Photo by Jimi Smith; Pool by Claffey Pools and Reid Schindler

Vanishing-edge pools have moved into the realm of classic.

In one form or another, they’ve taken on the status of everyday vernacular in the world of swimming pools and spas, both residential and commercial. The heart of these installations, weir flow technology, whereby water spills over an edge or weir into a lower vessel, has been around since the 1600s.

“So this is not new science, just a newer application,” says Terry Brannon, engineering faculty chair with Genesis.

While they appeared in a selection of very high-end pools well beforehand, they began to hit big in backyards about 25 years ago.

Their correct design and construction have become more crucial than ever, as designers create vanishing edges that are longer and more varied.

“Originally we might have seen just one edge; now we see whole pools surrounded by vanishing edges,” Brannon says. “And we’re getting more sophisticated materials. So we’re getting a little bit more creative.”

Here, learn what swimming pool/spa designers and builders can do to avoid some of the most commonly made mistakes on vanishing pools, as shared by Brannon, who also owns the consulting/instruction firm Brannon Aquatics and formerly was president with The Brannon Corp., both of Tyler, Texas.

1. The whole team must work toward a level weir.

For both the look and functionality of a vanishing edge, the weir must be level. Many pool/spa builders work within tolerances of 1/16 inch or less.

Without that level of precision, you might need to push more water to flood the edge, or end up with dry spots. “Any mistake you make in the level of a weir is going to show up badly,” Brannon says.

Crews at each stage of construction must work in service of this ideal. Unfortunately, some teams who perform the earlier construction stages, particularly shotcrete, work to their regular standards and leave it up to others to make up the difference and perform final leveling in later stages such as tile or masonry.

The impact can go beyond a sloppy spillway. “We have to make up the difference with these materials that don’t have the durability and don’t necessarily stick to the concrete well,” Brannon says. “So we see those tile walls failing and coming up because they put that on too thick.”

The levelness also becomes compromised if shotcrete applicators don’t use the proper tools. “They’re troweling it instead of using what we call a mule, which is used to drag the top of the weir so it’s always the same shape,” Brannon says. “They don’t use a proper screed to make the weir the same shape and therefore level. It’s like trying to draw a line by hand — you can’t draw it perfectly straight.”

2. Use the right leveling technology.

To help achieve a level weir, builders should rely on the right technology.

As vanishing edges become longer, and builders try to create the vanishing-edge effect with less water, construction tolerances continue to tighten. For many, building the walls within 1/16-inch of perfectly level has become the norm.

The laser level has become perhaps the most common tool builders use these days to reach that standard. But there’s a caveat here. One reason laser levels have become more commonplace is that they’ve become more affordable. While this may be a net positive for consumers, there is a reality that can complicate things for professional contractors — the less-expensive laser levels often lack the precision needed for the tight tolerances required in vanishing-edge construction.

“When we first came out with laser levels, they were very precise — and expensive,” Brannon explains. “You could spend $2,000. Now you can go to Home Depot and buy a laser level for $49. That is not nearly as precise as a $1,000 laser level. As time has passed, we’ve gotten cheaper equipment, and some pool builders have selected that cheaper equipment.”

When choosing a laser level, look at the manufacturer specifications for level tolerance, which generally will state the accuracy in a fraction of an inch for every 100 feet. Look for those whose accuracy will be less than 1/8 inch in 100 feet.

“We don’t want it to be any less precise, because we’re going to be building weirs, and we’re seeing people build weirs that are 40 to 50 feet long,” Brannon says.

That’s not to say that more basic technology can’t work. But here, too, it requires careful selection.

The bubble lasers often seen in carpentry usually will not provide enough accuracy, especially over longer distances. “Especially when you move from end to end, you can be out several fractions of an inch beyond tolerance,” Brannon says.

On the other hand, water levels often fit the bill, despite the fact that they have been around for thousands of years. These run off the precision of water seeking its own level, and they provide an accurate way to gauge the level over longer distances, making them ideal in most of these applications.

3. Correctly size the catch basin.

One of the most frequently made mistakes is also one of the most potentially problematic: undersizing the catch basin.

“It could ruin landscape, erode downhill slopes and undermine the pool structure itself,” Brannon says. “Nothing good comes from undersizing that basin.”

