Hot Topics: Tub Shower Tile and Backer Board Problem Hot Topics: Tub Shower Tile and Backer Board Problem

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Original Post: Tub shower tile and backer board problem

billzie Member

I need some advice on how to proceed with a tub / shower tile job. I hired a "contractor" to help me with a bathroom remodel. He was supposed to install the tub / shower backer board (HardieBacker) and install tile. He disappeared and I have to finish the job.

So, the HardieBacker has a gap around the tub that varies from 1/2 to 7/8." Is this excessive? If so, how would I handle filling this gap?

Also, he used paper tape and mud for the HardieBacker joints and screws—some of it is bubbled up and some of the screws are just proud of the board. Should I sand away all that and reapply with fiber mesh tape per HardieBacker's instructions? It seems like most of the screws are stripped, so pulling them tighter probably isn't an option.

For additional info: the tub is a standard 5' steel/iron tub and was the only original thing left in place for the remodel. It's installed right up against the studs. I will be using a combination of 4 1/8" sq and 4 1/8 x 8 1/4" tiles and some glass mosaics as a band or inlay (not decided what the design will be, but the tiles are on site already). I don't have mortar or grout yet, so advice on those as well, please.

Marq1 Member

I think we need a picture to understand what the gap is. Cement board uses fiberglass tape and mortar.

billzie Member

Gap as in the HardieBacker board was installed above the top edge of the tub creating a gap that varies in size 1/2-7/8", making me concerned that there won't be enough support behind the tile. 1/8-1/4" I could see being OK as that's not so much wider than tile spacing and the bead of silicone would do OK.

This steel/iron tub does not have an upward turned lip to mount it to the studs like some modern tubs I've seen, so there is nothing in this gap except the stud space.

Pilot Dane Group Moderator

I have seen tubs without an upturned lip leak many times. Even if there is not a full blown leak, it's easy for water to wick under caulk that has pulled. It's not the best to enclose for a shower. If you have a basement or crawl space below, it's a good idea to go underneath once a year and inspect for water damage. Often, it only manifests above when the tub starts to sink or the tile moves when pushed or pulled away from the tub.

The gap between your HardieBacker and tub is a bit much, but a lot depends on the tile you install. A large thick tile can better bridge the gap without support while there is no chance with a penny tile. Where some trouble will come is when you grout. With nothing backing up the joints in that area, the grout will push through. The grout in that area may also crack and fall out if hit or treated roughly since it's only sticking to the sides of the tile and may not have great adhesion as most tiles are glazed on the sides.

Definitely get rid of all the paper tape and sheet rock mud. It's totally useless in a shower/wet application. In fact, you can remove it with a wet sponge. Once you get back to bare clean backer, use fiberglass tape and Thinset mortar.

Screw heads often don't countersink fully into HardieBacker, so that's not a major issue. It's annoying when you go to spread Thinset to set tile, but it's not the end of the world. If the screws are stripped, then they aren't doing much to hold everything in place. Remove the stripped ones and install another screw an inch or two away and don't forget to fill the holes with Thinset to seal them off.

billzie Member

I'm using Daltile wall tile in the sizes mentioned earlier. Should I just fill the gap with the mortar I'll be using for the tile?

I will be buying my supplies from Lowes; they are basically the only game in town or within 45 miles. So, knowing their brands, what should I be looking for?

czizzi Form Topic Moderator

You can bridge the gap at the tub lip with tile, but major concerns that there is no tile lip on the tub. This makes you rely on caulk as your last defense against water; caulk fails all the time and you have no back-up safety net.

The bridge between Hardie and drywall needs to be cleaned up if regular joint compound was used. I would then go back with a mesh tape designed for cement board and a setting-type compound which is not subject to water erosion like regular joint compound.

Again, your backer can be saved. The issue I have is the non-tile ready tub. What thickness of Hardie did he use? If 1/4", then just tear it out as it's not designed for walls.

billzie Member

The tub has the slightest bit of a raised edge along its top, probably enough to keep moisture from going past, but certainly not enough for my rowdy 6-year-old's monsoon baths.

I was thinking of maybe getting some stainless steel finish nails and setting them as anchors in the studs in the gap and then running stainless steel wire between them to act as substrate for mortar to fill the gap. The only other idea I had was to take ~4" wide pieces of the HardieBacker and slide or wedge them into each stud space of the gap with construction adhesive buttered on each end and inside back edge. Or am I overthinking this too much? The HardieBacker is for walls, so 1/2".

Pilot Dane Group Moderator

There is only so much you can do with that tub. It's OK for a tub, but not good for a shower or heavy splashes. The best thing you can do is give the caulk joint between the tub and tile a thorough inspection once or twice a year. If you find any spots where it seems the caulk has come loose from the tile or tub, replace it with fresh caulk. As czizzi mentioned, the caulk is the main defense against water getting into the wall.

billzie Member

These are the only pics I had of the gap. It is worse elsewhere. You can see the slight top edge lip of the tub.

A tub with a gap around it.


A bathtub with a gap in the wall around it.

Pilot Dane Group Moderator

Has the wall/tub surround been painted? That will affect the adhesion of the Thinset. Putting tile over a painted wall may be OK for a kitchen back splash that will usually be dry and see no abuse, but a shower is a different story.

czizzi Form Topic Moderator

Looks painted with HydroBan, which is a waterproofing membrane similar to RedGard.

The gaps are normal and nothing to worry about. Just plan your tile layout so a vertical grout line falls well outside that area and the majority of the tile is adhered to the backer board. I see nothing wrong at this point. Enjoy your build.

billzie Member

The contractor painted the bathroom while I was at work (I was supposed to paint) and he painted into where the tiles would be, but most of the tub/shower area is still bare HardieBacker and the mud over tape. I guess it's a good thing he didn't mud the screws.

I'll be removing the mud and tape and the paint if needed. Then, using the fiberglass mesh tape with mortgage and mortaring that gap between the tub and HardieBacker.

So, "thinset" and "mortar" are basically interchangeable terms here, right?

My local Lowes carries Mapei brand tile products. I'm using Daltile 4" square and 4x8" rectangle tiles in the field and I'll be putting a glass tile mosaic in an inlay or a band (design not finalized yet). Suggestions please on tile gap spacing, sanded or unsanded grout, and does mortar color matter much?

czizzi Form Topic Moderator

Wall tile would be 1/8" spacing that would match your mosaic spacing. Unsanded on the walls for a spacing that small.

If some of the screws are proud of the HardieBacker, then do this: trowel on a v-notched trowel and then back butter and trowel each tile to give you a little more mortar thickness to bridge the screw heads. Start at row number two and nail a level ledger to the wall to work off of. Set the ledger just shy of a full tile and when the ledger is removed, custom cut each tile to fit tight to the tub with maybe a 1/16th of an inch gap that will be caulked in the end. Your gaps are normal. You can bridge them with your tile.

LFT mortar will give you a little more stick and less sag for setting wall tile. Other mortars will sag and droop on walls.

To read the rest of the thread, look here: https://www.doityourself.com/forum/walls-ceilings/587683-tub-shower-tile-backer-board-problem.html

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