What size backer rod should I for tub and wall tile?

QUESTION

Sealant and Backer Rod for Bath tub to wall tile joint transition - I would like to try using polyethylene backer rod to solve my problem of a crack at the joint between the tub and the tile. What size backer rod should I be trying to use? With water in the tub, the crack was opening up to close to 1/4 inch. (Without water it's negligible.)

ANSWER

ANSWER - The polyethylene backer rod is not likely the cause of your cracking at the tub to shower wall transition, but if there is room for it in that joint, then you should use it to fill the excess space and to gauge the thickness of the sealant you are using.

Generally the transition between the tub and tile on the wall is either wrongly filled with cementitious grout or the installer wrongly fills (caulks) it with a latex sealant sometimes referred to as a caulking. The latex sealants don't perform well, don't have warranties, and do not last long.

Our industry standards requires that an ASTM C920 sealant be used which is either a 100% silicone (not Siliconized latex) or a urethane sealant; there are other types. These types of sealants will last for years and have warranties.

Our industry standards says to use a polyethylene backer rod to fill up excess space in the joint. The backer rod should be larger than the joint and be compressed into it. The backer rod will fill up the excess space plus you can control how thick the sealant will be on top of the backer rod filling the joint. The sealant cannot be applied too thick or too thin for it to perform correctly. Follow the sealant manufacturer's instructions, but normally it should be at least 1/8" thick and not thicker than 1/2". Normally the manufacturer requires the use of a primer prior to apply the sealant.

Based on your description that when the tub is full of water the joint is less than 1/4" wide, I would put in the 1/4" backer rod with the tub full of water, and apply the sealant without water in the tub. This is based on the joint being deep enough so you can apply between a 1/8" to 1/4" thick sealant on top if allowed by the sealant manufacturer. If 1/4" backer rod is too thick then use a smaller one; you can get it as thin as 1/8". If 1/8" backer rod is too thick then don't use a backer rod.

Good Luck!

21 thoughts on “What size backer rod should I for tub and wall tile?

    • Donato Pompo says:

      You can install a 1/4″ thick backer rod and that will leave 1/4″ of space for the sealant. Be sure to use a sealant that meets ASTM C920, which is normally a silicone or urethane type of sealant. To make it easier to clean up when applying these types of sealants, first put down blue masking tape along the edges of the tile. right after applying the sealant and tooling it to the correct finish, immediately pull up the tape. Otherwise it can be a messy job to clean it up.

  1. Donna says:

    I have a 1 and 3/4 inch gap between my ceramic tile and the tub. Can I use a backer rod, and then use a silicon caulk over it? Thanks!

    • Donato Pompo says:

      That is a very wide gap for sealant. There are building sealants that are that wide, but the depth of the joint might not be deep enough. Normally the sealant manufacturers require a 2:1 ration of width to depth. So you should have a 1/2″ depth above the backer rod. the sealant manufacturers normally say never apply the sealant less than 1/4″ thick. So it depends on what the depth is of your joint.

  2. Robin says:

    I am about to redo the seal between our 30-year-old tub and the tiles. The tub was never square with the wall (vertically or horizontally), thus I have an irregular space to fill. You mention priming the backer rod before applying the caulk. What kind of primer would you recommend and how would you apply it?

    • Donato Pompo says:

      You would use the primer that the manufacturer of the sealant requires. Not all sealants used for caulking require a primer, so read their instructions and data sheets to make sure you follow their directions.

  3. Mark Miller says:

    Question- when using a polyethylene backer rod along a tub/tile joint before caulking, is there any benefit to using an adhesive in the joint to prevent the backer rod from eventually pushing the caulk away from the joint?

    • Donato Pompo says:

      If you are using the right diameter foam backer rod it should compress in the joint and fit without moving.

      The standards say to use a compressible closed cell polyethylene foam back up strip. You use the back up strip for two reasons. First because the caulking/sealant will not bond to it, which allows the sealant to have greater elongation and compression physical properties. You want two point contact between the two sides of the joint.

      The other reason is so you can gauge the thickness of the sealant being installed in the joint. Sealant manufacturers say the thickness should be a minimum of 1/4″ thick even though the ratio of joint width to depth is 2:1. If the movement joint is on a walking surface the traffic grade sealant should be 1/2″ thick in order to withstand the pressure of a high-heel.

      1/4″ diameter backer rod will fit in a 1/8″ wide movement joint. 3/8″ or even 1/2″ backer rod will fit into a 1/4″ wide movement joint.

      The only time I have seen the backer rod sticking up in a movement joint is when the sealant was applied less than 1/4″ thick and the joint is less than 1/2″ deep. In those cases you have to use bond breaker tape in those joints. For floors some will use sand to fill the lower depth of the joint and then caulk over that.

  4. Jay says:

    Newbie here, tiling my bathroom FLOOR – I left a 1/4″ gap between Hardibacker and the tub. I’m planning to use backer rod and silicone caulk there before I tile.

    Can the tile go slightly over the backer rod to leave a final gap of 1/8″ between tub and tile? (I’d prefer this for aesthetics but am worried thinset won’t adhere if tile is slightly over top of backer rod and silicone)

    Or should my tile end exactly where the Hardibacker ends with a 1/4″ gap between tub and tile? Then caulk that to finish.

    I really appreciate this post!

