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themccomasunit 09-09-2010 11:17 AM

Shower retile underlayment advice
We're retiling a tub shower. I have stripped the tile and old sheet rock to the studs all the way around. So far so good, no wood rot. Whenever the house was built, it appears that they used two layers of sheet rock for the tub/shower walls. I plan to use a hardy board type substrate going back, so the question is, since what was there was the thickness of two layers of sheet rock do I go back with two layers of hardy board or one layer of sheet rock with one layer of hardy board on top? I could go with double hardy board layers, however, I'm not sure if the special screws for hardy board are long enough to go through two layers.:)

Nestor_Kelebay 09-10-2010 10:57 AM

No, you don't need two layers of Hardibacker.

The purpose of the tile backer board (whatever it is; Hardibacker, Wonderboard, DenShield or even Greenboard) is to provide a dimensionally stable substrate for the tile that won't swell and shrink with changes in humidity and temperature like wood does. (And also won't disintegrate if it gets wet like drywall and greenboard do.)

So, if it were me, I would probably put up 1/2 inch DenShield (or even ordinary drywall) and then your 1/2 inch Hardibacker over the DenShield. The tile backer should not go behind or in front of the tub lip; it should stop 1/8 to 1/4 inch above the top of the tub lip. So, it's a good idea to go to any metal supply shop and get some 1/8 inch thick pieces of steel strapping about a foot long, put a board across your tub, put the 1/8 inch steel strapping across from the board to the tub lip, and set your heavy Hardiboard on the steel strapping to hold it a uniform 1/8 inch above the tub lip while you secure it in place.

If it were me, I'd just buy some stainless steel flat head screws of the length you need to dispose of the screw length problem. It might cost you $30 or so for 100 screws, but that's peanuts compared to what you're saving by doing the work yourself.

And, I would predrill the holes through the hardibacker with a 1/8 inch masonary bit, extend the holes into the studs with an X-tra long wood bit, and then counter-sink the holes into the Hardibacker with a 1/4 inch shank die grinder bit like the second one from the left:

Use a putty knife to schmear some thin set over the holes before you drive your #8 or #10 ss screws in, and the thin set will accomodate any difference in shape between the screw head and the countersink, and the excess thin set will get squished out of the hole and you can scrape it off with your putty knife.

Then paint over everything with a waterproof membrane like RedGard.

Then tile over the RedGard.

And if this is your first time tiling, you should know not to set your first row of tiles on the tub or shower pan because even though it might look straight and level, it might not be. So, if you set your first row of tiles on the tub, then all of your tiling is going to be as crooked as the tub was set.

What you want to do instead is nail or screw pieces of STRAIGHT wood molding to the wall around the shower area, and set your first row of tiles on that wood molding. Then, when all your tiles are set, you go back and remove the wood molding and cut your bottom course of tiles to fit down to the tub.

You want to choose the elevation you attach that board carefully. Lay out a row of tiles along the floor with spacers between them so that you can measure up from a hypothetical wood molding elevation to see what's going to happen at the ceiling. If it turns out that you're going to have to cut 1 inch tall tiles to fit to the ceiling, then you want to incorporate a horizontal border into your tiling that'll take up 1 1/2 or two inches of height so that you end up with almost full tiles along that top row too. And, you can choose various widths of border tiles to get the top row to be nearly full tiles.

(Similarily, you want to choose the starting position on that wood molding carefully too. Ideally, you'd like to be cutting almost full tiles to fit into the corners as well. However walls aren't aways straight and they aren't always vertical either, so you want to pick a starting position so that you end up cutting 1/2 inch to an inch off the tiles that fit into the corners as well. That way, if the walls aren't vertical, you don't end up with a 3/4 inch wide grout line in a corner because the wall and tiling diverged.)

Once you set the first 3 or 4 rows of tile on that wood molding, and allow the thin set to cure, then you can remove the wood molding as the tiles won't slip down the wall after the thin set has set up for a day. However, I always wait till the end and cut the bottom row of tiles last.

