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Taping Hardibacker seams

ckorto's picture

I'm currently building a tile shower.  Using 1/2" Hardibacker for the shower transitioning into 1/2" greenboard outside the shower.  I'm going to use mesh and mastic for the corner seam and two seams inside the shower but I was wondering what others use for the transition from cement board to drywall that will eventually be painted.  I've read some use a setting type drywall compound while others stick with the mastic.  Does this smooth well like mud.  Does mastic stick well to oil primed drywall or is it better to go right to the mud/unprimed greenboard.  I appreciate any opinions.

Thanks, Chuck

(post #105579, reply #1 of 5)

I would recommend using only thinset for all the seams and for setting the tile.   Check out , I switched over to their hydro-guard one  waterproofing product about 2 yrs ago. I have been using it on seams ,corners, and shower pans with great results. The  product not only waterproofs, but will act as an anti-fracture membrane (it is rated at 800% elongation)which has all but eliminated call backs for cracked corners and grout. 


(post #105579, reply #2 of 5)

mastic does not belong anywhere in a bathroom.

especially in a shower.



    Buck Construction

 Artistry In Carpentry

     Pittsburgh Pa

    Buck Construction

 Artistry In Carpentry

     Pittsburgh Pa

(post #105579, reply #3 of 5)

Like the others said, abandon the mastic immediately. Use thinset mortar to seam the panels in the shower.

I use setting mud for the seam where cement panel meets gypsum panel. The thinset is too gritty and forget about being able to sand it. Those transition seams are in an area where it should be pretty dry.

After you are all taped and mudded, you can coat the Hardi with Red Gard, bringing the RG just shy of the tile's outer boundary. This will keep the Hardi from holding water or from wicking it into the drywall. Or, you could do Kerdi.


(post #105579, reply #4 of 5)

As seajai says do not use mastic use a thinset mortar. I tape and mortar my seams as I tile one wall at a time as I go along. I used to hang the board tape the seam one day the tile the next. But doing so I found I was always fighting the dried mortar no mater how smooth I put it on, the tiles rocked on them or bulged out if the grout joint was near the center of a seam.

So with self adhesive tape I first run it up the corner dab a bit of mortar at the top to keep it from falling. Next I mortar the corner seam of the side I am working for about 18'-24" from the bottom making sure it is filling any gap between the two boards. Then start laying the tile, when I reach the top of what I've taped in the corner with tile I mortar the tape again the same distance tile and keep alternating.

Once the top row of tile reaches a horizontal seam then I tape it. If my backer board is butting up against sheetrock I work it the same as the corner. If there is a good size gap, any thing larger then 3/16 between the two, I fill it the night before with a drier mortar mix first then tape it putting barely any mortar over the tape almost just embedding the tape in the fill mortar running over it with a dry knife..

I usually try if I can to have a sheetrock to backer board seam fall so the tile over laps the sheetrock about an 1 1/2". If this can't be done and the last vertical row of tile falls on the backerboard, I still finish the joint between the two with mortar for the first one or two coats. Then once all the tile is set I go over the exposed mortar with quick setting drywall compound like USG Easy sand 45 feathering it out over the sheetrock.


(post #105579, reply #5 of 5)

I appreciate everyone's help, thank you.  Chuck