View Full Version : Backerboard for shower/tub

09-29-2004, 06:51 PM
Can anyone help?

Im redoing my shower/tub, but one side of the tub, the side where the valve is, sits about 1/2 inch away from the studs. So, when i go to put the backerboard on, it will sit on the lip of the tub. I believe the lip is suppose to be behind the backerboard and then i leave the backerboard 1/4 inch off the tub.

So my question is....do i just bring the backerboard down to the top of the tub lip? A friend suggested thats what I should do. He then said to fill the 1/2 inch below the backerboard up with the thin set mastic. Then tile over it and run the tile to the tub.

The lip I'm talking about is about a 1/2 inch up from the tub. This is where you nail the lip to the studs. The other sides are okay, just 1 side is way from the studs 1/2 inch.

Sooo...does that sound okay? Run backerboard to the lip, then fill the 1/2 inch below the backerboard with thin set mastic....then run my tile all the way down to the tub...

Any help is appreciated!...thanks

09-30-2004, 09:47 AM
Someone on Terry's site probably knows the answer but there is a similar site just for tile check it out John Bridge (http://www.johnbridge.com/vbulletin/)

09-30-2004, 04:54 PM
What you suggest is NOT OK. The backerboard MUST overlap down over the inside of the tub flange. Your situation is common, and the solution is to put furring strips on the studs. I always use some 1/8" lath strips jsut to get the backerboard to be plumb. In your case, you may find some lath strips of the proper thickness, or you could even rip some strips of 3/8 or 1/2 plywood...whatever it takes.

Bob's HandyGuy
09-30-2004, 05:01 PM
I had the same problem when I re-did the tile in my bathroom. In my case, the backerboard was actually behind the lip. I created a slope from the backerboard to the lip with plaster. I primed and painted the plaster then trimmed a bit away from the tub end to prevent wicking of any standing water. It's been several years now with no loosening of the tiles, even though I used green board instead of cement board. As long as the last course of tile is large enough to secure most of it to the board with mastic, I think you'll do fine.

09-30-2004, 08:38 PM
Thanks for the help guys....Im not sure what to do, it sounds like i should put furring strips on, but then it also sounds like i may be able to slope it away and be okay.

What if I put a couple of layers of roofing paper behind the backerboard and this allows the board to go over the lip...would that be okay? What do you guys think?

Thanks again!

Bob's HandyGuy
10-01-2004, 02:55 PM
If you can shim it with roofing paper, why not just rabbet the back edge with a router?

10-01-2004, 06:56 PM
I presume this is a fiberglass tub when you talk about nailing it into the studs. While I usually agree with Jim I do remember that he lived on a boat for a long time (or was it a ship?). I would center the tub for a 1/4" on each end and cut out the back of the concrete board. I have stopped the concrete board at the lip on cast iron tubs but they just have a little rise. Shimming the wall may make it hard to tie into the existing wall flat.

10-02-2004, 03:02 PM
Sounds like you've already set the tub and probably the waste and overflow and may not want to try to move it. I recently retiled a bath alcove and also had a 1/2 inch gap on the supply wall. My solution was what Jimbo suggested, i.e. using 1/2 inch firring strips on the studs (You could also use a layer of 1/2 inch greenboard behind the backer board). However, this pulled the tub section of the wall a 1/2 inch beyond the plane of the adjacent section of wall, making a step-down. My solution for this was to use a piece of 1/2 inch thick decorative moulding to bridge the two planes and run the tile in the tub alcove to it. I used the same moulding on the opposite wall to edge the tile for symmetry.
Don't forget to leave a small expansion gap to fill with flexible caulk between the tub's horizontal edge and the bottom of the backerboard and tile.

10-02-2004, 04:00 PM
Please, what ever you do, do not install the tile backer board without a waterproof membrane behind it. Wonderboard and Durock are cementitious products and therefore hygroscopic. They absorb moisture. I am a plumbing contractor, not a tile guy, but, I believe the product known as Denshield is the only actual waterproof material.

John Bridge
10-02-2004, 06:58 PM
Hello TD, :)

Actually, the backer board can stop just above the tub flange, and the tile will carry down to the tub deck. The joint between the tile and tub is caulked.