Typically, you'd want to be able to cut 1/2 inch off the bottom course of tiles so that that bottom course looks like full tiles but still allows a full half inch of wiggle room so that if the tub is 1/2 inch higher at one end than the other, you can still get the tiles to fit. The flatter and more level your tub and the straighter and more vertical your walls, the less wiggle room you need. Still it's best to lay some tiles out on the floor both end-to-end and side-to-side to see how things will work out with each hypothetical starting point on the elevation of the wood molding you choose.

I always use 6 X 8 inch tiles because that's the largest size you can comfortably handle with one hand. That's because the way I set tiles is different than most. Most people spread the mastic or thin set on the wall and then set the tiles with both hands. I spread thin set on the wall first, and then back butter each tile with wet thin set. That way, if the thin set on the wall dries and skins over, the moisture from the fresh thin set on the back of the tile will re-activate it so that I don't have an issue with the thin set not wetting and sticking to the tiles. And after 21 bathrooms after 25 years, I have yet to have a loose tile.

The reason why you might have that extra layer of drywall might be because you have an older house that originally had plaster walls. It may be that a previous owner took down the plaster and put up drywall. But, since plaster is typically 3/4 inch thick, he should have put up two layers of 3/8 inch drywall instead of two layers of 1/2 inch drywall. It could also be that the walls aren't straight and there's a transition from plaster to drywall somewhere in your bathroom. The last person to do any renovations in the bathroom may simply have noticed that the plaster was particularily thick and realized he needed two layers of 1/2 inch drywall for the drywall to be flush with the plaster. Plaster was all applied by hand, so it's not uncommon for plaster walls to vary in thickness by quite a bit. You can go from 5/8 inch thick to 1 1/4 inch, and everything in between.

Post again if you have any other questions.

(PS: The first thing you should do is use some masking tape to cover over the rubber stopper in your sink drain so that it doesn't accidentally pop out and you get a bunch of crap fall into your tub drain. Also, put a piece of scrap carpet in your tub pile side down to protect the finish of the tub from being scratched up during the work. You wanna use a thick carpet that won't grind any pieces of tile into the tub enamel.

themccomasunit 09-10-2010 11:12 AM

What's DenShield?

So tile should not go over the lip of the tub? It was this way originally. How come that's not a good practice?

The wood strip around the top edge of the can you remove it after the tile is set if tile is over top of it? What's the best practice on where to lay out your first row of tiles?

Sorry for the questions. I've tiled floor several times, and even a shower pan, but never around the top edge of a tub.

Nestor_Kelebay 09-10-2010 02:34 PM


I made some changes to my last post, so you may want to re-read it.

Take a look at this web page about Red Gard. It's made by Custom Building Products, the same company that makes Wonderboard.

DenShield is a tile backer board like Wonderboard or Hardibacker, but it's not made out of cement. It's made with a gypsum core, like drywall, but that gypsum is specially treated with a water repellant before it's used to make the board, so water just beads up if you put it on that gypsum, rather than being absorbed into it. Denshield is made by the Georgia Pacific Company and it's recommended as a tile backer for shower walls. You can buy it at any home center or drywall wholesaler that sells Georgia Pacific products.

The cadillac of all tiling backer boards is Hardibacker. It's the strongest tile backer board so it provides the best support for either wall or floor tiles. Dens-Shield is, in my opinion, 2nd best, or at least as good as the cement/fiberglass boards like Wonderboard or Aquaboard. Denshield has a waterproof plastic coating on one side, and the waterproof treated gypsum in the middle, and then a fiberglass-ish mesh on the back. You cut it just like drywall, and it's as easy to install as drywall, but it's very much more water resistant than drywall. I've used it on several bathrooms in my own building, and it's a good product.


So tile should not go over the lip of the tub?
You misunderstood what I wrote. Maybe re-read that part. The tile BACKER BOARD shouldn't go over the lip. It's bottom should be 1/8 to 1/4 inch above the lip of the tub and the front of the tile backer board should be in front of the lip of the tub. When you cut your bottom row of tiles, you will set them onto the bottom of the tile backer board, and they will hang down in front of the lip of the tub. When you set them, you want to provide about 1/16 of an inch of space between the bottoms of the bottom tiles and the tub.

You will eventually caulk over that tiny gap with mildew resistant silicone caulk.

