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Old 03-18-2009, 10:43 PM   #1
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Cement Board Install Question


Ok I have gutted my 9x8 bathroom months ago and have the old tub area walls down to the studs. Removed and disposed of the old tub surround and the molded sheetrock.

I recently bought several sheets of the Permabase cement board for the new tub/shower area and the mold-resistant drywall for the remaining walls. What I plan to do is:

I have a roll of 15lb. roofing felt that I can attach to the studs before putting on the cement board. I have the special cement board screws to screw it in to the studs. My question is, my tub I believe is a cast iron tub which is virtually impossible to move due to the weight. Would the cementboard be overlapped over the front of the flange and raised about 1/4" from the tub surface? From what I have been reading so far, this area will be caulked once the tile is in place.

Since the CB's come in 3x5 sheets, will I have to cut custom sizes to fill in the leftover wall space what isn't completely covered?

And since I do plan to put in ceramic tile on the CB, would it be best to install the tile up to the ceiling?

I know I will need some fiberglass tape and thinset (?) to fill in the joints between the CB.

I have recently installed a new tub/shower faucet and redid the plumbing myself for it (my area does not require permits nor inspections). I know I will have to score the holes out for the handles and showerhead, but what is the best way to accurately cut these holes out to ensure that it fits properly when the CB is put in place? I have one pic showing what I mean. The handles on the faucet was put on temporarily to test the faucet/shower for leaks.

Thanks for any help!
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Old 03-18-2009, 11:41 PM   #2
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Cement Board Install Question


Install 1/4" strips of wood on your studs to allow you to lap the backerboard over the lip of the tub without bending it. Otherwise that bend will show in your corners.

The 15# felt won't hurt. But, I'd really recommend waterproofing the backerboard itself to effectively manage water. Water definitely will permeate the tile and grout and will soak into that backerboard otherwise. RedGuard is a good product for this...You just paint it on after installing the backerboard.

Yes, tape your backerboard seams with fiberglass backerboard tape. Use thinset to fill the joint and bed the tape. You'll also use thinset for your tiles, not mastic. I do not recommend pre-mixed thinset for wet areas. Get a good quality dry mix and mix it yourself, it is better.

Cut the backerboard as needed, just be sure to land any vertical seams on studs. Sometimes it helps to double up the studs or add blocking where you know you'll have seams. That makes life much easier.

Ideally, use a $15 diamond tipped tile bit to cut perfect holes for the faucet. Otherwise you can use a utility knife and some elbow grease. Score the backerboard as deep as possible on both sides where you want the holes then use a hammer to remove the concrete.

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Old 03-19-2009, 05:43 AM   #3
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Cement Board Install Question


A hole saw will also cut these for you accurately. Keep the CBU up off the tub edge a 1/4"
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Old 03-19-2009, 12:59 PM   #4
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Normally drywall is hung all the way around the tub down to the upper tub lip. Then the 1/2" backerboard is installed on top of that to within 1/4" of the rim. That gives you the reveal you need for your bullnose tile around the edge. Going to the ceiling is a matter of personal preference.
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Old 03-19-2009, 01:41 PM   #5
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I agree with Thekctermite that the cement board should go over the tub rim. It's just common sense flashing detail. I stop the cement board about 3/8" short of the tub bottom. I started using the kerdi liner as an extra waterproofing step. It has a shallow learning curve and worth the extra time and cost.
Any reason you bought a non pressure balanced tub control?
Typically I install the controls up at about 4' off the floor. The sole use of a tub anymore is rare and it's easier to control the shower with the faucet higher up on the wall.
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Old 03-19-2009, 01:49 PM   #6
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Originally Posted by bjbatlanta View Post
Normally drywall is hung all the way around the tub down to the upper tub lip. Then the 1/2" backerboard is installed on top of that to within 1/4" of the rim. That gives you the reveal you need for your bullnose tile around the edge. Going to the ceiling is a matter of personal preference.
Really? That sure isn't done around here. The I-codes now prohibit gypsum wallboard behind tile in wet areas (tubs and showers). If the added thickness is needed to provide an interesting tile reveal detail then two layers of CBU could be used.
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Old 03-19-2009, 01:57 PM   #7
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That will work also, but drywall is still used in this area for the first layer.......
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Old 03-19-2009, 02:02 PM   #8
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Seems to me an interpretation? Yes we all can agree no drywall behind tiles since it is not waterproof or water resistant. But if it is behind the CBU it is used for no other use than a shim. And the regard waterproofs the CBU so why would the drywall be an issue in this installation? Wood shims should not get wet either?
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Old 03-19-2009, 02:39 PM   #9
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That will work also, but drywall is still used in this area for the first layer.......
Must be a regional thing. Not done around here.
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Old 03-19-2009, 02:45 PM   #10
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Like Bob said, the drywall is more or less just a shim and the CBU protects it from moisture. And it probably is a regional thing.....

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