# Backerboard under Fiberglass Shower Pan?



## RippySkippy (Feb 9, 2007)

BillyDIY said:


> ... is there anything intrinsically wrong with putting Durock or Hardy backer cement board under a fiberglass shower pan like this?


No...BUT...just putting the two materials next to each other will NOT make the area water proof. Typically one uses 1/4" backer board for floor applications.



BillyDIY said:


> As an aside to this, the instructions do not indicate any means by which you should secure the shower pan in the alcove. They indicate use of clips for a wall surround but not if you are using ceramic tile. The previous shower pan had some large head nails securing it in place above the flange (obviously not through it). Am I to assume that since the cement board will overlap the flange will be enough to secure the shower pan?


In addition to the nails above the flange, set the pan in thinset to provide support which helps to secure it. 

Outta curosity, what's your plan to water proof the cement board on the wall under the tile?


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## BillyDIY (Aug 24, 2008)

Thanks RippySkippy, and I agree that the cement board also needs a waterproof membrane. And I was planning on a coat of RedGuard for that.

So can I assume that the RedGuard or other waterproof membrane is the key to preventing future rot, especially at a transition like this? (or course there is caulking at those seems as a first line of defense.

I really need to put something underneath the shower pan because the previous plywood has a bunch of gouges and partially rotted areas. Do you recommend against using the cement board? I can easily put a 32x32 1/4 inch piece of plywood there too which has the side benefit of making the floor 7/8 instead of the existing 5/8 and also why for the floor I plan on using 1/2 inch cement board because it is recommended 1 1/4 subfloor to put tile on it. You need to put thinset between the plywood and cement board (plus screws). Any reason I could not do the same with the plywood to plywood combo?

I don't think that ripping up the entire subfloor is an option because this butts up against the 2x4 framing to the exterior wall and the 5/8 plywood goes under it and would be difficult to make the cut that close to the 2x4 stud box frame and also leave yet another seal.

Thanks again.


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