Go With The Overflow
Perimeter-overflow Pools Are Both Beautiful And Technically Challenging.
Few things in the backyard will inspire the instant awe of a well-done perimeter-overflow pool. They cast an ethereal air over just about any space.
The vessel essentially overflows and spills across the bond beam. Deck and water merge into one so the pool appears to be a large mirror. As a side benefit, these pools can make a yard look larger since there is no coping or raised bond beam to interrupt your gaze.
But these installations aren't for every homeowner. “A perimeter-overflow pool is not particularly family-friendly,” says Steve Sargent, president of Elite Custom Pools in Lake Forest, Calif. “Kids doing cannonballs are going to create a lot of splashout.” This can leave the deck slippery and create a hazard for children.
Perimeter-overflow technology is complicated and requires a high level of technical expertise along with an acute eye for detail. Below is a primer outlining the main challenges of these installations. However, no single article can teach anyone to build a perimeter-overflow pool. It's important to take a class or consult with an experienced professional before attempting this type of project.
How they work. The basic workings of a perimeter-overflow pool are deceptively simple. It's essentially a vanishing edge around the entire pool. Water fills the vessel until it overflows and spills into a receptacle, generally an underground gutter that runs around the pool. In some systems, the gutters are large enough for storage, but in most cases they direct water to a remote holding tank.
Most perimeter-overflow pools sit flush with the deck. This version is also referred to as a rim-flow, slot-overflow, or wet edge. After spilling over the top of the pool, the water enters the gutter through a grate or a ¼- to 1-inch slot. The latter is becoming more popular as many consider grates less attractive. However, some manufacturers now offer grates in stone.
Another type of perimeter-overflow installation sits above the ground and looks more like an overflowing bowl. While rarely used for pools, this option is becoming increasingly popular for spas. Water falls over a raised wall and into a surrounding moat-like catch basin, or a slot system for the wet-deck effect. Designing access into the pool or spa can be tricky, because you have to interrupt the flow with a set of steps leading up the wall.
Breaking it down. There are five concerns when it comes to engineering and constructing perimeter-overflow pools:
The Edge. The edge refers to the weir where water breaks from the surface and falls into a gutter or catch basin. In that regard, a rim-flow pool is like a vanishing edge times four, since water has to sheet over the entire perimeter.
Those concerned with energy efficiency will want to overflow the pool using the least amount of water possible, which allows you to power the system with a smaller pump. However, it also results in a thinner film wetting the edge. If aiming for this effect, you need the entire perimeter to be as close to perfectly level as possible. Otherwise, water will take the path of least resistance, spilling in a thicker sheet over lower parts of the pool and leaving higher spots dry. Pool builders recommend working with a ¼-inch “tolerance” at most. This means the entire perimeter must fall within ¼ inch of level. Most experts shoot for closer to a 1/16- or even 1/32-inch tolerance.
To accomplish this, every crew member must work as close to the plans as possible. Only the most exact excavation, steel, and shotcrete application will enable the masons who set the edge tile or stone to achieve perfection. If using tile, setters must be sure to make it flush with the grout joints. Some like to use stone because high spots can be ground down. When using a rougher variety, however, you'll need to feed more water to wet the edge.
The Sounds. One characteristic of perimeter-overflow pools is the possibility for a noise hazard. This happens because water moves through plumbing via gravity from the gutter to the holding tank. There's no pump to create a vacuum, so air exists in the lines. As water drops into the pipe, that air is displaced and needs a place to go. If not handled properly, the pool can make the same gurgling sound as a 5-gallon water bottle when it's propped up on a dispenser.
Pool builders solve this problem using several methods. Gravity-feed lines must always be larger than those utilized in a pressurized system. The standard 2- to 3-inch lines would become full, and water would force the air up and out. Instead, size the pipes so they are only one-third full of water at any given time. You will probably need 4-, 6-, or even 8-inch plumbing. These lines should not run vertically downward. Rather, install them at a ¼-inch-per-foot slant. This way, water can amble slowly.
On top of that, it's a good idea to include some type of noise-elimination device. Many builders place air vents in the pipe leading from gutter to tank. You can also oversize the gutters, so the water has plenty of room to move. Another option is to place drains on the walls of the gutters instead of the floors, so that water exits horizontally.
The Storage. Builders can create the tank themselves from concrete or install a prefabricated unit suitable for holding potable water. While some like the ease of a manufactured product, others feel more confident in their own construction
Either way, the tank must sit below the gutters so that water can gravity-feed downward to it. The container must be large enough to hold all the water that's not directly in the pool. This includes rain and water in transit (that which is moving through the plumbing when the overflow pump is running). Once the system shuts off, the pipes and gutters will drain everything to the tank, unless the plumbing is used for storage.
Outfit the tank with both automatic-fill and overflow devices to maintain the right amount of water in the system.
The Cleaning. Perimeter-overflow pools have unique maintenance concerns. The pool builder should design the vessel to facilitate cleaning as much as possible. Because of their high water level, these pools have no skimmers to help sift out leaves and other debris. Dirt will head straight for the edges and hit the slot or grate. While some pool builders prefer to design a system for minimal flow, you can also feed a little more water over the top, so that its force helps direct matter over into the gutter, says William Palmer, president of Las Vegas-based Prestige Pools.
The choice of slot or grate can affect the ability to clean the pool. Grates are generally easier to remove and have more open area in which dirt and particles can fall, says Lew Akins, president of Ocean Quest Pools by Lew Akins in Belton, Texas.
But some like the look of a slot better. You can size this space on the larger side to help debris fall through. Sargent creates a 1-inch gap between the edge of the slot and the deck. This not only allows more debris to fall through, but it also accommodates a small nozzle so a service tech can spray down the gutters. Some don't like the way this looks, however, and prefer narrower openings. You can also install some of the coping stones immediately surrounding the slot so they can be removed for access.
Many pool builders slope the gutters to direct water and particulate into the drains. The holding tank can then be devised to sift out the unwanted materials, Palmer says. Solid matter will settle to the bottom anyway, so pick up water from higher in the tank. Place the feeds in the top 18 to 24 inches so that it pulls the cleanest water back into the pool. The container will need occasional cleaning, too. For that reason, the manhole leading to it should be put in an accessible area.
The Splashout. As long as people use the pool, there will be splashout, especially if children are jumping into the water. For this reason, builders should subtly slope the deck toward the vessel so that water falls into the slot. The 3 feet of surface immediately adjacent should tilt about ¼ inch per foot.
Their sleek design merges water and deck, giving perimeter-overflow pools an ethereal quality.
Raised perimeter-overflow pools can pose an access challenge. This design solves that issue with a step leading up to the pool and a deck overhang on the spa.