A Mud Job is a labor intense job performed by experienced tile setters-plasterers. Many tile setters at one time were also plasterers because the two processes involve many of the same procedures and skills. A mud job requires troweling a cement mortar onto the walls and ceiling and then beating tiles into this cement base before the cement is cured. The grout color on a mud job was usually white (white cement) for the walls and a grey (grey cement) for the floor. Many mud showers that are still functioning after many years of use with no issues and are being replaced only for a change in tile design and tile color. A mud job is the best foundation to start with as it creates a seamless base that is square, level and plumb.
The introduction of new products has reduced and changed the mud job process. No longer is there a need to beat the tile into the brown coat. The brown coat (mixture of sand cement/ masonry lime) can cure and tile installed better using a thinset. This enables the homeowner to pick from a variety of grout colors. The use of pre fabricated cement board has also limited the use of mud work. Installing cement board is basically one step process versus a three step process for a mud job. Therefore for budget reasons, cement board is used versus a mud job.
The 1st step of a mud job starts with fastening rib lath (ribwire) to your bare studs. The ribwire is cut with a pair of wire snips. (Use caution when handling the ribwire as it is extremely sharp.) There is an up position when fastening the ribwire using 1 – 1 ¼ inch roofing nails. If the rib wire is installed upside down the scratch coat (sand /cement mixture) will roll off the wire as you start to apply. Ribwire at one time was a 2.54 black steel wire mesh. Today you can purchase 2.76 galvanized wire mesh. The ribwire comes in sheets 97 inches x 27 inches. The wire mesh is overlapped approximately 2 inches and enough nails are use to secure the wire and keep it taut. If the wire mesh is not fastened taut it will be more difficult to apply the scratch coat (step 2) it could also lead to a heavier brown coat (step 3) because of the indentation that will form between studs. When fastening the wire mesh to the ceiling install it going in one direction. This will make applying the scratch coat easier.
The 2nd step is called the scratch coat. This is a sand (washed fine sand)/ cement mixture with the consistency of mortar. The scratch coat stiffens the wire providing a rigid, strong base for the brown coat (3rd step). When applying the scratch coat using a hawk and trowel, care must be given not to press too hard otherwise most of the cement will fall in back, between the studs. If a vapor barrier (15 lb felt paper) is used this will help from losing most of the cement. As the scratch coat continues to be applied to the rib wire it is scratched using a scarifer or a piece of rib wire cut to create a key into the scratch coat. Run the tool key left to right parallel to the rib wire. This will insure your brown coat will adhere to the scratch coat. Applying your scratch coat can be challenging especially when applying it to the ceiling and should only be attempted by a professional.
The 3rd step is to apply a brown coat. The brown coat is a mixture of sand (washed fine sand) cement and masonry lime. The brown coat adds strength and thickness to the walls creating a plumb, level and square shower enclosure. Before applying the brown coat, the scratch coat must be dry (24-48 hours). If the scratch coat becomes too dry it may have to be dampened with water for adhesion. Too much water and the brown coat will roll off the wall. Rip temporary screeds using 6/4 inch board that is 1/8 inch thick. Cut them the desired length from floor to ceiling. Then apply an amount of brown coat to your back wall 6 inches wide. Insert one of your slightly dampened temporary screeds into the brown coat. Placing your level parallel on the screed tap it using a rubber mallet until plumb. This screed should be embedded into the brown coat a minimum of ½ inch in depth from the scratch coat. Clean excess mud from the edge of the screed using the edge of the trowel. Continue installing a second screed on the same wall. Try to stay away from the adjacent wall approximately 4-6 inches. This will help you run a smaller square on the screeds. Continue installing temporary screeds on your adjacent walls. Check for square with your back wall to insure that you will have the proper thickness of your brown coat. Remember that your shower valves were set and given a finish depth. If you have a shower jamb without a return you can fasten a ¾ board to the outside shower jamb as your second screed. If you have return jambs with a shower door application the ¾ inch board will have to be placed perpendicular to the return wall and used as a screed. The ¾ inch board should be the width of the finished browned jamb. This ¾ inch board is embedded into the brown coat just like the temporary screed with the edge serving as the screed surface. The face of the ¾ board must be plumb and edge square to the adjacent wall. You must take into account the width needed for the shower door. Once you have your screeds in place you can start applying the brown coat. Apply tight the first coat and then double up. Take a straight edge and run up the wall staying on the screeds. Remove excess from the straight edge and return it to the mortar bed. The brown coat mixture may require remixing depending on how quickly it is used. Chances are you will rescreed the wall several times before it becomes flat. Proceed applying the brown coat to adjacent walls. With the walls done run a square into your corners riding on the adjacent screeds. More mud may have to be added to the corners. After the brown coat has taken up, float the walls using a wood float. Small voids aren’t a problem and will only aid in the adhesion of the tile. Remove the temporary screeds and fill the void with mud and float. The walls should be set enough to act as a screed.
The ceiling can be browned using the same method as the walls using temporary screeds. A tight coat is recommended for the ceiling otherwise it might just roll off if applied too heavy. If you need to apply a heavy coat to the ceiling apply a first coat, scratch it and wait until it sets up slightly and then reapply a second coat.
You now have a plumb, square, and level shower ready to apply the tile or stone.
This is just a brief description of what a mud job entails. It truly requires a craftsman to do it correctly. And should only be attempted by a professional.