the correct backerboard joint. They are held fast to the wall studs with (minimum) 1-5/8” backerboard screws at 6” on-center. This allows a minimum of ¾” penetration into the stud once the backerboard and furring strips have been penetrated (STB40-42).
Dr’s Tip: It is a good idea to have 2” #10 corrosion resistant screws on hand for the 90° corner joints as these fasteners will be driven at an angle and need a little more length to penetrate adequately.
Note: If at any time it is necessary to have a vertical backerboard joint in the field, a nailing stud for both sides of the joint must be provided. This generally would be if the tub had a seat or some similar arrangement where the walls exceed the normal 5 foot length. Horizontal joints do not need solid backing. For clarity, vertical joints must be solidly backed.
Note the nice fitting around the exposed plumbing fixtures. This is accomplished with a dry cut masonry saw. The smaller holes are cut with a masonry hole saw available at most home improvement stores.
The final step in a backerboard installation is the preparation of the backerboard joints. The basic steps are: the joints (1/8”) are filled with thinset completely, a layer of alkali resistant backerboard tape is placed in the wet thinset, and a skim coat of thinset is applied over the freshly set tape. The joint is smoothed over and made as flat as possible so that the joint is not out of plane with the rest of the board.
Once the backerboard joints have cured the installation can be tiled. In this case, the first course of tile was set on top of the tub as it was a very level surface. If the tub is not level, an alternate method for setting the first courses of tile must be used.
This system can include the use of a ledger system as seen in these photos. The ledgers simply establish a level starting point for the lower courses of tile. Basically, the position of the ledgers is so that a full tile is at the lowest point of the tub. All the rest of the tiles in the bottom course will be cut. This will also work with a ½ tile if preferred.
In any thinset tile installation, the thinset is keyed into the backerboard with the flat side of the trowel, then an even thinset bed is established with the notched side, and then the tile is placed into the fresh bed. The tile is pressed or beaten into the freshly combed thinset and aligned. In this case,spacers were used to keep the tile aligned during their curing time.
While the process is not shown in photos, the ledgers are removed and tile is cut and placed in the space left by the ledgers. The finished tile is cleaned and grouted and allowed to cure. A sealant is used in place of hard grout at the 90° corners and tub to tile interface. Following the grout cure a sealer is used on the grout joints.