Ask the Experts – April 2016


I am installing an indoor/outdoor patio for a continuous look on a cement substrate. The tile is 16” x 48” porcelain. The outdoor portion will be raised approximately 2” with a mud float. Should I use a bonded mud float or install a cleavage membrane? Additionally should I reinforce the float with lathe or not? Finally, I am in Austin, Texas, so I am wondering if I should put a membrane on top of the float to help prevent problems when there is a rare freeze. Thank you for your expertise.


Thank you for contacting the National Tile Contractors Association.

The closest TCNA Handbook for Ceramic, Glass and Stone Tile Installation method to follow for the exterior portion of this project is F103B. This method details a wire-reinforced mortar bed (maximum 2” thick) on top of a drainage mat/system. A drainage mat makes an ideal cleavage membrane since it promotes quicker evacuation of water from the tile/mortar bed system. Refer to TCNA Handbook methods F103B, F111 and F121 for details including specifications for cementitious bond coat mortar.

Sloping away from the structure is required for any exterior area, both for the substrate and the mortar bed. A minimum slope of 1/4” in 12” of horizontal run is required and is the typical specification to meet code. The slope of the substrate and the mortar bed need to match each other to produce a uniform thickness mortar bed. Prevention of water flooding back to the structure is the main reason, but slope and drainage are also a great help in reducing efflorescence issues.

A waterproof membrane is a good idea as long as there is not excessive moisture in or beneath the slab, which can be caused by poor landscape drainage design or clay-type soils for an on-ground installation. Moisture in the slab should be checked before deciding to install a waterproofing or crack-isolation membrane. Most membranes are suitable for moisture presence up 5 lbs/1,000 sq. ft. Some manufacturers state their product can be used up to 12 lbs/1,000 sq. ft. Check with the manufacturer of the membrane to determine what their product can handle. Moisture in excess of 12 lbs/1,000 sq. ft. will require significant mitigation before the project can proceed.

It is critical to pay strict attention to the planning and installing of soft joints and expansion joints. Installation of a membrane does not eliminate the need for proper movement joint placement. Soft joints must be installed in the mud bed and tile in line with the control joints. Expansion joints must be installed in the tile every 8’ – 10’ in the exterior and in the interior section if it is adjacent to a large surface area of windows and glass doors. Expansion joints are also required at the interior/exterior dividing wall and all other interior and exterior walls, cabinetry etc. See TCNA Handbook method EJ-171 for more details.

To ensure acceptable lippage with the installation of the large format tile it is very important to begin with a flat substrate. Your mud bed flatness must ensure a maximum variation in plane of 1/8” in 10’. You will also want to consider a 1/3rd offset layout if the tiles have any warpage. A lippage tuning system may also be beneficial.

I hope this helps!
Mark Heinlein,
NTCA technical trainer