Tile How-To: >> Showers 5
Backerboard application
This section will cover the application of backerboard in a shower installation. The backerboard basically takes the place of the mortar bed as the substrate for the tile. In this section we are not going into a great deal of detail as we did in the original section on shower construction. It would still be helpful for the reader to go through the original section.
Many of the currently manufactured backerboards are suitable for a shower installation. In this particular job, we are using a cement backerboard. Please visit our section on showers and tubs for additional information.
The installation covered in this section is new construction. This means that no shower or tub existed in this space prior to this installation. This means that the installer will first have to check the previous trades work to determine if the work was done satisfactorily.
The first step is exactly that, inspect that the previous work was done properly. Therefore, the installer will first make sure that the plumbing, both drain and in-wall valve assemblies, are at the proper level or placement. Next the installer will check the studs for being positioned properly (16” on-center), plumb, and on-plane with each other. Remember, backerboard will follow whatever the studs are doing. If the studs are not on-plane or plumb, neither will the backerboard.
Note from these photos how the installer is checking the studs for these issues. The installer determined that the shower drain was as close to the middle of the shower floor as possible. Next, the installer verified that the shower valves and related plumbing were at the proper distance from the stud face to allow proper function of plumbing fixtures.
This installation includes the building of a tile lined mortar bed shower pan. Solid blocking is applied to the stud cavities 12 inches above the floor level and four two-by-fours are stacked and nailed in place to form the shower curb.
Next, the installer will pre-slope the area beneath the shower pan membrane by installing a wood ledger that will set the proper elevation for adequate drainage. In this case, the height needed to be ¾” to establish the required ¼” per foot pre-slope to drain from the side walls.
Next the 15lb roofing felt and 2.5lb metal lath are installed and stapled in place. Wall mortar is then used to establish the pre-slope over which the shower pan membrane will be installed.
Once the pre-slope is dry, the installer will set about the task of applying the shower pan membrane. The installer measures the space and marks a level line around the perimeter of the solid backing where the membrane needs to extend.
The membrane should extend a minimum of 3” above dam or curb height and lap over the dam face.
The membrane is loosely placed inside the space and the drain flange bolts are punctured through the membrane (STB64). A bead of membrane caulking is applied between the membrane and lower flange and then the upper flange is clamped tightly in place.
The membrane is then fastened to the solid blocking with staples to facilitate the proper positioning of the membrane with all necessary folds. Finally, fasten the membrane every 6-8” with 1” galvanized roofing nails. Be sure to use membrane caulking at any cuts made near the dam interface to prevent leaks.
Fill the pan with water for 24 hours and check the underside, if possible, for leaks. After 24 hours, carefully remove the membrane from the drain opening and allow the water to evacuate.
If local codes mandate a drain plug for the water test, simply remove the plug.
Dr’s Tip: From this point on, great care must be exercised to avoid puncturing the membrane. All movement, the use of tools, other trades in the area, must be closely scrutinized to avoid a puncture.
In this installation, a mortar bed will be applied to the curb or dam that explains the 2.5lb metal lath seen in that area. Due to the thickness of the membrane and its folds, furring strips (1/4” thick) are applied to all bearing stud faces and cavities.
Next 15lb roofing felt is applied over those furring strips in roofing style. This means that each upper layer overlaps the one below approximately 4”. The roofing felt is simply stapled in place. The roofing felt needs to overlap the shower pan membrane 4”.
Dr’s Tip: Make sure to somehow mark or measure the positioning of all studs or fastening surfaces that are beneath the roofing felt. Normally, the installer can feel the studs and furring strips below the felt. In the subsequent steps, the backerboard fasteners will need to penetrate the studs as close to the center as possible.
Next the backerboard panels are cut to fit and installed. Be absolutely certain that the backerboard panels do not contact the membrane at the bottom at any time during their installation. This can be accomplished by using pieces of ¾” wood without sharp edges to rest the boards on while installed.
Fasten the panels with corrosion resistant backerboard screws 1-5/8” (minimum) in length at 6” on-center. Note that the joints are horizontal, have a minimum of 1/8” gap between sheets, and no vertical joints in the field are present. If it is necessary to have in field vertical joints, a stud(s) must be provided so that the joint between the two panels starts and stops on solid backing.
It is sometimes helpful to have 2” corrosion resistant (#10) screws on hand for the 90° corner joints as these fasteners will need to go in the boards at an angle to hit the corner studs.
These 1/8” joints between the panels and the corner joints are treated as follows: The joints are completely filled with thinset, alkali resistant backerboard tape is applied on the first layer of thinset, and that is followed by a finished skim coat of thinset. Make these joints as flat as possible to avoid high and low spots on the finished wall.
This installation included a somewhat new technology in the use of foam core niches used as shampoo and shower accessory shelves. The location for these niches was framed out prior to the application of the backerboard. The edges around them received the same furring strips and treatment as other areas. A principle difference in this product is that the niches are imbedded in silicon adhesive and are watertight when installed.
All the backerboard surfaces that will receive tile are now finished. Wall mortar is prepared and the dam surfaces receive their mortar bed. This was easy on this job as the backerboard provided the proper elevation to use a piece of wood to cut off the excess. For more info on this portion, review the original section on showers.
The next step is to apply the mortar bed for the shower floor. The mortar mixture (dry-pack) is prepared (1 part cement to 4 parts damp sand) with an ad-mixture (anti-hydro, etc.). Next corrosion resistant wire mesh is cut to fit leaving room the shower drain assembly.
Before the first dry-pack mortar goes on the floor, crushed tile or stone is added around the drain weep holes to keep them open under the finished mortar bed. This will allow accumulated moisture to run down the drain and not be trapped in the finished mortar bed.
Half the mortar bed goes on top of the membrane and is followed by the corrosion resistant wire mesh (the only type available in this locale). The reason for this step is to place the wire in the approximate middle of the mortar bed for strength and shrinkage resistance.
Next columns of mortar are erected at the perimeter of the bed as a means of cutting the bed with wood or metal screeds to the proper plane and slope (1/4” per foot).
Use levels to accurately place these erected columns. Note how a combination of wood screeds and a wood float trowel are used to tamp and finish the floor mortar.
The backerboard shower installation is now ready for tile. Most backerboard manufacturers require the use of modified thinset mortar for the tiling process. Check the recommendations for the backerboard chosen. Also, a good practice is to tile the shower floor first and then tile the walls above.
