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


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 - 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!

8 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.

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