    • Donato Pompo says:

      The tile should always be supported by the backer board and the thinset bonding to it. Otherwise it makes that unsupported edge of the tile susceptible to cracking if there is a concentrated load applied to that area of the tile. Being next to the tub and only cantilevering over the joint 1/8″ maybe it would never be subjected to a load, but that would be a risk…

      You could always put a plastic molding over that joint at the tub to cover it up so you can’t see it. You wouldn’t want to use a wood molding because it will get wet at a bathtub.

  5. Jan Willem Rosenboom says:

    I am in the process of replacing tile wall to bathtub caulking and found a wet mess when I removed the old caulk. Should I also try to replace the old backer rod as part of the process, or can I just let things dry out before proceeding? (Old backer rod is closed cell). Thanks for the advice, great post!

    • Donato Pompo says:

      You need to determine how the water got behind the caulking sealant? If the water got in because the caulking failed and allowed water in, then you can let it dry out and then install the new sealant.

      If the caulking in the joint wasn’t breached and you found water behind the caulking then I would be concerned about how did the water get there? You have to find that water source so you can prevent water to get behind the tile. Otherwise if you caulk that transition joint you will trap water in and that could lead to other problems.

      I would let it dry out and then put in the new closed cell backer rod, which its diameter must be wider than the joint itself so when you stuff in the backer rod it will have a tight fit. They test out the shower several times and then remove the backer rod to see if there is any water behind it. If there is, then you have to forensically investigate the source to block the breach.

      If it is dry behind the new backer rod then go ahead and thoroughly clean out the joint. The reinstall the backer rod and caulk the joint so that the thickness of the sealant is at least 1/4″ thick. Use an ASTM C920 sealant as the caulking, which will either be a 100% silicone or a polyurethane sealant.

      Be sure to use blue masking tape on each side of the joint before you install the sealant. Tool the joint to a slight concave surface and then immediately remove the blue tape by pulling it out and away from the joint. If you do this right you will have little clean up to contend with. If you do it wrong it gets messy….

      • Jan-Willem Rosenboom says:

        Fantastic, thank you. I really appreciate your speedy reply… (I think the original backer may not have been big enough, which then led to issues when the sealant failed. But I will test this out as you suggest).

  6. Abby says:

    Hi,
    I have a gap between my tub and tile that ranges from 1/4-3/4 inch. How should this be covered? I’m confused about if i need to fill in with backer Rod, grout over that and then a transition trim to cover it up. Please help. Thank you for your time.

    • Donato Pompo says:

      If the gap is more than 1/4″ then whomever installed the tile didn’t do a good job. Particularly if the gap is not a consistent width. 3/4″ is way to wide….

      That joint should be cleaned out and dried. The it should be filled with an ASTM C920 sealant such as a pure silicone or polyurethane sealant. The manufactures of the sealants require that you first install a closed cell polyethylene foam backup strip or a thin bond breaker tape. If the open joint is deep then the foam strip allows you to gauge the thickness of the sealant so it isn’t too thin or too thick. The foam diameter should be wider than the width of the joint so it compresses into the joint and doesn’t move. It needs to be at least 1/4″ thick. The recommendation is that the thickness ratio is 2:1 width to depth of the joint. The backer foam or bond breaker tape is to prevent the sealant from bonding to the substrate and only to the two adjacent edges of the joint in order to achieve the specified expansion and contraction of the joint.

      You do not use a cementitous or epoxy grout in this joint. It has to be a resilient sealant that meets ASTM C920 in order to perform properly over time. For walls it needs to be a non-sag sealant. For floors it needs to be a traffic grade sealant with a shore A hardness of 25 or greater.

  7. Rob says:

    I have recaulked this tub 3 times and the caulk always cracks. The gap is 3/8″ to 7/16″ not quite as wide as some of the other people posting. What size backer rod should I use? I’m planning to use the Dap Kwik Seal Ultra due to its mold protection.

  8. Donato Pompo says:

    The Dap product does not meet ASTM C920. You should be using a sealant that meets ASTM C920 that is either a polyurethane product or a structural sealant, which performs substantially better than the acrylic caulkings.

    The sealant has to be applied at least 1/4″ thick. If a closed-cell polyethylene foam backer rod won’t fit in, then you have to use a bond breaker tape at the bottom of the sealant joint. The edges that you are bonding to should be thoroughly cleaned with a final wiping with an acetone. Follow the data sheet directions of the sealant that you end up using.

  9. Rob says:

    I started my demo and discovered the grout was in fact added, at the time of tile installation, right up to the edge of the tub. Maybe that’s been my problem all along. I’ve just been covering the grout with caulk and it cracks. Should I remove the grout to expose the edge of the tile (chip it out) and use backer rod to fill the gap between the tile and the tub? Also I’m struggling to find a C920 sealant but found this site. https://www.buildsite.com/astm/C920 Hopefully one of these will be the white color I’m looking for. Thanks

    • Donato Pompo says:

      Putting sealant over a cementitious grout does nothing other than cover the grout. That is not a resilient movement joint. All of the grout has to be removed.
      The ASTM C920 sealants are polyurethanes and 100% silicone sealants. You can buy them a Home Depot or other hardware stores as well as tile distributor locations.

  10. Rob says:

    Oops Just realized at the time of the original tile installation we used Spectrum Siliconized Acrylic Latex Caulk between the tile and tub (to match the grout color). That Caulk has a 1/4″ minium gap and I doubt that is C920. So back to my last question should I remove it, use the backer rod and use a C920 sealant?

    • Donato Pompo says:

      Siliconized Acrylic Latex caulk is a marketing term. It is an inexpensive sealant that is not long lasting and doesn’t perform well. It does not meet ASTM C920.
      Read the previous answers to your questions as your question was previously answered.

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