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Question DetailsAsked on 1/8/2018

Will thin set mortar hold 1/4" durarock on top of durarock already on wall? Wall is out of level over 1/4".

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If this is a personal appearance judgment thing, so be it - your $, your call. But if dealing with a contractor, tile (per Tile Council standards) should have a finish surface within 1/4" of level/plumb per 10' so you are actually pretty close (assuming 8' wall height) to that generally accepted standard of acceptability already. And of course, which way it is out of plumb matters too - a wall tilting "away" at the top will not look so bad to most people as one tilting "in" at the top, and of course if it is the "back" wall (perpendicular to normal bathroom line-of-sight) it will not be so noticeable as if it is an "end wall" on a normal tub or shower set perpendicular to the room. And remember, if getting shower doors, the door frame at the wall will (if plumbed up properly) will uuslaly make the wall tilt less noticeable, though you will have a noticeable wedge-shaped caulk seam there.

OK - following all assumes you want to go ahead with truing this up.

Since you said wall, I presume you mean the durarock (actually spelled durock but so many people call it durarock) is "out of plumb" (i.e. not vertical and maybe "licked out" at one side - i.e. not aligned correctly relative to the centerline of the wall), but presumably not out of "level".

Bottom line, what you are talking about is not something I would consider doing. I would do the job right (but that is my general principle anyway) - tear the existing durarock off and shim out the studs to plumb, then put new backer board on so you don't risk future regrets. And if paying for this to be done, unless the durarock is getting a mudcoat on it as a base for the tile (which can easily accomodate a 1/4" variation in thickness but is hard to get perfectly plumb on a vertical surface, as discussed below), trying to correct it with shimming or mudding is going to cost a LOT more than just removing and replacing the existing board with proper shimming. Plus of course, if this problem was created by someone you were paying, HE should be doing the rework for free if it is out of spec (marginally so maybe, from what you say).

Several things I see wrong with your idea. First, while a very good tile layer could lay up a plastic portland cement layer on the durarock to make the surface plumb, it takes a good bit of skill to get a surface like that both plumb and flat, and a 1/4" thick layer would likely not adhere well without a mud-coat screening under it to keep it from slumping or peeling off the surface - especially since thinset does not really adhere well to durock - the special concrete backer board tile setting adhesives are designed to do that. To avoid gaps under the durock you would then probably want to thinly thinset coat this cured smoothing coat immediately before putting on the durarock to smooth out the bearing surface (or use a concrete backer board adhesive), but I would not use thinset to adhere it, it just does not adhere to durarock that well. I would still use screws to adhere the overlying sheet to the studs so you don't have a mass peeloff in the future.

However, two layers like that is asking for trouble - not only hard to get trued up right and avoid ripples in it (which 1/4" backer board will do), but depending on the materials used it sounds like an invitation to trapping any water leakage through the grout inside that double-layered wall, and any penetration would try to mostly run down between the layers or stay under the tile rather than penetrating to and running down the waterproof membrane - just sounds liek an invitation to trouble down the road, because ALL tile joints eventually develop at least a few leak spots - ditto to almost all types of sheet surround joints, though with them there are a few which are designed to be almost totally impervious to water penetration as long as water cannot actually stand against them and pool.

Shimming out a second layer of durarock could be done in theory - but that gap between the sheets, especially with 1/4" durarock, would provide a wet moldy zone in the event of any water leakage through the surround or tile. Plus leaving a gap in there between sheets, with 1/4" backer, is out - one slip in the tub causing someone to lean hard against the wall to support themselves could easily crack and deform the board permanently, causing ripples in the surround (or even breakage in thin or brittle materials), or popping off of tile.

Trying to use thinset to adjust for the wall profile when tiling over the durock would NOT be a good idea - tough enough to get a good tile surface when working on a flat surface - trying to get it plumb and flat AND getting the tile surface good at same time would be a problem - plus the additional 1/4" of thinset would definitely cause slumping problems, and also squeeze out and deform when tapping the tile into place to properly seat and adjust it into place. Doing that would be pretty much assured of causing a warped wall and deformed (out of plane) individual tiles - just too much thickness of thinset to work with.

In a typical shower (roughly 3x6 feet, 3 sided) you have about 72-96 SF of surface, so at about $0.60-0.90/SF for 1/4" board that is maybe $40-100 in backer board and fasteners - more around $150 range if replaced with 1/2" board. I would take it out and redo it with proper shims on the studs to true them up first - and given that you are talking only 1/4" or less thick shimming, use hardwood strips to reduce splitting. Another common practice is to staple on scrap wood shim strips of the right thickness, then just before board installation glop on thick lines of waterproof construction adhesive between (but not on top of) each shim to act as the actual back support (like the stud does) when the board is put on - using the shims only for final location, and the adhesive to provide the actual "bearing" surface to support the backer board at each stud. Other people use construction adhesive to glue shim lath to the studs, let it cure, then use a power door planer (hand planing is VERY tough on the bod and hard to control on the vertical) to trim the thickness to the correct profile - but do NOT go all the way through the shim - hitting construction adhesive with a planer will at least badly dull and may total the blades.

One major concern I have which might also dictate how you solve this - 1/4" backer board is too thin unless you are putting in an unusually heavy-weight panel-type or stone (real or "cultured" (instead of normal sheet) surround over it. Under tile I would demand 1/2" minimum for standard 16" max stud spacing, which is the industry standard minimum thickness for walls and ceilings. I could not get the USG instructions page to come up, but here they are courtesy of Lowes website - can't guarantee they are current but probably do not change much. Note fastener spacing is 8" max - shims (or construction adhesive strips) should be continuous on the studs.

One other thought - realizing my tile laying experience began about a half century ago so I grew up on mudcoats over screening or perforated mesh, and I really have not seen good results (myself or by any other tile layer) with mastics - especially under wet surface tile. Can be OK for hearths and such, just not good in potentially wet environments even if supposedly designed for that environment.

I presume you have properly taken care of your waterproof barrier (membrane) and tying it into the rim of the tub or shower so any leakage through the surround or tile runs down the membrane and into the tub or shower. If you forgot that, after tearing the durarock off would be a good time to get that done right - because to do it right the membrane should be replaced too, otherwise it will have holes in it. Some people would not worry about that it it is not damaged in taking the durarock off, given that the holes are small and can be caulk filled.

And of course be sure you properly tape and waterproof the sheet joints in the durock with proper concrete backer board joint compound - do NOT use standard drywall gypsum compound or tape.

Answered 10 months ago by LCD

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