It’s easy to underestimate how much water they need to hold, because there are so many factors. When the system shuts off, the catch basin must contain the water that would be in motion while the vanishing edge is in operation. This is called the water in transit. It’s not just what actually spills over the weir, but also the added depth required in the entire pool to cause the overspill. If it’s engineered to spill a 1/8-inch-thick sheet, then the entire pool needs to be 1/8-inch deeper.

“When the pump is turned off, [the water in transition] is going to drain down to the weir level and stop,” Brannon explains. “That water has to go somewhere.”

The basin also must hold makeup water for evaporation of the entire pool. This adds up deceptively quickly. Because the catch basin is so much smaller than the main pool, accommodating the entire pool’s evaporation has a greater impact on its size.

“As an example, let’s say the swimming pool is 1,000 square feet, but the [catch] basin is only 100 square feet,” Brannon says. “If you lost 1 inch of water in the pool, you would have to have 10 inches of water [in the catch basin], because it’s 10 times smaller. It could amount to several inches of water that has to be accommodated.”

The basin also must hold splashout and displacement when people are using the main pool.

Additionally, the miniature pool must reach far enough away from the spillway to capture all the water that spills over it. So an edge with a roaring sheet or whitewater likely will require a catch basin that measures more front to back than if a wafer-thin sheet glides down the wall.

Finally, the water in the catch basin must always be deep enough to properly cover the drain. This is required even when vanishing-edge operation causes the catch basin water level to drop. This generally means at least 6 to 12 inches when it’s at its shallowest during operation.

4. Don’t make this shotcrete application error.

A monolithic shoot is crucial in just about any pool. If you interrupt the application, and then pick up where you left off, it is no longer one piece. This can cause structural problems down the road.

This can be more difficult to accomplish when shooting the raised wall of a standard vanishing-edge pool. The applicator isn’t shooting against the earth, but rather forms.

To do this, some applicators try a method that they presumably believe is a monolithic shoot but isn’t: They might tie a temporary form, such as pegboard, on the backside of the rebar, shoot only part of the wall’s thickness, then peel off the form and finish the shoot from the other side of the steel.

“Sometimes somebody will hold up a piece of plywood and shoot against it,” Brannon says. “Then they remove the plywood and shoot from the other side. They are shooting in two directions.”

This doesn’t leave a monolithic wall. “That creates two thin walls instead of one thick wall, which causes bonding and delamination problems,” Brannon says.

Instead, applicators should shoot the entire thickness of the wall from one side only, then pull the forms.

5. Provide proper waterproofing.

In examining vanishing-pools as a consultant, Brannon sees one problem probably most often: efflorescence as a result of water migrating from inside the pool, through the weir wall to the side exposed to air. This results in efflorescence forming on the outside wall — an eyesore in any application, but especially problematic when trying to showcase the falling water. Worse, these calcium carbonate deposits can break the bond between tile and the weir wall, causing the tiles to pop off.

“This is because they’re not waterproofing the pool adequately, either by using an integral waterproofing or putting on a topical waterproofing on both sides of the wall,” Brannon says.

Both sides of the weir wall must be waterproofed, as well as the top.

As for the pool interior, it’s best to waterproof the whole vessel. At the very least, waterproofing should be applied 4 feet past the weir wall on all sides — 4 feet to its left, its right, and toward the floor. (This is the minimum needed to prevent efflorescence on a vanishing-edge pool. Other aspects of the design, such as placement on a rooftop, would carry their own requirements.)

Brannon also encourages waterproofing the entire catch basin, not only for structural reasons, but also to reduce the chance of water loss from water migration or leakage.

And always consultant instructions and specifications from the manufacture of the waterproofing system, to ensure the product is suitable for swimming pools, spas and waterfeatures, and that it’s compatible with the surface materials being used.

6. Make the Catch Basin Safe

Because catch basins are smaller, it may be easy to overlook their safety. But they still qualify as pools and pose a drowning hazard, especially to children and pets.

Because of this, many jurisdictions require drowning-prevention measures similar to those used in the main pool. In some cases, for instance, Brannon has had to build stairs inside the basin them to provide an exit route should somebody fall in. Other times fencing may be required, or other barriers to prevent people randomly walking by from falling in.

In some cases, Brannon has addressed this issue by covering the catch basin with grating and, perhaps, placing decorative rock over the grating. This way, water spills over the weir, through grating that can support people standing on it, and into the basin below it.