• Stay in range
When the edges are not completely level, it takes substantially more water to wet the edge and avoid dry spots, consequently requiring larger pumps. When dry spots do appear, they destroy the visual effect.

Paul Benedetti says he must use a tight edge tolerance because he aims to flow 2 gallons per minute per linear foot over the edge. This allows him to use smaller pumps. “The goal is no more than 1/16th inches from the high to low points,” says Benedetti, vice president of Aquatic Technology Pool and Spa in Morgan Hill, Calif. “Dead-on level is ideal!”

• Tolerant materials
Not all materials are the same, however. When choosing an edge treatment, builders should consider the measurement between the high and low points of the chosen material. This directly affects your edge tolerance.

“Ceramic tile can have an edge tolerance of 1/8- to 1/16-inch,” Benedetti says. “The top of the tile is flat, but there are dips [at] the grout lines between the tiles. Italian granite has an edge tolerance of 1/16-inch — it is bumpy. Polished marble is absolutely flat ... therefore it has 0 edge variance.”

Generally, builders concur that natural stone materials are the easiest choice for achieving a tight edge tolerance. “Natural stone is faster to set,” “says Benedetti. “It can be installed in large pieces. Most stones are available as ‘gauged,’ or machined to a consistent thickness. And the edge of natural stone can be fine-tuned by polishing or grinding down high spots if necessary, without leaving any telltale signs that it was reworked.”

Glass mosaic tile, on the other hand, can impose a higher edge tolerance because of it is difficult to install: It is small, hard to set and tends to slide around, which can hamper precision. So when using this material on the edge, the pipe size and pump size must be increased to maintain a normal water flow.

• Sizing up bather surge
The splash created by bather surge needs to be addressed during the construction of vanishing-edge pools.

“The splash-out of a negative-edge pool can reach as far as six feet,” says Dave Schneider, president of Natures Creations in Santa Fe, N.M. “Water loss also occurs as people exit the pool.” So designers must equip the collection system to catch the displaced water and store it during pool use.

“If the catch basin [or gutter] is not built to accommodate this water, there will be significant water loss which then must be made up,” Schneider says.

• Collection box
To collect all the water properly, your catch basin or surge tank must be large enough to handle water displacement.

The size of the surge tank, according to Benedetti, depends on a few factors: the volume of water needed to satisfy water in transit, that is, the amount necessary to fill the supply and return pipes; the amount of room necessary for bather surge, which is the water each bather displaces with movement; and the volume that is needed to raise the level of water in the pool above the edge and overcome variances in edge tolerance.

Many builders complete hydraulic calculations to determine the amount of water in transit and then oversize the surge pit accordingly. The surge tank is generally outfitted with an autofill device that allows it to always maintain a set water level even when evaporation is a threat.

Even with these precautions, the collection system needs a backup to ensure that the water level falls within an acceptable range. For one thing, builders should use autofill devices to supply water to the catch basin or surge tank when it runs low enough to place the pump at risk. An overflow device should also be installed to prevent the catch basin from overflowing.

For a complete article on perimeter-overflow pools, see "A New Edge" available in the July 18, 2001 issue of Pool & Spa News.

For a primer on vanishing-edge design, see "Mastering the Illusion," Jan. 25, 1993, and "Mastering the Edge," Jan. 27, 1998.