What Is an Underside Lip? How Do I Know If I Need One?

Ordering registers sometimes comes with several options, like an underside lip. What is an underside lip? How do you know whether you should get one or not?


Read on. Here’s everything you need to make the right decision.


What Is an Underside Lip?

Flange, frame, underside lip . . . these all describe a feature of a register that helps it stay in place. Flip the register over so you’re looking at the bottom side. See that thing with a rectangular outline sticking out? That’s the underside lip.

It is used for floor installations only and keeps the register from sliding around. The underside lip may be all you need to keep your floor registers in place, especially if you’ve selected a weightier material like cast iron, steel, or cast aluminum.

Without an underside lip, you’ll have to rely on one of two ways to secure your registers: mounting them to the floor with screws or recessing them so they’re flush with the floor.

Underside of a cast iron register showing the underside lip.

How Do You Know If You Need an Underside Lip?

An underside lip is a must have if you have floors in which you do not want to, or cannot drill holes to secure the register with screws. For instance, a tile floor works best with a register with an underside lip because it’s likely you won’t be able to secure it with screws.

Likewise, on a wood floor (though you could secure your register with screws) you probably don’t want to drill into that beautiful hardwood.

Another option if you don’t want to drill holes is to cut/router the opening in your floor to be the same size as your register then place the entire register down into the floor. This is called a recessed register. It sits flush with the floor and is held in place by the floor itself. You can still order an underside lip for a recessed register, but it is not generally necessary.

If you’re not going to drill holes in the floor or recess your registers, an underside lip is absolutely necessary to keep them in place.


Can You Use an Underside Lip and Secure It with Screws?

If you feel your register needs some extra securing, you can use both the underside lip and the screws to do so. This is a good idea for high traffic areas where passersby may catch an edge and accidentally lift the register out of the hole.

It’s also a good idea if you have children or curious pets who might try to lift the registers off the holes.

You can order any register at Reggio Registers with mounting holes and/or an underside lip. If you don’t see the option to order with an underside lip, call us directly to place your order at 800-880-3090.


When Is An Underside Lip Not Recommended?

An underside lip doesn't work very well if the opening cut into the floor is uneven or is the wrong size or shape. In these instances, the underside lip wouldn’t fit. Instead, secure the register with screws in the mounting holes.

If you have a very deep flooring material like a high-pile carpet, the underside lip may not be deep enough to keep the register in place. You would want to use screws and mounting holes in these cases too.


Can You Use an Underside Lip for the Wall or Ceiling Registers?

You don’t need an underside lip for the wall or ceiling. They are just for floor registers. However, you can generally use them on the wall or ceiling without issue.

Learn more about the making of registers.

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