So, the bottom row of tiles hangs down in front of the lip of the tub, but the tile backer board does NOT. It stops just above the lip of the tub so that it can be screwed flat to the wall studs.


The wood strip around the top edge of the can you remove it after the tile is set if tile is over top of it? What's the best practice on where to lay out your first row of tiles?
No. You don't tile over it. When I said you set all your tiles ABOVE it, I meant that you set all the tiles that will be at a higher elevation than the molding first, and then remove it to set your last row to fit down to the tub. I didn't mean that you tile over the wood molding. That wouldn't work. You rest your bottom row of full tiles on the wood molding so that they don't slip down the wall as the thin set cures and so you know those tiles are straight and level just like the wood molding is, and hopefully your tub is. You set all subsequent rows of tiles on the previous row of tiles with tile spacers under them. Once you've set all your tiles that are at a higher elevation than the wood molding (including any cut tiles), you then remove the wood molding and cut your bottom row of tiles to fit down to the tub.

Imagine you get a 1 1/4 inch wide by 1/4 inch thick piece of wood molding. Sight along it's length to ensure it's straight. Set a spirit level on top of it and hold that spirit level in place with your thumbs while a helper marks the top of the wood molding on the wall with a pencil at both ends. Now, if you have a stud finder, locate the stud locations and screw that wood molding flat against the wall with some screws so that it's top edge is on those pencil marks. Alternatively, just STICK the wood molding to the wall with double sided mirror mounting tape, and only set ONE row of tiles above it. Wait for the thin set holding those tiles to cure before setting any subsequent rows of tiles.

Do the same at the front and back walls so that you have a 1 1/4 inch wide by 1/4 inch thick band of wood going all around the base of the wall area you want to tile. And, of course, don't just screw it on anywhere. Decide where it should be so that you just have to trim the bottom row of tiles a bit to fit down to the tub. That means you have to measure from the tub to the top of the molding (and allow for the width of a grout joint) to ensure you'll have to CUT those bottom tiles to fit to the tub rather than STRETCH them.

And, of course, you have to spend some time deciding where to start tiling along that wood strip, and you have to make that same starting point decision for the side wall, and both the front and back walls.

Now you start tiling, and the way I like to do this is to set out some tiles on the floor so that I'm covering about 3 to 4 square feet of wall area. I measure from my starting point over about horizontally about 3 feet, and measure from that point and my starting point up about 2 feet and draw a rectangle. The horizontal and vertical dimensions of that rectangle will be determined by the size of your tiles; you want to have the tiles cover the rectangle completely, but not have any thin set sticking out of the rectange that's not covered by tile.

So, you lay out your tiles both side to side and end to end on the floor with tile spacers between them, and lets say that you find that 5 tiles wide is 40 1/2 inches and three tiles high is 18 1/2 inches. You would draw a rectangle from your starting point 40 3/8 inches wide by 18 and 3/8 inches high. Then you put 2 inch wide painter's masking tape both on the wood molding and on the wall so that only that rectangle is exposed. That way you can spread the thin set on your wall quickly without being careful to stay inside the lines. You want to use about a 1/4 inch V notch trowel for tiling wall tiles with thin set... you don't use a square notch trowel for wall tiles like you do for floor tiles... you use a V notch trowel.

...continued on next post.

Nestor_Kelebay 09-10-2010 02:43 PM

Sorry, here's the web page for Red Gard.

Nestor_Kelebay 09-10-2010 02:46 PM

Now, what I like to do is use 6X8 inch tiles in the LANDSCAPE orientation. You almost NEVER see rectangular tiles set in the landscape orientation, but there's absolutely no reason not to set them that way. So, that allows me to get a cool "custom" look using plain jane tiles. Anyone seeing that tiling will notice that it's different because of the orientation of the tiles, and it sends the message: "Whomever did this tiling had the confidence in his knowledge and skill to venture off the well beaten path."

So, you have your thin set drying on the wall. You pull the tape off, and back butter each tile with fresh thin set, and press it snugly into the thin set on the wall. That way you don't spread more thin set than you can cover in 20 or 30 minutes, so that way you can stop and take a break or run an errand or whatever every 20 or 30 minutes. You want to set the tile spacers PERPENDICULAR to the tiling so they're sticking out so that they can be easily removed once the thin set has set up in a day or two. You set the first row of tiles directly on the wood after removing the masking tape, and have plastic spacers between all the tiles, both between the rows of tiles and between the columns of tiles.

