Tile Installation: >> Methods/Preparation (Walls)
Let's start out by choosing a method. The method includes how the substrate will be prepared to receive the tile, the necessary materials, and can include the type of tile used. This will depend on the area in which you wish to tile and the level of performance required.
- What is the existing material and how is it supported?
The most common types of supporting material for walls are masonry, concrete, drywall, and open wood and steel framing.
In the section "surface prep-wall & floor" the discussion focused on the supporting materials and how they should be prepared for tile. What were not discussed were the actual methods used over the suitable solid framing structure. In choosing a method, the old saying "good, better, best" applies. Pick the best method your budget will allow. Failed installations are costly and avoidable. I will never forget the first time I heard, "Why is there never enough time to do it right but always time to do it over?"
In selecting the best method, select the method that will suit the space and will provide the most stable surface for the tile. In addition, select a method that will give lasting performance. There are many variables. The very best method for the installation of wall tile is a direct bond to a mortar bed. For a mortar bed, this is not always the most practical approach. Depending on the application requirements, it may be better to chose one of the other acceptable methods.
This is the primary reason so many methods have been tested and are approved. Tile can be set successfully in a variety of ways and on a variety of surfaces. In addition to the normal substrates like masonry, mortar, cementations backer boards, and drywall, there are adhesive Manufacturers that indicate within their literature that their adhesives will bond tile to tile, paint, epoxy coatings, and steel plate. It is mandatory that the Manufacturers instructions be closely followed in this case.
Let's look at some typical residential and light commercial installations and what methods might suit your needs. Remember to look in the section covering expansion joints for specific requirements in the installation of wall tile. Especially as it relates to the treatment of corner joints and abutting surfaces.
In Figure A, we see a typical "one coat" mortar bed method. The reason it is called a "one coat method" is that the mortar is floated in one coat. There is not a scratch coat involved in this method. The principal element with this method is that it needs to be supported by solid backing.
You will notice that the method involves the use of a vapor barrier membrane. This separates it from the solid backing material of drywall, plaster, masonry, or other backing that will allow the solid fastening of membrane and metal lath. This is a very good method for tub and shower enclosures over drywall. In wet areas using drywall for solid backing, the drywall should be the water-resistant type.
The method is actually quite simple. The wall area must have solid backing in place. A continuous layer of 15 pound roofing felt or 4 mil. Polyethylene sheeting is applied to the backing. Build the vapor barrier membrane in the "roofing" manner meaning that the membrane starts at the bottom and upper layers are applied up the wall. This is designed to allow moisture, if any, to run down the membrane to lower elevations without running behind the membrane into the wall space.
The membrane must be overlapped at any joint a minimum of 2" horizontal and 6" vertical. Also, the membrane must flow over the tub flange or shower waterproof membrane. Go to "Showers & Tubs" for specific details.
The membrane can be held in place by a few staples located at strategic places. This is a good time to discuss layout. The membrane can be an excellent guide to indicate the outside perimeter of the installation. In other words have the membrane edge serve as the outside edges of the tile work. The mortar bed would extend to the end of the membrane and reinforcement and trim tile would be installed at that point.
For example, when doing three-wall shower or tub enclosures, calculate full tile measurements prior to installing the membrane. This allows the installer to float the walls, described later, and know that there would be no need to cut tiles on at least two walls. It also allows the installer to use the membrane edge to place the float strips accurately.
For all one-coat methods the following procedure needs to be followed. Once the membrane is installed, the 2.5 lb (Min) metal lath is fastened to the solid backing. This needs to be accomplished with corrosion resistant nails, screws, or in the case of masonry, powder actuated nails. The metal lath needs to be cut at all 90° corners.
Regardless of the fasteners chosen, the fasteners need to catch at least three strands of the lath and penetrate the solid backing and studs, if any, to at least a depth of 3/4". For screws and powder actuated nails, this might mean the use of a corrosion resistant washer is necessary. Be sure to check local building codes for requirements in fastening membranes and reinforcing wire. The local codes could be different than those shown here.
Once the wire reinforcing is in place, the wood float strips are cut to the desired length. Many tile mechanics place their wood floats vertically in the installation. The float strips can also be placed horizontally. In either case, the strips are cut and pre-moistened so that they do not wick too much moisture out of the mortar bed making adjustment difficult.
If the installation is a tub or shower, case must be taken to not damage the tub or installed shower pan. They must be protected with sufficient material to guard the surfaces from damage.
Now it is time to mix the mortar used for a one-coat method. ANSI recommends that mortar for walls have 1 part Portland cement, 1/2 part hydrated (type s) lime and 5 parts damp sand up to 1 part Portland cement, 1 part hydrated lime and 7 parts damp sand. When hand mixing, combine the dry ingredients first then add the potable water. If using a mixer place water in first.
