Installing Fiberglass Bathtubs

The first thing you need to do when installing fiberglass bathtubs is to assemble the parts of the trip lever/waste components. Refer to your manufacturer's instructions. At this time leave all the connections sufficiently loose so that you can reposition the components with ease.


Next, from the back side of the bathtub, seat the rubber gasket in the upper part of the drain assembly in the topmost hole. This is used for the trip lever. Align the lower portion with the bathtub drain hole. Now install the tub drain in the tub’s bottom. First, dab a heavy bead of plumber's putty at the base of the drain flange. Now screw the drain into place but leave it somewhat loose.

Install the trip lever now by sticking the entire assembly in through the top hole of the tub. Secure the trip lever to the bathtub with screws.


We are now ready to mount the fiberglass tub. Carefully set the bathtub in place. Check it to ensure that it is plumb and level. If not, use wooden shims where needed to adjust the bathtub. With that done, secure the bathtub to the wall using screws.

Your next step is to install the diverter and spout. To get it just right, draw a perpendicular line with a pencil on the center of the back of the bathtub. Next draw a horizontal line. The height where you draw this line should allow the spout to be one inch above the top of the fiberglass bathtub.

Now you are ready to mark the point above the spout where you will place your diverter, used to turn the water to the tub off and on. The pilot holes should be drilled from the back of the bathtub. Using hole saws, cut the correct size holes from the front side of the bathtub.

Diverter Fittings

You are now going to install the copper fittings in the diverter. First, wrap Teflon tape around the threads. Then snap the plastic supporting device onto the diverter itself and insert the diverter in through the back of the bathtub. Fasten the diverter to the tub by securing the escutcheon plate at the front side of the bathtub.

You will now cut a piece of pipe to use in connecting the bottom part of the diverter to the spout. As you take your measurement, be sure to subtract an inch to make up for the size of the fittings.

Valve off the principal water supply to the house. If there are any caps on the ends of the water lines, remove from. You might find caps soldered in place. If so, use a torch to heat them and then pull them off. Cut as many pieces of pipe as you need to install water supply lines at the diverter’s sides. Be certain that you connect the lines to the correct fittings.

Shower Riser Pipe

It’s time to install the shower’s riser pipe from the upper part of the diverter. Fasten the upper part of the shower’s riser to a supporting block attached between two studs.

Next, install greenboard around the bathtub’s enclosure. You will have to install quarter inch shims on the studs nearest to the bathtub to make up for the dimensions of the unit’s flange.

From bottom side of the bathtub, dry fit the P trap to ensure that everything will fits correctly. Put the brass adapter fitting onto the drain tailpiece. Connect the brass adapter to the bottom side of the bathtub loosely. Connect the P-trap to the tailpiece. Connect a horizontal section of pipe to the P trap. Attach the opposite end of this pipe to the master drain line.

You’ve now completed installing your fiberglass bathtub. Turn on the main water to the house, run the tub water, and check for leaks.

See Also:

Installing a Bathrub Overflow Seal