Vaults can solve a variety of pool “problems.” In a swim-up bar, a sunken vault resolves one common dilemma: how to put the “bartender” on the same level as the other swimmers. The same concept — only completely buried and enclosed on top — helps conceal and silence the equipment pad in a tight space.

But to make the vault work, a designer must account for its structural makeup, drainage and access. Here, experts explain how.

Create a drainage plan

Whether it’s from rain-water or a broken pipe, vaults are susceptible to flooding and ideally should have at least one drain.

For sunken bars, experts prefer regular surface drains with the smallest openings possible.

“If you have a big grate, then a woman will get her heels caught in it,” notes Brian Cullingworth, president of Brian Cullingworth Custom Pools & Spas in Temecula, Calif.

You can then send the excess water to the sewer or street.

If elevations allow, you can also use gravity to remove the drainage. Just look for a spot where the vault can sit above street level or another major drainage destination.

Don Goldstone recalls one recent client who was adamant that the drainage be handled by gravity.

“I said, ‘Have your general contractor give me a drain line that’s low enough to make it work with gravity,’” says the president of Ultimate Water Creations in Los Angeles.

“[The GC] put in a 10-inch drain line for the whole backyard, which runs out to the street, which is lower than the house,” Goldstone adds.

If gravity isn’t an option, you’ll need a sump and sump pump beneath the drain.

Make the sump large enough. If it’s too small, it will undoubtedly overflow.

“What if the sump pump has dirt clogged in it and isn’t working properly?” Goldstone says. “Water would fill that hole faster than the pump can move it out.” So he makes his sumps at least 2-by-2-by-2-feet, which allows for some extra space for water to fill up.

It’s also important to isolate the pit from other drainage in the yard. Cullingworth always adds a vacuum break between the sump pump’s evacuation lines and other drains in the yard.

“If you connected the drains, the possibility exists that the drains could back up and everything would siphon down into your pit,” he says.

Local building officials often pay close attention to a sunken bar with a sump. If the sump pump fails or there’s a power outage, the vault may flood and the overflow could seep into neighboring yards, or erode nearby hillsides.

In Goldstone’s area, some building departments require signatures from neighbors acknowledging that a sump is close by. In addition, some mandate a second sump pump for backup, and an alarm in case both fail.

If a sump is necessary, always explain these complications to the client, Goldstone advises. Some will resist using a sump, especially if it requires them to jump through too many regulatory hoops.

Make it structurally sound

For a swim-up bar, the vault is generally constructed in the same fashion as the pool. Buried rooms, on the other hand, can be built out of shotcrete, gunite, poured concrete or block.

“It’s basically a four-sided retaining wall that has a lid on it,” says Steve Sargent, president of Elite Custom Pools and Spas in Lake Forrest, Calif. “We use retaining wall engineering when we build them.”

As such, experts strongly suggest consulting an engineer when building a buried vault.

“We need to provide a structural design to make sure that the lid can support any sort of loads that are going to be on top of it,” says Todd Lacher, associate engineer at Pool Engineering Inc. in Anaheim, Calif.

If you plan to cover the vault with grass, for instance, you’ll need to place at least 12- to 18 inches of topsoil over the ceiling — more if boxed trees will sit on top of it.

The configuration and depth of the vault must be considered, too. Structurally speaking, a vault does not operate in the same way as a pool.

“The way the vault resists [soil] pressure is different than the way a swimming pool resists it,” Lacher says.

Further, a vault lacks the structural support inherent in most pools. The vault is basically a box with a floor, four walls and a ceiling. It doesn’t have the rounded contours and thicker concrete of a swimming-pool cove.

“We also have to design the wall to make sure that it is stable and that its connection to its foundation is adequate,” Lacher says. “With a pool, there’s no definitive wall and foundation. It’s all one shell — the wall just monolithically tapers into the floor of the pool. With a vault, it’s not like that. You would go in and pour a foundation slab, and then you’d come in and construct walls.”

The engineer also must consider depth and design accordingly because pressure increases the deeper you go. A wall that may only need to be 6- or 8 inches thick above grade, for instance, may need to start at 12 inches thick when buried 10 feet underground.

Consider access

Buried equipment vaults can be placed beneath a deck or grassy area. Just make sure the access is adequate and convenient.

You may even use a simple manhole.

“For some of the access holes, we just use metal grates, and then locate the heater directly underneath them, and that vents out,” Sargent says. “It’s not ideal, but it seems to work pretty well.”

Another option involves creating an access door out of the deck material, similar to concealed cover-box lids.

“We make a stainless steel or metal doorway, and then have the stone inlaid down into it,” Sargent says. “It’s not completely invisible, but it’s pretty darn close.”

To set the stone, crews just use a mortar base. Then they hinge the tray so it can be easily lifted up and laid down on the deck when fully opened.

But, Sargent warns, if you use a concrete sub-base beneath the decking, don’t place it on the metal lid — it’ll likely become too heavy.

And regardless of how you build the access door, make sure it’s wide enough to allow the largest piece of equipment to fit through, lest it needs replacing.

“Heaters have come down in size so much that you can get away with about a 3-by-3-foot access hole,” Sargent says. “If it were a high-efficiency heater, you’d need a little more room, but not much.”

Also, install a ladder close to or directly underneath the opening.

Sargent likes to place the door at the corner of the room and mount metal rungs to the wall. It’s a space-saving alternative to a pre-manufactured ladder.