If the plywood extends under the pan someone can still pull out the exposed area 20 years from now and cut it flush at the edge of the tile, leaving the shower untouched.

OTOH, how much might a piece of inherently dimensionally stable plywood shrink? 1/16"? Even 1/8"? That's why they make caulk. It seems unlikely that your meeting between linoleum and shower is going to be so amazingly perfect that it doesn't need to be caulked from day one.

What are you using for a shower surround? Purple drywall is not acceptable as a substrate even with a membrane.

> Purple drywall is not acceptable as a substrate even with a membrane

It is when the shower surround is solid surface.

I am not a pro, but I researched the heck out of solid surface showers before my builder did mine a year and a half ago.
Which solid surface pan are you using? What are the installation instructions? Many (most?) are set into a bed of mortar. It fills all the voids under the pan, making it more solid and preventing any flex or movement. I know that my sister’s acrylic shower should have been done that way (but wasn’t - brothers!) and it cracked and is no longer usable. Mine is a 300# solid surface one (Transolid) and it still went in a mortar bed.

Where you put your subfloor for this depends on a couple things. Do you want the shower pan 3/4” closer to the height of the room floor, or does that not matter to you? If you want that 3/4”, then cut your subfloor under the shower to go in strips between the floor joists. If it does not matter, just do subfloor across the whole room. I cannot for the life of me remember what Jim used for my subflooor - plywood or cement board. Your shower receptor manufacturer should say. If you did not get installation instructions, go to the web site of the seller or manufacturer and look for a PDF download. There should be one for specs and one for instructions.

I have a PDF on the sunken subfloor in my big computer holler if you want it.

I don't believe drywall is approved in a shower area period even behind solid surface. Drywall manufacturers for years have said absolutely not to use but.........

I would likely choose to do the shower first and the floor after.

Millworkman, true solid surface (thick panels that have color through and-through and can have surface repairs done on-site - Corian and the like) can be installed over drywall. Not fiberglass, acrylic or other sheet material that is not solid surface.

Greg Wellman, I forgot to mention that my builder, who was using Kerdi products to make a waterproof floor with a drain outside of my flat-entry wheelchair-accessible shower, used the product Kerdi Fix to seal the gap between my shower pan and the floor sub flooring. I don’t know for sure if this product is that much better than regular caulk, but in this spot that will not be accessible to replace the caulk in the future, he bought a tube of Fix for just this application.

Shower pan is usually installed long before drywall. It goes in with topout plumbing.

The Lauan plywood is NOT an acceptable substrate under tile as it doesn't have the strength to properly support tile. The grade of plywood accepted by the NTCA is CC or better. Any plywood marked "sheathing" is not acceptable either.

As far as the purple drywall, the manufacturer disallows it in shower situations unless a surface applied membrane is installed over it - generally rendering the purple part of the drywall useless. Might as well use the cheaper white drywall.

6. Not suitable for use as a substrate for tile in wet areas such as tubs and showers, gang showers and other areas subject to direct water exposure. 7. Use as a tile substrate is limited to tile installed according to the most current TCNA and ANSI specifications.

Even with solid surface materials as a finish surface, the shower MUST be waterproofed to industry standards.

Not according to the manufacturers of SS showers. They are good with the drywall.

When installing a traditional solid surface shower, you are correct that the shower floor is installed first. Then the drywall is hung so that it leaves a gap above the floor (which may be caulked before the shower wall is hung, I don’t think the pictograph instructions for mine made that entirely clear) and the shower walls are hung on the drywall hanging below that gap and in front of the shower floor’s flange. This gap ensures that if any water gets behind the caulk at the floor/wall joint, the water never reaches the drywall to be wicked up. In the shower, any horizontal seams are interlocked so that water does not enter. Corner joints are either caulked or are caulked and covered with corner molding that seals out the water. Both Swanstone and Transolid instructions say that water resistant drywall is fine. One style of Transolid wall is already drilled out for all fixtures and handrails and the 1/4” SS material is backed with an aluminum mesh to make sheets rigid and that is glued directly onto studs. Ferrules are provided for all of the screw holes and studs must be placed according to the instructions.

