What kind of material should I use behind my new shower system: The manufacture of these waterproofing systems say use a dimensional board, lets call these : DBs

This means: MR Drywall/Drywall - Hardi Board - Plywood ...most boards that are dimensional bonded by anything other than just gravity.

What is not dimensional is  standard Concrete board. There is a big misconception in the industry that says; sure go ahead and use concrete board... this is a fallacy. I will explain why. Lets call it CB for this conversation.

CB will flex between your studs, CB is the most non moisture repellent material available for tile setting... other than cardboard and if you are setting tile on cardboard there is a loose screw somewhere and it ain't in the CB...

I would advise you to never use CB on your walls. ( my pinion)

MR Drywall AKA; Greenboard - Blueboard - Yellowboard - etc...

I actually prefer MR Drywall MR= Moisture Resistant. You don't need MR but due to code in many states it is required in any wet set or moisture prone application. The screws are easy to countersink, its easy to work with and its the lesser expensive of all the DBs. Do not tape your seams with drywall mud if it is not already done, use a 3-5 pattern on your screws- use treated screws, not drywall screws as they are made of a high carbon steel and are prone to rusting, the lime and moisture content in your thinset will eat/rust the heads off the standard drywall screw in short time.

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Drywall: This is an acceptable substrate if no MR is available or if you already have it installed or you just feel like it. Follow the instructions for MR drywall and understand the 3-5 pattern means 3 screws in the field and 5 screws on the edges. The reason you do not want to tape the seams (unless they are already taped) is that thinset and gypsum do not play well when mixed together with a barrier of ; say paint or primer.
 

Hardi-board: This is premium application, however it is not needed at all, it is redundant much like these new foam boards on the market. It works very well but you will have a very hard time countersinking your screws. Most tile ready niches have a 1/2 thickness and most of the time the hardi you use on walls is 1/4. This can create an issue when you try to run your tile from wall to niche interior edge. Hardi also has very specific instructions that 90% of people do not follow and this mean professional installers... Hardi states that you must damp mop each sheet prior to putting the thinset up as the gypsum material in Hardi will wick the moisture out of the thinset before it has time to dry/cure. The damping also removes the excessive layer of cement and gypsum dust that Hardi produces, and opend the pores of the hardi-board to allow the thinset to bond. Pores and hardiboard together are an oxymoron to me , but that is what they say...

Who is they...and why are we listen to people that call themselves...they...

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Plywood: This is an acceptable substrate. It is actually the most superior substrate than can be used it has a R-value that non of the others have... so....R- Value means while your showering in a house with thin walls your visiting family members that should have left after the third day... (Benjamin Franklin; Three days, fish and houseguest)... will be less likely you hear slapping soap on your armpit while gleefully singing showtoons...

However if you use plywood be prepared to break a few screwheads off and break a few screw tip bits while you attempt to countersink your screws. You will a tank like shower! (FYI: In hurricane prone areas they use 3/4 plywood as wall support under the drywall.)

Can I trim my shower base?

Yes, trim it as you need it. You can use a utility knife and cut both side and break it off or just grab lunch and cut through it with a hand saw. I say grab a lunch because by the time you get done everybody else will have eaten their lunches...

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If my tray is too small can I add mud to the edges?

No, not my first choice and never a good choice as now you are bringing something of greater density into the mix, with greater density comes higher PSI ratings, with this come fragility. I had people brag about how they used quik-crete and made their own mudbed, they say "I never had a problem"... that you know of...

High PSI concrete will break unless it has bedding under it that allows movement, sure it sounds strong; 'I got 5800 PSI concrete on my shower floor!'  You have just set this shower up for failure, as the walls and floor expand and contract the concrete will not, it will just break.

This why I developed the tray extensions you see in the "My Kits".

It is important you have equal density material on the floor you will be spraying hot water on for 5-10 minutes each morning and night, or if you one of my kids it seems to be 30 minutes, or more...

My Tray Extensions come in several sizes 6"- 12" and 24" they are pre-sloped and designed to match the slope of the tray you purchased.

With any other greater density material you can get pressure issues that lead to what we call Picture Framing, this means the greater density material will move with your substrate or walls and cause a separation between the styrofoam tray and the mudpack that will show through the grout lines in accordance with the application below it. Hence the term;  Frame:The mudpack has moved while the tray has not and created a framed separation that has transferred through to the grout joint. 

Picture: You see the broke grout line

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