Nevertheless, add only enough water to bring the mortar to the following consistency. When you draw a tool through the mortar a groove will form and not collapse. The mortar needs to be workable and not too wet. Remember the more water in the mix, the more will have to evaporate and cause shrinkage.
Let's talk about the application of wall mortar. Some installers bring the mortar to the wall site in 5- gallon buckets. They then use a combination of a margin trowel and finish trowel to apply the mortar to the surface. In the industry it is faster to master the use of a "hawk" in your non-working hand and the finish trowel in the other. In this way a larger amount of mortar is at hand on top of the hawk to apply to the wall. Some workers have been known to use both at once on large walls to speed the process.
Nevertheless, force the mortar into the wire mesh a tightly as possible with the initial coat. The idea here is to imbed the mortar fully into and behind the reinforcement. If the reinforcement can be fully encased, it will have a better chance of resisting corrosion. Once the wall area has been initially coated, the float strips can be set in place. Build a column of mortar for the float strips at the desired locations. The number of strips is dependent on the size and length of the wall.
If setting the strips vertically, the strips must be positioned to allow the installer to reach all ends of the wall surface with the metal featheredge or screed. The same is true of the horizontal method of setting the float strips. My horizontal method is only useful in tub and shower enclosures that involve a back wall.
In a three-wall tub of shower enclosure, start at the back wall first. Set the lower float strip measuring from the edge of the membrane as discussed earlier. Use a level or straight edge to imbed the float strip to insure that it is flat along its length. Failure to set the float strips flat will result in the wall being floated in an uneven plane. Remember the float strip sets the final dimension of the finished floated mortar bed.
After the lower float strip is set and adjusted, set the upper strip in a similar manner. However, use a level to plumb the strip accurately. Next set the sidewall float strips in a similar fashion starting at the bottom first. However, use a square between the back and sidewall float strips to insure that the two walls are square to one another.
Squaring the side and back walls is critical when building an installation that will receive a tiled floor. The beauty of the mortar system is that walls can be built square to one another. This allows the tile on the floor to be installed square, straight, and full if desired.
If setting the float strips vertically, simply imbed the strips using a level to the desired depth. In this way the plumb of the strip can be accomplished along with the depth measurements. In long walls, like wainscoting's or gang showers, the vertical float strips work best.
Dr's Tip: In long walls, use a "dry line" to keep the float strips aligned along the surface area. A "dry line" is masonry string positioned by nails to represent a straight line in front of the wall. This line can be flush with the intended final wall surface or backed off to a known distance to measure from.
In either case, imbed the float strips into the column of mortar to represent the desired finished plane of the mortar bed. This is accomplished by knowing the desired thickness of the bed. In the one coat method, the mortar bed can be as thin as 3/8" and as thick as 3/4". If the solid backing is plumb and in accurate plane, the mortar bed should be a consistent depth throughout. It would be best to keep the bed in the 1/2" to 3/4" range for strength.
Once the float strips are in place, "scratch" the imbedded first coat of mortar to allow the final finish coat to grip better. This can be accomplished with the use of a heavy 1/2" X 1/2" notched trowel of a scratch coat tool described and pictured in the "Tools" section.
At this point, the installer begins the process of filling in the spaces between the float strips. Starting from the bottom, build the mortar by eye to a depth a little deeper that the float strip depth. Then use the featheredge or screed to cut off the excess. While this step is in progress, be sure to fill in any holes or depressions encountered. You will be surprised at how fast this technique can be mastered.
While the mortar bed is still plastic, the float strips need to be removed and the recesses filled in with mortar. Accomplish this by using a margin trowel to cut along both edges of the float strip to loosen them. Gently pry the strip from one end carefully loosening it as you go. Fill in the recess with a combined use of a margin trowel; finish trowel, or wood float cutting the excess mortar off the top of the recess. Once this has been accomplished, give the mortar bed a smoothing trowelling with the finish trowel or wood float.
Tile can be applied directly to a fresh mortar bed with a trowelable paste made from Portland cement and water. A common preference is to allow the mortar bed to cure and set the tile using the thin set method. ANSI A 108.1 A-3.2.1 recommends that a mortar bed cure at least 20 hours at 70° F prior to the thin set method. The longer the mortar bed cures the better. In fact, some Manufacturers of thin set products recommend longer curing time. Consult the product literature for the instructions for the product that you wish to use.