European manufacturers of flatter shower floors , like Duravit, do not use a flange and instructions do not have them set into mortar. If I remember the one set of instructions that we were able to figure out, they even used the shower drain weep holes to take care of water that gets behind the walls and under the shower receptor. The joint at the wall/shower floor interface was just caulked, with no flange. I imagine these need waterproofing. I never understood the construction of these showers. They were not solid surface, I think, but more like acrylic. With manufacturers using pictograph instructions instead of words, even study of them can leave you perplexed.

Nancy can you show us where the drywall manufacturer says this is suitable?

The drywall is not acceptable according to drywall manufacturer. Even with a waterproof membrane applied. Does not matter what Schluter, Laticrete, Custom and others say. I was a Tech Consultant for USG. Their shower membrane was far superior to Schluter yet they still would not allow drywall as a substrate in a wet area period.

It's simple. Why install a product that can be damaged by moisture in a setting where moisture is present on a daily basis?

No, just where the shower manufacturer does.

A shower manufacturer can NOT approve another manufactures product to be applied in a manner that is unapproved by that product manufacturer themselves. Sure the shower folks say they can bond to it. That's it!!!

A leaking mixing valve, control valve, pipe or face plate gasket, moisture in the wall cavity etc. will break down the dry wall from behind the shower system. Now who you gonna call? Ghost Busters:) Shower manufacturers will say their system did not fail. What failed was a product used in a area that it was not approved for.

A similar scenario is the prefab tub and shower base manufacturers instructions showing wallboard ending above the flange which is a detail that does not exist in the tile world. It is a movement joint and would need to be honored through the entire tile assembly. The board being shimmed so it continues past the flange is the approved method.

Yet these products provide instructions for unapproved methods. Buyer beware!!

Thanks Mike. I knew that but wanted Nancy to hear it.

I'm having all the solid surface done by a solid surface pro. The pan is a custom construction made from sheets of the solid surface bonded together and then shaped/milled/whatever. He does this for a living, so I figure he knows what he's doing. He works with all the brands of solid surface (Corian, HiMacs, etc.) I'm using Avonite because my wife preferred their "Summit" pattern to less expensive but similar HiMacs patterns. The solid surface is crazy-expensive, which is one reason I'm trying to do all the less-precise stuff like demolition and under-surface prep myself.

My solid surface pro said to use the purple drywall. There isn't a huge amount of information online about doing solid surface. I had assumed I'd be putting in cement board, but he said the solid surface doesn't adhere well to cement board making the water resistant drywall the material of choice. Hey, it beats what was there before - the old three-wall one-piece fiberglass shower was installed on studs, period. I pulled it out and was looking at the insulation. So good thing the fiberglass never cracked. The solid surface material should never leak, so the only remaining danger is the plumbing ... and let's face it, a house is full of plumbing behind regular drywall.

My solid surface pro didn't seem to have a strong opinion on the question I asked here about the plywood underlayment. He was fully prepared to put the pan on the OSB subfloor, but when I pressed him about the plywood-for-the-linoleum he said to put the plywood in first and he'd put the pan on top of that. So that's what I plan to do, but it doesn't seem like there's really a right and wrong answer there.

"The solid surface is crazy-expensive, which is one reason I'm trying to do all the less-precise stuff like demolition and under-surface prep myself."

All the more reason to have the substrate be correct.

When we had the shower/tub one piece thing removed in order for a cast iron tub and then tiled shower surround walls put in, the guy who did the demo of the old stuff was the one who had the qualifications to also correctly install the cast iron tub - the difficult part came when the guy doing the tile got involved in the project. If the cat hadn't managed to find her way inside the walls around/underneath the lip of the tub while he was out at lunch, and was too scared to move when he came back and got started up again making all the noise and whatnot - I probably would have never known that the water-resistant drywall he was putting up wasn't actually attached to any of the wall studs (he used random pieces to attach it to instead, but none of that was secured to wall studs either...yeah, I know...makes no sense but that's how it was!) ...so in a way I'm thankful to that cat for getting into a situation that required me to carefully take down each and every piece of drywall that he'd finished by the end of that first day, he'd only left a space around 10" wide by 18" tall with only some kind of paper stuff there which is how I'd figured out where the cat had gotten to in the first place (the paper wasn't as muffling as the drywall was so while I knew it sounded like she was somewhere in between the lower and upper floors, I had to go in and listen by the papered section to really figure it out). If the cat hadn't gotten trapped I'd have gone in to take a shower one day not long after the job had been completed and had an avalanche of tile fall down on me because the drywall wasn't secured and was installed with all that tile weight in a 'wet area'.

