How to Caulk a Large Gap Between Tub and Tile

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  • 1-3 hours
  • Intermediate
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It’s one thing to read a step-by-step tutorial regarding DIY projects such as retiling a tub surround, and another to have your project go as perfectly as it does in said tutorial. The truth is, when you get into a project you’ll likely identify ways your project is different, and many times that means challenging.

One example of this is a popular topic in our forum regarding how to deal with a large gap between the top of the bathtub and the bottom of the tile coming down the wall. You may find this during a renovation, remodel, or simple caulking replacement. Whatever instigates it, knowing the right technique for caulking a larger than average gap will save you time and protect your investment from water leaks.

Step 1 - Determine the Reason

When you find a large gap, say ½”, evaluate why. If your bathtub has sunk you’re dealing with a bigger issue. However, if the gap is due to original shoddy craftsmanship or material shortage, the mistake can be covered up, providing a completed look that hides the flaw behind.

Having said that, if your gap is excessively large you may need to add an additional row of cut tile or raise the bathtub to meet the bottom of the existing tile. You could also select a smaller, matching or complimentary tile to fill in the bottom row.

bathroom with two kinds of tile next to bathtub and sink

Step 2 - Use a Backer Rod

A product referred to as backer rod is basically a roll of foam that looks like a coiled garden soaker hose. Backer rod is designed to fill gaps larger than the thickness of a bead of standard caulk, which is about ¼”. It’s easy to use, cut, and manipulate into place.

Measure the thickness of your gap and purchase a roll of backer rod slightly larger in thickness. The backer rod is flexible foam, so it will compress into place. Cut the length you need for one side of the bathtub. Using a putty knife, start at one end, pressing the backer rod into place as you move down the length. Position the backer rod slightly behind the surface of the tiles. Continue around the tub until the gap is filled on all three sides. Sometimes your gap is larger on one side of the tub than the other due to the imprecise square of a wall, ceiling, or floor. Simply fill the spaces with the appropriate backer rod as needed.

Step 3 - Caulk

With the backer rod in place, the caulking now has a surface to stick to and rest against. This will keep the caulking from disappearing into the gap. Cut the end off the tube of caulking. The further up on the tube you cut, the thicker your bead of caulk will be so use the thickness at the neck of the tube to estimate.

hands cutting caulk gun

Place the caulk into a caulking gun and secure it in place. Start on the outer corner of the tub surround. Pull the trigger in a consistent stream while moving at a steady pace. Once the bead is in place, apply light pressure with a moistened finger and run it across the surface. This creates the curved, finished look while filling all the gaps where water could enter the space. Keep a rag handy to wipe excess caulking on as you work around the tub.


Another option is to skip the backer rod and use ceramic quarter round or PVC quarter round molding. For either product, cut to size and dry fit against the gap. Rather than filling the space, this option will place material in front of it, essentially blocking the gap. Use tub adhesive to attach the material once you have them cut to size. Once the quarter round is dry and secure, apply a thin bead of caulking around the outside top and bottom to block any water access points.

Hint: While selecting your caulking, make sure it is recommended for bathrooms. It will need to be waterproof. You can select from a variety of colors depending on the color of the tub, tile, or quarter round you are using. Clear and white are the most common options.