If the wall area does not have solid backing a different method will have to be chosen. A common preference when encountering open stud areas is to install solid backing then proceed with the one coat method. The difference in floating a mortar bed without backing is found in the addition of a scratch coat of mortar and a thicker final "brown coat" of mortar. This can be seen in Figure B.
In Figure B, note that the mortar bed thickness is greater than that of the one coat method. In many cases, this fact alone makes the one coat system more desirable. Also, the initial "scratch coat" adds one more day to the job since it needs to cure at least 24 hours.
In practice, the metal lath and membrane are applied to the studs in the same manner as in Figure A. The scratch coat is similar to the initial coat indicated in Figure A. However, the work then ceases for 24 hours while the scratch coat cures. If the framing members are not plumb within 1/4" in 8' a leveling coat must be applied to bring this coat into plumb. This leveling coat should then be scratched using a scratching tool as described in the tool section. The float strips are not applied until the next work begins prior to the final leveling coat or "brown coat."
Following the curing of the scratch coat, the float strips and final coat are applied by the installer in the same way as indicated in Figure A.
Note: Figure A, B, and D can be used on exterior applications. Especially important is the requirement of properly designed and placed expansion joints. Please refer to The Tile Doctor content related to "Expansion Joints".
|Dr's Tip: The mortar bed system is probably the best if the budget and time will allow it. However, the tile, grout, and mortar bed do not keep water from penetrating the wall. The proper installation of the vapor barrier is paramount behind these installations.|
In Figure C, we see a wall installation where tile is applied in the thin set method directly to gypsum board. This method is not recommended for wet areas. Gypsum board can deteriorate when exposed to water. For dry areas, the tile can be applied to gypsum board using Portland cement based thin set mortars and organic thin set adhesives.
Like backer board installations, discussed in Figure D, the framing members must be true, square, and in plane. The maximum variation in plane must not exceed 1/8" in 8 feet. The reason for this stringent tolerance is simple. The drywall is installed as the backing element to receive tile. That means that any variance in the drywalls surface will be noticeable in the tile surface. This is a key element in selecting the mortar method as the small variances can be eliminated with the final leveling coat of mortar.
Sometimes the drywall methods are pre-selected for the installer by virtue of drywall that is already in place and satisfactorily sound. In this case tile can be applied using the thin set method. The installer must insure that the drywall is installed correctly.
The drywall should be nailed or screwed at 6"-8" on center into the studs. Metal studs should be at least 3-5/8" and 20 gauge or heavier. Like wood studs they should be spaced at 16" on center. The drywall joints should receive tape and joint compound in one coat only. The nails or screws should receive one coat of joint compound only. This is to insure that the taping, nail heads, and joint compound do not interfere with the tiling process.
To check nail heads, simply run a taping knife down the line of nails. If there is an audible click, the nail head is protruding out too far and needs to be adjusted. Do not apply a water-resistant membrane behind drywall. This will only tend to collect moisture and concentrate it into the drywall causing deterioration.
In Figure D, we see a backer board installation. There are several different types of backer boards available today. There are CBU (cementations backer units), fiber cement underlayments, and glass mat water-resistant gypsum backer board. In their installation, they are very similar. Their ability to be used in wet areas can differ greatly. Some are designed for use in wet areas and others are not. Be sure to select the right type of backer board that the installation calls for.
The backer boards are available in a variety of thickness' and sizes. The most common are in the 3' X 5' size and 1/2" thickness. They have been around for about 30 years and were developed to replace the mortar bed method. Like the drywall method, the framing must be true, square, and in plane to within 1/8" in 8 feet.
The units should be fastened to wood or metal studs using corrosion resistant screws or ring shank nails spaced 6" on center. The fasteners should not be countersunk as the strength of the unit is most often in the top layer of the unit. Manufacturers will recommend a certain type of fastener that will work best with their product. The units can be applied over drywall.
When installing the units, a 1/8" gap should be left between the sheets, which is filled with bonding mortar. The joints and 90° corners should be taped using alkali resistant tape and bonding mortar. This step is designed to give the units a monolithic performance. Or performances like a mortar bed.
For wet installations, a water-resistant membrane is required. For tub and shower installations, this might mean the use of 15-pound roofing felt or 4 mil. Polyethylene sheeting is required. If the unit is to be installed over drywall in a wet area, the same membrane might be required. This will depend on the Manufacturers instructions. Remember that not all backer boards are recommended for wet areas. If the backer board abuts drywall in a wet area, a 1/8" gap should be left and filled with a silicon sealant. This step is designed to deter water from wicking from the backer board into the drywall.
Once the backer board is properly installed, the area can be tiled using the thin set method using Portland cement mortars, organic, and epoxy adhesives.