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Gap Between Backer Board and Shower Pan: The Complete Guide

One of the most important things you can do to ensure your new shower is watertight is to install a backer board. However, there are specific structural guidelines that you must adhere to. So, how large should the gap between the backer board and the shower pan be? And, what do you fill the gap with?

There should be a gap of 1/4 inch between the backer board and the shower pan in your bathroom. This space ensures there is no pressure on the pan, which could result in cracks. In this way, it also helps prevent water damage.

Gap Between Backer Board and Shower Pan

Read on to find out all of the details and my best tips about the job!

Why Should There Be a Gap Between the Backer Board and Shower Pan?

For a few reasons, there should be a gap between a backer board and the shower pan.

Shower Pans Flex

Shower pans made of acrylic, fiberglass, or composite expand and contract with changes in temperature, weight, and age as your house settles.

The shower pan could crack from the pressure without a gap. If this happens, it’s not just going to look bad—it could also leak water into your house.

The Backer Board Stays Rigid

A backer board, or cement board, is a hard, flat material used to set tile. It prevents the warping and buckling of other materials like plywood.

A backer board is a better choice than drywall for use in damp areas like bathrooms and laundry rooms because it stands up to moisture.

Thinset mortar, used in conjunction with the stability of a backer board, makes it relatively easy to create a tile surface that’s level and consistent.

This gap allows for relief points that can help prevent water damage. Trust me—it’s much easier to fill the gap than repair a cracked shower pan.

Installation Ease

Shower pans may not match up perfectly with the wall behind them or the floor under them. Shimming is essential to make sure your shower pan is level and flush. But not all installations are perfect 100% of the time.

A gap is created between the backer board and shower pan to make up for these slight measurement adjustments.

Shower Pan

How Large Should the Gap Between the Backer Board and the Shower Pan Be?

You should not install the backer board with seams touching the shower pan. Ensure there is a 1/4 inch of space between the pan and the backer panel.

This 1/4-inch gap allows for expansion and contraction of the materials over time, preventing water from getting in behind them.

Filling the Gap

It’s best to use grout in certain spots around the shower and caulk in others. Though caulk would be the better choice for sealing the space between the shower pan and backer board, some builders opt to use grout, which can cause even more issues.

Fill the gap between the shower pan and the backer board with caulk.

Grout vs. Caulk

Caulk and grout are used for different purposes. Caulk is a sealant, while grout forms part of the structure that holds tiles to walls or other surfaces.

Grout is used for adhering tiles to the wall and will not be worn down by water. Caulk, on the other hand, is temperature resistant but can become unreliable over time.

Caulk is not meant for adhering tile directly onto a solid surface such as concrete or backer board.

What Type of Caulk To Use in the Shower

Because of the humid environment in your bathroom, I recommend you invest in a higher-than-usual degree of water protection for your shower. Silicone caulk is a great option for watertight seals. Silicone is an excellent material since it lasts a long time, is versatile, and won’t deteriorate.

Silicone caulk can’t be painted over. Use one made from acrylic to prevent the caulk from staining the new paint on your tub or shower. If you’re going to use caulk, it should be silicone. That silicone coating keeps it pliable and watertight.

This caulk goes on white, but it will dry clear. Wait at least a day after caulking to apply the first layer of paint.

Fill the Gap

Steps To Fill the Gap

The first step to filling the gap between the backer board and shower pan is to collect your materials.

You will need:

  • Silicone caulk
  • Caulking gun
  • Utility knife
  • Silicone tool
  • Paper towels

Load the caulking tube into the caulking gun. Cut the tube tip at a 45-degree angle using your utility knife.

Maintain a firm grip on the gun’s handle. The caulk must be extruded along the seam at a gradual and consistent pressure rate when using a caulking gun. You can use gloved fingers or a silicone tool to smooth the caulk after it has been applied.

Push the caulk into the seam and smooth it over to make a more aesthetically pleasing connection. If you have a more secure seal, you will have better protection against leaks.


Here are the most common questions about the gaps between backer boards and shower pans.

How do I ensure I’m leaving enough of a gap between the backer board and shower pan?

The best way to ensure you have left enough space is by using spacers.

How do I know when to replace caulk in the gap?

Caulk’s fundamental flaw is that it shrinks and dries up with time. It loses its waterproofing properties when it shrinks. Caulk in the shower needs occasional maintenance and replacement. Caulk must be replaced within five years or anytime mold or mildew appears.


With some care and attention, you can ensure your shower is solid and waterproof. Make sure any gaps are filled with caulk before you use your shower. If you see any signs of mildew or mold, remove the caulk and replace it with new. This will keep your shower in tip-top shape for years to come.

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