Just because someone does installation of a given thing for the house for a living doesn't mean that they're necessarily going to know all the things there are to know about a particular thing that happens to be the thing you want them to install for you. It's also possible for someone who has been in the business for decades to get into certain habits when they do a job and become extreeeeeeeeeeeeeemely resistant to any newfangled ideas about how things should be done - ideally this is where the instructions from a manufacturer would help out, but considering how...unclear...those things can be, and how strongly manufacturers feel about having to shell out to customers when something goes wrong (why else would they provide hazy-at-best details on how to install things, along with a bunch of fine print that trust me, IS ALL THE EXCLUSIONS that will mean they don't have to pay anything for the failure of their product...and homeowners must sift through an assortment of professionals, put those professionals through an obstacle course designed to weed out all but the best for their particular job, pray that they can afford that best option, etc etc)... :P

Tear out the drywall now while you have a chance. Look closer at your PRO, and said advice.

Cost of drywall. 8x4 AVG $8-$10

Cement board 3x5 $10

Waterproof foam board $25

do the math on your shower and tub area sqft. and see how much your PRO has saved you on the front end here.(never mind the back end for now :))

if all the rotted mushy black mold drywall ive pulled out of showers and tub surrounds has anything to say about pricing, its that the slightly more expensive cement board will save you money twice., the foam 3 times.

How anyone can argue the drywall point baffles

@cabot CC? BC?

Thanks. I wish this had been discussed in the five years I was researching this before my shower was installed. I don't recall ever reading anywhere that manufacturers' instructions should NOT be followed. The concept of leaving that gap is clearly stated in my product's installation instructions. The instructions call for 1/2" water resistant substrate for walls that ends 1/2" above the lip of the shower pan.

It sounds like those composite shims (discussed in another thread) are what you want in the space between the lip extending up from the shower pan and where the "water resistant substrate" ends. I do not understand exactly what that looks like. I cannot envision a 1/4" thick shim going only that short distance the whole perimeter of the shower pan. I agree that it sounds like the only proper underlayment for shower walls is the waterproof foam board. Then the gap is unnecessary. I do not remember what my contractor used. Lets hope the Kerdi guy who helped him plan the bathroom floor with drain also suggested their boards for behind the walls if Jim was not already planning on them. I am not sure that I knew that waterproof foam wall boards existed when we were planning the shower.

The more you pros speak up about these issues in the industry, the better for all of us consumers.

Speaking up comes at a cost here Nancy.

"The instructions call for 1/2" water resistant substrate"

Here is the rub with that statement and what people often over look. Purple drywall is MOISTURE resistant not WATER resistant. Yes there is a difference. And I will guarantee that when the Moisture Resistant Drywall fails the Solid Surface company will throw that right back at you

When lawyers start playing the semantics games, we are all lost.

The other issue with the drywall; there is a probability you could end up with black mold and not know you have it.

Nancy, It's not semantics its just not reading and comprehending part. Water resistant is not water proof. And Purple or Green Board are Moisture Resistant and you even wrote above from the Solid Surface Mfr that does not say Drywall of any type is not ok it says Water Proof Surface. I used to deliver and later sold bldg materials for years and saw more drywall ruined by not being tarped properly, bldg roof leaks, water splashing, etc, etc, even Moisture Resistant Drywall turned to mush.

Yes, you are right, but when you are not in the industry, there is no difference between moisture resistant and water resistant until you really stop and think about it. Okay, so it can get moist, but not wet? How does THAT work? What is the dividing line between moist and wet? See, at some point you are into semantics. We are supposed to research the building terms so that we can learn what we do not know we don't know, but we might not even realize there is a difference between the words "moist" and "wet" when we read them. Those are the times that even careful homeowners can get caught.

Often, we are not even doing our own work, but are trying to learn so that we can make sure that our hired contractors are doing things right. So it is not even a case of trying to do another professional's job by DIY and that being the cause of the problem. Then you add in this news that we cannot trust the instructions of the product manufacturer, and it begins to feel like there is no way to win.