It may be an exaggeration to refer to waterproofing failures as “nightmares.” But sometimes it’s barely a stretch.

Consultants get to hear the horror stories. Designer and consultant David Peterson spoke with a plasterer dealing with one disaster. Without telling anybody, the general contractor had applied a waterproofing system to the pool shell before the plasterers came in. The finish material was then applied. “The next spring, when they go to open up the pool for summer, the plaster is coming off in giant sheets,” says Peterson, president of San Diego-based Watershapes Consulting.

With features such as vanishing edges and raised walls becoming commonplace, waterproofing is more important than ever. If the installation directly abuts the home or is suspended on the second floor or higher, there is no room for error. And there are so many systems on the market. Not only must a builder choose the right one for the application, but then it must be applied in the right conditions.

There is still quite a bit to be learned about waterproofing pools and spas. But here, some of the best contractors in North America share what they’ve observed so far about what types of compounds to use in various applications, and they discuss commonly made mistakes to avoid.

A conundrum

Waterproofing pools and spas to protect them from efflorescence, popped-off tiles, or delaminations is to walk a tight rope. The product must truly waterproof the vessel — in a way that preserves the aesthetics of the veneer going over it.

Some are tempted to look at products that have worked with related industries. But they don’t really compare.

“I don’t care if the product has been used for 30 years behind retaining walls, it’s not the same thing as a swimming pool,” Peterson says. “And if you look at water-treatment plants, they’re not putting plaster in their vessels — it’s just a purely functional water-tight tank. That’s completely different from trying to waterproof something that’s got a pretty-looking finish on it.”

Additionally, shells sometimes need protection against water on the positive side — on the face of the surface you’re trying to protect. Other times, it must be protected from seepage coming from behind the wall — or the negative side.

Negative-side protection has become more important with the popularity of vanishing-edge pools. “We’ve learned that a lot of glass tile failures have to do with water coming through from the backside of the pools, pulling contaminants from the concrete and mortar beds along with it and [building up] calcium deposits and efflorescence around and behind the glass and wreaking havoc on the glass,” says Greg Andrews, cofounder of National Tile and Stone Authority, a consulting and research organization.

The selection of systems can become overwhelming.

The right fit

Certain products do a better job of each task. Each has its own strengths and weaknesses.

Integral systems, mixed in the concrete before it is poured or shot, often prove effective with negative side pressure, Andrews says. He characterizes these as a first line of defense. “These react with water or alkalinity, and they grow either a hard gel-like material or crystals within the matrix of the concrete,” he says.

This is especially important for shotcrete, since there is no access to apply a product to the back of the shell.

Some spray-on and brush-on products perform the same task, penetrating the concrete and closing microscopic holes.

Cementitious systems applied to the shell also work as densifiers, helping protect the concrete and steel, and holding back hydrostatic pressure. These also work well with negative pressure, Andrews says.

“They densify, produce a really hard, dense film over the concrete and close up the pores on a topical level,” he says.

These can work with plaster or a float coat for tile. They will not bridge fissures and are more likely to crack with movement, Andrews says. However, they are more easily compatible with plaster and pebble because, as cementitious products, they will behave more similarly to movement and other conditions.

Elastomeric or membrane systems are, as the name implies, very elastic. Generally intended for tile, Andrews says, these show higher waterproofing characteristics and can better bridge over cracks and flex with slight movement.

But these systems are too flexible for cementitious finishes such as plaster or pebble to be applied directly over them. They will move at a different rate than the veneer material, so they can come apart. “As that plaster wants to shrink back, its going to sheer off the membrane,” Peterson says.

Most of these must be used over the entire vessel to create a monolithic barrier. So using them just for a tile line or vanishing-edge wall will defeat the purpose.

These systems often are appropriate for positive pressure. “You roll it or spray it on, but after it cures, it’s like a sheet of rubber,” Peterson says. “The water [inside the pool] pushes that sheet of rubber against the concrete substrate … actually helping the rubber sheet stick to the wall.”

