Cement Board Under The Shower Pan. (Necessary!?)

Bathrooms and showers are high moist places where all the elements in the room, including the shower pan, are subjected to a high degree of water and moisture. This is why it is important to follow the right installation procedures and use building materials that can withstand high amounts of moisture without breaking apart.

A cement board is a combination of reinforcing fibers and cement transformed into thick sheets that are used as a tile backing board. In addition, a cement board can be used as a sub-surface under the shower pan to provide support and help the shower pan comfortably sit. Despite the several advantages of cement board, some enthusiastic DIY homeowners are still confused as to whether they really need cement board under their shower pan.

This article tries to break down the advantages of the cement board and helps you reach a conclusion on whether you truly need a cement board under your shower pan or not.  

The cement board has many advantages; however, you don’t necessarily need to place a cement board under your shower pan. The most important step to take is to make sure your shower pan is installed correctly over the subfloor with a mortar bed. A cement board, in most cases, isn’t a necessity, but if you have the mean, it’s a good thing to add it to your shower pan installation plan. 

Does cement board go into shower pan?

The choice is up to the house owner or the building contractor, and it is very subjective. If you prefer the cement board under the shower tray, that’s fine, and if you don’t want it under your shower tray, it is also fine.

As earlier stated, the inclusion of a cement board under the shower pan isn’t a must. However, its inclusion may help the shower pan sit comfortably on the subfloor and cushion the mortar bed’s arch effect on the shower pan. In essence, you could decide to add it and not add it, and your shower pan will still function effectively if properly installed. In addition, you can also use a cement board under the shower pan as long as the waterproof membrane is placed over the cement board to give the cement board a strong backing underneath and help the cement board retain its strength.

Why do I need a cement board for my shower pan?

Cement boards are generally used in wet areas, such as showers and bathrooms. Usually, shower and bathroom areas are exposed to water and moist; therefore, without protecting the subfloor with cement board, the mortar bud may start to rot and deteriorate. The cement board also helps prevent the growth of molds and mildew. In addition, cement boards are a good substitute for tiles and are commonly installed in showers as well.

Do you install a cement board before the shower pan?

The cement board must be installed before the shower pan. If the shower pan is not correctly placed, it might lead to other complications in the future. Therefore, it is important the person in charge of the installation gets it right at once. The wallboard should be placed 1/8 inch over the shower pan. In case the shower pan is not available, the wallboard can be guessed. It is important to note guessing the height may lead to inaccuracies and an expansion gap that may be too big or too small.  

Do you need a cement board under a shower base?

If your shower base or shower pan is installed on a wood floor covered with plywood or a similar type of particle board, then you don’t need cement as long as the flooring is strong and flat. However, some shower bases require a cement bed to facilitate drainage and support the shower base floor. Another possible reason is if the shower pan is placed in mortar. In this case, even if the cement board subfloor is leveled, you may not necessarily need a cement board under the shower pan; however, the structure needs a level solid surface. This means, in a case where the board isn’t placed under the shower, the most important thing to look out for is leveled cement. The cement should be thicker than 1/2″.

What do you put under the shower pan for support?

The bedded mortar is the universal acceptable support structure for the shower pan. Building contractors use bedded mortar to cover areas where the shower base will sit. Once the mortar is set, it hardens and prevents the tub from flexing.  

Does the cement board need to be sealed after using it under the shower pan?

While the cement board doesn’t always need to be sealed when used under the shower pan, it sometimes makes sense to seal them. If you’re installing a cement board in a bathroom or shower, then it makes sense to put a waterproofing sealer like Redguard on it, to act as the membrane and help solidify its effectiveness.

Does it matter which side of the cement board faces out?

The cement board has 2 sides :
1. smooth side
2. rough side

 So, if you will be using a thin-set mortar adhesive to install the tile, face the rough side out
. But, if you will be using the latex mastic, face the smooth side out. It’s important to note that it doesn’t really matter which side of the cement board faces out. You will most likely get a similar result regardless of the side that faces out.


A cement board is a combination of reinforcing fibers and cement transformed into thick sheets used as a tile backing board, and it has many advantages, especially in wet areas. The shower is mostly wet and moist due to exposure to water. Hence the need for a cement board under the shower pan to help drain water from the shower and protects the floor from water intrusion, rot, and mold. But it is important to note that you can do without cement board. The most important step is to ensure your shower pan is installed correctly over the subfloor with a mortar bed.