10-03-2004, 04:02 AM
I've never used the Denshield product to which breplum referred, but I have heard many people speak of it negatively. My understanding is that it's a gypsum product like sheetrock, but with a waterproof coating and, once you nail or screw it in place, you've pierced the the coating and made it ineffective as a moisture barrier. I imagine also that, if your bottom caulk joint fails, it can wick up water much faster, hold it longer, and deteriorate more quickly than a cementitious backer board. The commonly used product that I know of for a moisture barrier is 15 pound felt or roofing paper tacked onto the studs (fasteners sealed over with roofing cement or waterproof caulk). Your backer board is nailed on over that (My preference is Hardibacker - I find it easier to work with than Durock or other cement boards). However, I was always puzzled by the fact that no one ever seems to mention waterproofing the backerboard fasteners, which would seem to create a pathway into the studs for moisture to travel. I'm not at all convinced that well done tile work, with sealed grout and properly cualked tub and corner joints that are maintained, ever creates a damaging moisture problem. I've taken apart a number of old tiled shower/bath alcoves that were put together in the 'old days' using no moisture barrier and tiled directly onto regular sheetrock or plywood. I never found moisture damage except in spots where the grout or caulking failed or was damaged and was never repaired properly. I just don't see how enough water can seep through an intact tile/sealed grout/caulked barrier to cause moisture damage. I have some experience with this, though not an enormous amount, so I would defer to any professional tilesetters who may have different experiences.

10-03-2004, 11:02 PM
Thanks for all the help guys...one more question.....I have some 1/4 inch gaps between my backerboard and the sheet rock, most of these areas are hign enough where water wont get on it too much....should I just use the backerboard tape and call it good...or should i use some wall joint compound to fill it then use the backerboard tape...or something else?


10-04-2004, 01:32 AM

I assume you are talking about an untiled area with a backer board/sheetrock joint between it and the top of your tile field. If the joint or "gap" will be covered by the top edge of your tile, then I would use fiberglass mesh tape and thinset fill, which can be done with your thinset mortar when you set the tile. If the joint is above the top edge of the tile field, then I would use tape (paper or mesh) and wallboard compound (make certain this joint is properly sealed with your susequent paint layers).
Good luck!

10-06-2004, 10:23 AM
When putting my tile on, do i run the bottom row of tile all the way down to the tub or do i leave the tile off the tub a 1/4 inch?

Also, what do i use to finish off the bottom row of tile....do i use caulk or grout to fill the area between the tile and tub?


10-06-2004, 11:44 AM
I'm not Charlie but caulk not grout. I put a ledger board system around the tub box about three inches off the tub and build up from there with large cut pieces at the tub because of its propensity to not be level. I prefer tile to within an 1/8" of the tub.
Is this how you do it John Bridge? (reading us?)

10-06-2004, 12:51 PM

I'm in agreement with what LonnythePlumber says, though the size of my cut tiles at the bottom row depend on the size tile I'm using and the height of my tile field. But that has more to do with one's sense of aesthetics rather than any hard and fast rule (but maybe professional tilesetters have a rule of thumb?). I also like to leave a 1/4 inch between the tile bottom and tub, although this is a harder gap to fill smoothly than the 1/8 inch that Lonny prefers, which is perfectly adequate.
Though not always necessary, I like to know the purpose of doing things a certain way. Thus, an explanation for using caulk versus grout between tile and tub. Your grouted tile field cemented to your backerboard is a solid, inflexible sheet. But it's attached to your wooden wall studs, which are subject to expansion and contraction, especially along their length. Although this movement is tiny, if you you had inflexible grout at your tile/tub joint, it could either move your entire tile field away from your tub, opening a slight gap, or toward your your tub, causing compression and cracking. The flexible caulk prevents these scenarios from occuring. The same applies to the vertical corners where your tiled walls meet.
I hope that doesn't sound like a lecture. I just think that, if we understand why things are done a certain way, we do them better.

10-06-2004, 02:44 PM
Charlie and Lonnie.....thanks for the help....and the explanation is great.

10-06-2004, 02:58 PM
I agree with Charlie about the caulk which should be used whenever different planes or materials intersect. The tub expands and contracts at different rates than the walls which can cause the grout to crack and admit water. I try to leave at least a 3" tile on the bottom row that touches the tub. I feel that too small of tile at the tub makes the wall weak. I plan my bathroom tile job from the tub out.

10-06-2004, 04:46 PM
You're very welcome. Sharing our information/knowledge/eperience helps all of us to do better. Lonny has done quite a bit of that and I appreciate his input as well as the other professionals who also give advice on this forum. I've found many answers to my questions on the forum's current and past discussions. It's a great resource and we ought to frequently thank Terry Love for sponsoring it.
Thanks Terry!http://terrylove.com/forums/images/icons/icon14.gif

03-08-2006, 05:56 PM
All, much appreciated. I'm remodelling one of the bathrooms in my house and it has been 15+ years since I've done any trades work. We have the tub in place and the room is completely enclosed in thick poly (small 5x7 bathroom). Getting ready to start hanging backerboard. I had two questions that were answered in this thread!

1. How to get the backer to 1/8" of tub over lip (especially when opening is about 1" larger than tub)? Answer, use furring strips or rabbit backer.

2. When using greenboard for non-tiled areas and backer for tiled areas, do you joint compound or thin-set or what? Answer, compound if no tiles, thinset behind tiles.

This has been extremeley helpful guys. Thanks so much! Hope any more questions I have are already answered!