It never hurts a pool designer or contractor to become known for doing something special and exceptionally well.
Through the years, for example, my firm has gained a reputation — to an extent that sometimes surprises me — for our perimeter-overflow systems.
This has led a large number of you to pull me aside at trade shows or drop me email messages asking just how we design and install these features.
The answer is neither quick nor simple.
These edges are a design element I have learned over the years from my good friend and Genesis 3 partner, Skip Phillips. Fortunately for me, Skip taught me enough that he now feels comfortable with me as his co-instructor for one of our most popular Genesis programs, “The Beauty of the Edge”.
The program will be given at this year’s International Pool | Spa | Patio Expo in New Orleans. If you haven’t taken the course in a while, or ever, I would highly recommend it. The program is constantly evolving and has never had the same material twice. Skip and I update and improve it by drawing from our network of Genesis Platinum members, some of whom are reaching new heights in their perimeter-overflow designs. We also use our own work as example for instruction.
My aim here is to give you basic information so you can make intelligent decisions about how to proceed when clients seek your input about this increasingly popular look.
Before we begin, let me say that perimeter-overflow pools are not for beginners. In addition to tremendous skill in developing clear and explicit plans, they require expertise in forming shells, configuring plumbing systems and shooting or pouring concrete.
It’s also a costly look, so if you miss the mark in either design or installation, your nightmares are only just beginning. Setting things straight after the fact can be supremely difficult without starting over again entirely.
What’s in a name?
Water-in-transit effects have become incredibly popular in recent years. If you look at brochures for high-end resorts or upscale properties, for example, you’ll almost always see a vanishing-edge or perimeter-overflow system of some type. There may be many reasons for this trend, but I think they can all be summed up by saying that these details spell “luxury” and “drop-dead gorgeous” in big, bold letters.
Ironically, one of the problems with these systems is the imprecise terminology. Consider the “vanishing edge,” for example: Despite the fact that this has been the most published term used to describe these effects for more than 20 years, some people still call them “infinity pools” or “infinite edges,” as well as “negative edges.”
Perimeter-overflow effects face the same issue. Are they properly known as “deck-level-overflow pools,” “knife-edge pools,” “rim-flow pools” or “slot-overflow pools”? The answer is yes and no. Indeed, in my practice, none of those names is descriptive enough, so we’ve come up with different terms for clarity’s sake.
One of these effects, for example, we have dubbed the “underwater-coping, perimeter-slot overflow.” (See diagram.) That’s a hyphen-laden mouthful, but it’s the clearest way we’ve found of describing a project where we press the pool into the ground and give it a slightly angled coping that lets water flow across it to the back edge, where we conceal a narrow slot. Once it flows into the slot, the water is driven by gravity into a gutter hidden beneath the cantilevered deck and coping material.
Or there’s the “radius-wall overflow,” in which we raise the edge of the pool above the decking. Here, we create a radius edge over which water flows before going down the side of the raised beam and through a deck-level slot to reach a concealed gutter.
There is also the elevated perimeter overflow, another vessel constructed out of the ground. In this version, water flows over all sides, clinging to the exterior riser — a dramatic effect indeed.
Finally, there’s what we call the “Lautner knife-edge perimeter-overflow,” named for John Lautner, the architect and Frank Lloyd Wright protégé who initiated the concept. (See diagram.)
The Lautner knife-edge is the most popular of the perimeter-overflow types we work with, and one of the most challenging. In this detail, water flows to the edge of the pool at deck level and spills into a slot right at the edge of a dry deck.
The treatment of the coping makes a visual difference between the various types. When you walk up to a pool with submerged coping (as with the underwater-coping variety), you look down and see wet decking or coping material at the edge of the pool.
But when you approach a knife-edge perimeter-overflow pool, you look down inside the pool to the full depth of water and your feet remain dry. It’s a genuinely cool detail that creates a certain “wow” factor, because it mystifies, intrigues and delights those who know nothing about how these systems are made.
Regardless which type is used, a perimeter-overflow system is an expensive detail — one that can cost almost as much as the rest of the pool, in some cases — and requires extremely tight tolerances.
As suggested above, these pools must be done correctly on initial installation because the consequence of getting it wrong or having the structure fail are almost too grim to consider.
Behind the edge
The key to success with any perimeter-overflow system is achieving a dead-level edge. In my operation, we call for a tolerance of plus or minus 1/32 of an inch. In doing so, we recommend use of a water level (gravity never lies), but I’d expect a laser level to work just as well.
To define the edge, we typically specify a 3/8-inch-thick, 12-by-12-inch granite tile with a bevel on the leading edge. These tiles are installed inside the gutter at an angle that falls steeply away from the pool’s interior. The top of the bevel, the only visible part of the granite tile, is the critical point and must be dead-level.
In using this material, we butt the tiles up against one another as tightly as possible. We do so because grouted joints make it tough to establish and maintain perfectly level lines; moreover, we want to do as little as possible to disrupt the visual integrity of the knife-edge line.
In already considering the edge, however, we’ve jumped a bit ahead of ourselves: Before that tile can be placed, you need to form and pour or shoot the gutter.
This process begins with designing a bond beam that is wider than what you’d find in a typical pool. The beam must contain the angled wall that falls back from the pool’s interior, as well as accommodate the gutter. The gutter’s bottom must be wide enough to house what I call drop pipes — lines that drop directly out of the gutter and to the trunk line flowing to the surge tank.
