« Blog

What are In Ground Pool Unblockable Drains – Explained

If you install or spend time around pool drains and suctions, hopefully you are fully aware of the safety factors involved. Specifically, the regulations required by the VGBA – Virginia Graeme Baker Act. First instituted in 2008 and updated a few times since, VGBA establishes important guidelines for suction safety to protect swimmers.

One of the most well known updates was to require all pools to use two drain suctions as a part of the installation (assuming a drain is installed). The reasoning for this is that if one drain is blocked by a person’s body, the other drain can compensate. Swimmers are better protected from entrapment with two drains. Each single drain must be capable of carrying the full required flow for the pools you see, so even if one drain is blocked it can still operate. 

pool drain group

Drains come in all shapes and sizes


There are some downsides to this, as no approach can be 100% perfect. Even with the second suction, a blocked drain can still cause a large amount of suction force to be applied to a body. There can still be some danger of injury or temporary entrapment. But the danger is greatly reduced.

A new option that companies like CMP have introduced is the Unblockable Drain. This means exactly what it says – a drain that cannot be blocked by someone’s body. According to VGBA Section 3.7.3: “Covers or grates marked unblockable may be installed in pools with multiple SOFA (Suction Outlet Fitting Assembly) systems. Covers or grates marked unblockable may also be installed in pools with single SOFA systems when this use is authorized by the cover/grate manufacturer.”

Because of this the pool can have a single drain again, but without the danger of blockage. And it’s even better than two drains because again, even a single blocked drain can pose a hazard. In this case blocking the drain completely and causing suction danger is impossible.


At its simplest, the unblockable drain is exactly what it says. A drain that can’t be blocked. But it is important to actually know how that is defined. Luckily, there are guidelines to follow.

In testing, a specially construction body blocking element is used to simulate a human body covering the drain. This allows testing of suction forces to understand the safety of the drain, and to determine if it truly meets the “unblockable” standard.

unblockable pool drain

Excerpt image from VGBA documentation

According to VGBA, “A suction outlet fitting assembly that, when installed according to the manufacturer’s instructions, cannot be shadowed by an 18 in. x 23 in. (457 mm x 584 mm) Body Blocking Element, and has a rated flow through the remaining open area beyond the shadowed portion that cannot create a suction force in excess of the force calculated in Equation 2.”

The standard continues: “If the sump opening can be completely shadowed by the body-blocking element (BBE) when viewed perpendicular to the pool mounting surface in which the SOFA is installed, it shall be categorized as blockable; those that cannot be completely shadowed shall be categorized as unblockable.”

As you can see, the standard is VERY clear about what is required for the unblockable suction or drain.

We’ve got some great ongoing education videos about pool drains, check them out!

2021/2022 Rules for Pool Drains Explained

How to Check Pool Drain Covers for Safety


While the flow and sizing are the standard we associate with unblockable suctions, these are not the only things that matter.

An unblockable drain must have specific markings to identify it based on the latest VGBA rules. Don’t just assume a drain appears unblockable because it is large. Check the markings. If the suction is designed for single drain use with no other amenities, it will be marked “This is an UNBLOCKABLE VGBA Suction Outlet that may be installed as the sole source of water for a single suction outlet.”

The unblockable drains must also meet all of the current standards for VGBA drains, in addition to unblockable requirements. They don’t get a pass on other rules just because they might be installed as a single drain. It’s important to understand these requirements. We outline these simply in our article about VGBA Standards, What’s Changing in 2021.

cmp unblockable vgba in ground ring drain

The CMP ring drain is unblockable and almost disappears in the pool


Regardless of what kind of suction you choose to use, safety is the priority. A pool drain creates a unique challenge because of its ability to grab a body under water and hold it in place. But tragedy can be avoided simply by staying up to date and taking these regulations seriously. 

Read the full set of rules yourself online – just check out ANSI/APSP.ICC-16 2017. For daily checks we also created this simple resource outlining how to maintain and check pool covers. It’s a short video so you’ve got no reason not to check it out! Interested in learning about CMP Unblockable Drain options? Check out the details on our latest pool drain product, the Unblockable Ring Drain.

If we all work together − manufacturers, builders and the pool service community − we can help completely rid pools of suction hazard dangers. It takes all of us to keep the pool safe, and now you have the resources to do it! Just keep up with the latest VGBA standards and you will be doing your part for pool suction safety.

« Blog