However, they may not work as well against negative pressure — at least not when applied inside the pool: If water seeps through the wall from outside, it can accumulate behind the membrane and push it away from the wall, causing delamination or other issues, Peterson says.

The situation is different for pools that are poured in place, because contractors can apply the membrane to the backside of the shell. Barry Justus, president of Poolscape in the Toronto area, combines that with a sump system at the pool’s lowest point, to keep groundwater away from the shell.

These systems generally must be applied when the ambient air temperature falls within certain range.

Highly modified cementitious products blend certain characteristics from elastomeric and cementitious systems. “It has a more tenacious, tougher finish than the elastormeric systems, but it’s more resilient than the cementitious systems,” Andrews says. They can have plaster placed over them, however they will need the correct preparation. Tile also can be set on top of these.

They can be used over targeted areas or the whole shell.

Touchy collaboration

These professionals now find it rare for a single waterproofing system to accomplish everything — even on a single job. In one area, you may need to control positive pressure, in another negative. One spot may require tile, the other pebble.

“I don’t know of a single product that acts as a magic bullet,” Andrews says. “And I don’t know of a company that makes a bumper-to-bumper system for a pool.”

Complicating matters more, manufacturers generally don’t want their products combined with others. If you do, it may void the warranty. Be sure to consult with the producer before combining.

Some builders resign themselves to the liability. Justus has found the combinations that work for him in different situations, but he knows the deal. “I’m going to use the best product that’s suitable for our application,” he says. “Sometimes we mix manufacturers, but you’re going out on your own.”

To make sure these combinations will work over the long haul, he tests his products in vanishing-edge surge tanks that are hidden from view. This way, if issues arise, the problems are concealed. He also tests new materials, such as exotic or new tiles, to see how they hold up.

“It’s not going to be a failure, it would just be something cosmetic, and nobody can see it,” he says. “We would do tile tests in them, so we would use different adhesive inside a surge tank, different waterproofing methods, and we could go in and look at it for three or four years. ”

When layering different waterproofing agents —no matter how well they get along with others — perform surface prep before applying the second system.

“You can’t just spray [one product] on and then roll another right over it,” Peterson says. “You have to prepare the surface again by pressure washing off any latent [product].”

Builder responsibilities

Waterproofing problems spur a lot of fingerpointing. And there’s a reason: So many factors determine its success.

It’s not always a product failure, say some, who believe some contractors place too much responsibility on the systems.

“Everybody’s looking for a Band-Aid to stick over inadequate concrete to rectify their poor placement,” says Paolo Benedetti, president of Aquatic Technology Pool & Spa in Morgan Hill, Calif.

Builders can begin to ensure waterproofing success with a well-placed shell. Begin with shotcrete at a compressive strength of at least 4,000 psi, following American Concrete Institute standards. Shoot or pour it so there are no honeycombs or other voids, since many systems only bridge over microscopic holes.

“The best shotcrete guys have small issues here and there with their projects,” Benedetti says. “But they go back and make remedial repairs where they have honeycombs, voids or other small issues. ”

Seal all penetrations, which can undermine the best waterproofing system.

When working with a general contractor or other trades, make sure nobody else applies a waterproofing agent without discussion. If you’re a designer specifying a system, make sure contractors follow that spec — or review suggested alternatives. Contractors should follow the spec.

In choosing a product, research its track record with pools and spas. While some have earned strong loyalty among pool contractors, others enter the market amid claims that don’t always bear out.

Before applying a system, make sure it is suitable specifically for pools and spas, with their chemically treated water, and for the finish you have in mind.

Seek manufacturer guidance, especially the first few times using the product. “They may look at it and say, ‘The substrate is too rough,’ or, ‘This wall’s taking direct heat,’” Peterson says.

Check for restrictions on ambient temperature during application, since extremes can compromise some systems. Note many products are not to be placed over moisture — the shell must be dry.

Finally, thoroughly document projects. Take photos showing each step and how it was performed. To record temperatures, Peterson uses a laser thermometer that connects to a cell phone and works with an app. Made by Ryobe and sold at hardware stores, it reads temperatures and records them with photos that show the readings.