How to Install a Drop-in Tub as an Undermount Tub in a Shower


Spa bathtub Prefabricated tubs such as this drop-in are commonly made of acrylic. Image Credit: Jupiterimages/ Images

Bathtubs come in many types, including prefabricated units. Prefabricated baths consist of fully formed tubs designed by manufactures to fit into an existing tub frame. A variety of choices exist for prefabricated bathtubs, including undermount tubs and drop-in tubs. Determining whether and how you can install a drop-in tub as an undermount in a shower requires an understanding of each tub type and tub installation procedures.

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Drop-In Tubs

Drop-in tubs get their name from the fact that they drop into place. When installing a drop-in tub, you place the unit into a tub frame from above. Drop-in tubs invariably possess rims, which catch on the rim of the tub frame to hold it in place. The rim sits above the level of the tub and tub frame. After dropping the tub into place, connect the plumbing and applying a waterproof sealant or caulk around the rim of the unit to prevent leakage.

Undermount Tubs

Undermount tubs install from beneath, rather than above. With bathtubs, the installation of an undermount tub requires access to the frame of the tub, usually by removing the top of the frame. Upon gaining access to the inside of the frame, place an undermount tub in the fame and position it so the lip of the unit sits underneath the lip of the top of the tub frame. Apply caulk or sealant around the lip of the tub when you attach it to the frame to prevent leaks, and replace all framing elements removed to install the tub. After installing and undermount tub, apply more caulk around the juncture of the tub and rim for additional leakage protection.

Installation Considerations

The actual tub in an undermount bathtub differs little, it at all, from the tub of a drop-in bathtub. However, the rim of a drop-in tub differs significantly from the lip of an undermount tub in design. While manufacturers build drop-in tub rims to sit on top of a frame and hold the tub in place, the lip of an undermount tub must attach to the bottom of a surface and anchor the tub in place. Because of this, the shape of a drop-in rims are often not conducive to undermount installation and also not designed to prevent leakage in the same direction as undermount tubs.

Drop-in Undermount Installation

You can ostensibly install a drop-in tub as an undermount by attaching it to the underside of a tub frame and caulking or sealing it in place. However, this may lead to problems like water leakage – the tub will fit, but it may not work properly. You can try this solution, but keep the risks of it in mind. Or, many companies produce prefabricated tubs designed to install either as a drop-in or an undermount. Purchasing one of these tubs can solve this problem. If you already own a tub, contact the tub manufacturer to determine if your tub can in fact install either way.


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