In addition, the beam needs to accommodate the outside wall of the gutter and its vent or “snorkel” lines, which we use to prevent an annoying slurping sound. The vent lines aren’t necessary if we choose to use horizontal drops, but that feature necessitates a wider pool beam.
In most situations, these beams end up with widths in the 20-to-24-inch range. The beam doesn’t necessarily need to extend all the way to the bottom of the pool (although it’s recommended in areas where freeze/thaw conditions prevail), but with all that’s contained within, it definitely needs to extend down a good way toward the floor of the pool.
In our designs, we prefer to encase the perimeter trunk line within the beam, which is one reason they get so deep. We do so because these are gravity-fed systems in which the trunk line typically runs at a pitch of about a 1/4-inch per foot. As a result, the beam must run deep enough to accommodate the trunk line at its lowest point, which in a 40-foot run would amount to a 10-inch drop. Typically, this means the beam will extend downward from the edge by about 2 feet.
All of this also depends, of course, on the hydraulic system to be contained within the wall, since the desired flow rate determines the size of the plumbing. And in all cases, the structural elements of these designs, including the size and frequency of the rebar and the thickness of the wall, must be specified by a licensed structural engineer in view of prevailing soil conditions and the needs of the hydraulic system.
Minding the gutter
In the forming and concrete-application stages, perhaps the trickiest area has to do with setting up the gutters. There are a number of ways to do so, and you also have the choice of pouring them in place or shooting them with shotcrete or gunite.
Some people fashion their gutters in two pours. The first creates the pool wall, with rebar left stubbed out on the outside and coated with a bonding agent. Then the crew comes back and forms the outside of the gutter before finishing the operation with a second pour.
I’ve tried that approach myself, but I prefer using just one shoot or pour because I don’t want to accept the risk of leaks developing at the junction of the two pours. In addition, we come down in favor of shooting these details rather than pouring them: When you pour the system in place, the crew must come back later to build up the knife edge by hand.
There’s no doubt, of course, that forming these trapezoidal gutters is a challenge. If the pool has a straight-line wall, the best and easiest way to do it is with a wooden form suspended over the bond beam’s form and structural steel. You can use wood with a curvilinear edge as well, but it’s obviously going to be far more difficult. That’s why some people use Styrofoam forms suspended over the beam to shape their curved gutters.
Another critical element has to do with providing structural support for the wedge of angled wall that forms the forward face of the gutter and to which the granite tile will be applied. We always specify the use of stainless steel rebar stubbed up at an angle or insertion of threaded stainless steel bar stock because it isn’t possible to get adequate concrete coverage in this narrow section. (See the drawing.)
Whatever concrete-application strategy you use, the plumbing for the gutter must be integrated into the forms. We typically use either 3- or 4-inch drop pipes to accommodate the desired flow rate and anticipated bather surge, and to allow debris to move into the trunk line and, ultimately, the surge tank. (We never grate the tops of the drop lines for that reason.) We run the drop pipes at 5-foot intervals, so the gutter form must have holes in it where the plumbing will be stubbed up into the gutter opening and capped, preferably under pressure.
On the drop pipes, we install tees with reducer bushings that create, typically, 1-1/2-inch lines that we elbow up and then elbow back into the side of the gutter. This creates a vent or snorkel that allows air to flow through the system, thereby reducing the gurgling noises that can result when the line velocity is faster than water can enter the piping.
This is an extremely important detail: As a rule, clients who are paying tens of thousands of dollars for this visual detail are not happy when the system becomes inordinately loud.
Waterproofing is another crucial factor when installing perimeter-overflow gutters, because you can’t afford to have them leak. Indeed, I believe in using waterproofing systems, specifying a concrete surface waterproofing agent that will effect a seal (such Flexcrete or Aquafin).
Again, this is critical because the wedge-shaped slice of concrete just inside the wall doesn’t have adequate concrete coverage near the edge to ensure sufficient structural waterproofing – another reason we use stainless steel bars at these points.
Another key question has to do with how you support the deck that will be cantilevered over the top of the gutter. If you’re using a material that’s structurally strong on its own, you can attach it directly to the exposed top area of the outer beam. In the project shown on page 34, however, we used stainless steel plating that’s 3/16-inches thick and ten inches wide, with holes drilled 3-1/2 inches from the outside edge, which allow us to bolt it to the gutter wall’s outside edge. Again, all of this hardware needs to be non-corrosive!
The top of the stainless steel plating is treated with a steel-to-concrete bonding agent. A number of suitable products are available, including Sikadur’s Sika 32 Hi-Mod or an epoxy from Laticrete. We’ll take the decking material and cut its forward edge at an angle matching the angle inside the gutter. Finally, we place the coping to leave a ¾- or 1-inch-wide slot – just enough space to allow insertion of a hose to clear away debris. This edge detail is one of many we employ, depending on design parameters, site conditions and client desires.
There are many other factors involved in creating this effect: collector-tank sizing, hydraulic-system design, flow rates, auto-leveling and redundant safety devices and gravity-flow design. All of this will be covered in our program, “The Beauty of the Edge,” Wednesday, Nov. 7, 8-11 a.m. I hope to see you there!
Brian Van Bower runs Aquatic Consultants Inc., a design firm based in Miami, and is president and co-founder of the Genesis 3 Design Group. Dedicated to top-of-the-line performance in aquatic design and construction, this organization conducts schools for like-minded pool designers and builders. He can be reached at firstname.lastname@example.org and will be presenting several classes at the IPSPE in New Orleans, Nov. 2-8, 2012.