Herrick Kimball, a remodeler and kitchen contractor in Moravia, New York, replies: The challenge here is to attach something that must be rock solid (a grab bar) to something that is thin and flexible (the wall of a fiberglass tub/shower unit). My solution is to screw the grab bar directly into blocks attached to the wood framing behind the wall. This approach works only if you can access the back side of the shower wall and may entail removing and replacing drywall.
After exposing the framing and the back of the shower wall, drill small lead holes through the shower wall where the grab-bar fasteners will penetrate so that you know where to install the blocks. Now toenail or screw in a piece of 2x blocking between the studs. I use short lengths of 1/4-in. galvanized pipe as rigid spacers between the block and the grab bar’s mounting plates.
With the blocking in place, drill holes slightly bigger than the outside diameter of the pipe through the shower wall in each mounting-screw location, and cut lengths of galvanized pipe so that each will be just proud of the shower wall. The pipe must be long enough so that the mounting plates attach snugly to the wall surface without distorting it.
Replace the attachment screws supplied with the grab bar with stainless-steel machine bolts long enough to go through the clearance holes and through the blocking installed in the stud wall. (I don’t believe long screws are strong enough to attach a grab bar.)
Once the clearance holes are drilled and the pipe cut, drill holes for the bolts in the blocking from inside the shower wall using the pipe as a guide. The bolts have to be nutted to be effective, and T-nuts driven into the blocking ensure that you can retorque the bolts if they loosen in the future.
I seal out water with rubber gaskets cut from 1/8-in.-thick gasket material that I found in my local hardware store’s plumbing department.
To stiffen the wall, apply a liberal amount of expanding foam directly behind each attachment point. When the foam cures, the installation is done.