CTEF Tile Tip: Preventing "Ponding" in Mosaic Tile Installations
June 7, 2010
For years tile installers have complained about the thinset squeeze up experienced when installing mosaic tiles. Their solution is to use a 5/32” or 3/16” V notch, but this only compounds the problem.
While this trowel may eliminate the squeeze up problem, it does not provide the required 95% coverage as called for in the ANSI Specifications and creates what is known as “ponding”. Ponding occurs when the V notch trowel used to install the tile lets voids between the unflattened mortar ridges. Water collects in these hollow areas and then wicks back up through the grout joint causing it to be darker than the surrounding joints. The dark joints will go away eventually, but unfortunately will reappear as soon as the grout gets wet the next time.
The remedy is simple and straight forward. Accepted industry practices require that the surface be wiped with a wet sponge to remove the surface dirt and dampen the dry surface to keep it from pulling the moisture from the thinset prematurely. The thinset mortar is then “keyed” or “burned” into the substrate with the flat side of the trowel to provide a good mechanical bond. The mortar is then combed with a ¼” x ¼”square or U- notched trowel in one direction as described in the NTCA Trowel and Error video. The flat side of the trowel is then moved across the mortar ridges perpendicularly, flattening the ridges into the valleys; yielding a smooth and flat 1/8” setting bed. The mosaic is then placed onto the flattened mortar and beaten in using a beating block and mallet. The thinset mortar will rise up about one third to one half of the thickness of the tile providing full coverage under the tile with no voids.
This method eliminates the need to rake out excess thinset between the mosaic tiles, eliminates the ponding problem and provides the 95% coverage required by ANSI. It is a win – win – win situation and makes the installation of mosaic tile much easier and less time consuming. Better methods using less time equal more money in your pocket.