Typically, I will pick a horizontal starting point on the wood molding, and then determine where the grout vertical grout line closest to the middle of the side wall will be. I'll draw a vertical line through that point using a spirit level, and then tile on one side of that line and then tile on the other side to the same elevation, leaving the final tiles that fit into the corner till last, after I tile all the way up to the ceiling.

So, I choose the elevation of my wood molding, and determine if I need or want a horizontal row of accent tiles.

Then I fasten the wood molding to all three walls with screws.

Then I choose a starting point along that wood molding for each wall.

Then I draw vertical lines through those starting points up to near the ceiling using a spirit level.

Then I measure off on the tiles I have arrayed on the floor where the vertical grout line closest to the middle of the side wall will be and draw a vertical line there as well.

Now, again using the tiles I have arrayed on the floor, determine how big a rectangle to draw on the wall so that I can just barely cover that rectangle with tiles.

I draw that rectangle and mask it off with 2 inch wide painter's or regular masking tape.

Then I mix up some thin set mortar (by going to any small appliance repair shop and asking for a used kitchen mixer blade an putting that kitchen mixer blade in an electric drill and using that to mix my thin set. Mix inside a cardboard box or plastic pail so that any thin set that goes flying will remain inside the box or pail so that it can be easily cleaned up.) You mix the mortar to taste, kinda. It should be thick enough so the trowel ridges hold their shape, but not so thick that the mortar dries out on you while your working.

Then I trowel that mortar onto the rectangle, and pull of the tape, and then back butter each tile with fresh mortar and gently press it onto the wall. The first row of tiles will sit directly on the wood molding.

I tile on both sides of the middle grout line on the side wall first. Then I tile the front and back walls. At this stage I'm only setting FULL tiles. I'm not cutting any tiles yet. The only exception to this is the holes I have to cut on the front wall for the tub spout and faucet, and also the hole for the shower arm at the top.

Once I get all the full tiles in, I cut the tiles that fit into the corners both at the front and back corners. You want to leave a good 1/8 inch of space between the cut tile and the hardibacker on the other wall or the cut tile and the glazed surface of the tile on the other wall so that you have a good "L" shaped root for the grout you put into the corners. If the tiles on both walls at the front and back corners meet perfectly, then you can't grout the corners because the grout won't stick well to a glazed surface. In that case, you'd have to silicone caulk the corners. In my view, it's much better to grout the corners and seal the grout rather than caulk the corners and have to deal with the mildew that grows on that caulk.

(I've done enough of this kind of work I can teach you how to clean mildew off of silicone caulk and also how to recaulk so that you don't have the problems most newbies do when it comes to replacing silicone caulk.)

Ok, so now all my full tiles are up, and my corners are done. Now I cut tiles to fit to the ceiling, and I've chose a border width to ensure that that last row of tiles are nearly full tiles too. I don't spread thin set on the walls for either the top row of tiles or the cut corner tiles; I simply back butter those tiles before setting them. You will NEVER notice that the tiles aren't flush with all the others, or in 25 years, not a single tenants in a 21 unit building ever has.

Now I remove the wood molding and cut my tiles to fit down to the tub. I stick toothpics, pieces of paper or match sticks under them to allow a 1/16 inch gap under that bottom row of tiles.

Finally, I take a bright light and using a piece of sandpaper folded in half, I go along each horizontal and vertical grout line and remove any thin set that may have oozed out from under the tile and partially blocked the grout gap between tiles.

Now, after removing all the tile spacers, I'm ready to grout. That;s another subject, and it's another post.

Also, I live in an area that never has earthquakes. If you don't need to use a "polymer modified" thin set, then I wouldn't. "Polymer modified" simply means "glue added" to make the thin set stick better to everything and dry much harder. In 21 bathrooms over 25 years I have never had a tile come off of it's own accord. So, using a polymer modified thin set in my case wouldn't help the tiles stay on any better, it would only make it harder to take the tiles off in future to redo the tiling should I need/want to.

Post again with any questions.

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