Do you need mortar under fiberglass shower pan?

When installing a fiberglass shower pan, one of the key considerations is whether you need to put mortar under it or not. This is an important decision that impacts how level and sturdy your shower pan will be. In this article, we’ll look at the pros and cons of using mortar under a fiberglass shower pan and provide recommendations on when it should or should not be used.

What is a fiberglass shower pan?

A fiberglass shower pan (also called a fiberglass shower base) is a preformed, one-piece unit that consists of fiberglass reinforced plastic. Fiberglass shower pans come ready to install and provide a watertight basin for your shower. They are available in many different sizes, shapes, and colors to fit your bathroom design.

Compared to building a custom tile shower, installing a fiberglass shower pan is a much quicker and easier process. Since they are preformed, you don’t have to do as much framing, sloping, or waterproofing work. Fiberglass shower pans are also more affordable than custom tile showers.

Do you need mortar under a fiberglass shower pan?

Whether you need mortar under a fiberglass shower pan depends on the type of installation and the manufacturer recommendations. Here are some general guidelines:

• For drop-in installation, mortar is not required under the shower pan. Drop-in fiberglass shower pans are designed to simply drop into place within a framed shower base.
• For three-wall alcove installation, mortar is usually recommended under the shower pan. Applying a mortar bed helps level, support, and strengthen the shower pan within the framing.
• Always check the manufacturer’s instructions. Some fiberglass shower pans may specifically require or recommend mortar for proper installation and performance.

Pros of using mortar under fiberglass shower pan

Here are some of the benefits of putting a mortar bed under a fiberglass shower pan prior to installation:

• Creates a level surface – Mortar allows you to create an even, sloped surface for the shower pan to sit on. This provides proper drainage.
• Supports weight – A mortar bed helps bear the weight of the shower pan and any people standing in it.
• Reduces movement – Mortar bonds the shower pan to the subfloor below, minimizing shifts, cracks, and flexing.
• Seals the edges – Acts as a sealant around shower pan edges to prevent moisture seepage.

Overall, the mortar provides a very stable, secure base for the shower pan to rest on within the framing. This helps prevent sagging or damage over time and improves the installation quality.

Cons of using mortar under fiberglass shower pan

There are also some downsides or challenges with putting a mortar bed under a fiberglass shower pan:

• More labor intensive – Preparing and pouring a mortar bed takes more effort and time.
• Difficult to modify or remove – The mortar makes it very difficult to replace or access anything under the shower pan in the future.
• Added cost – Mortar mix and additional materials can add more expense to the project.
• Requires skill – Properly creating a sloped mortar bed takes some skill and practice.

For these reasons, a mortar base may be passed on if not absolutely required. But in many cases, the added stability and quality outweigh the downsides.

When should you use mortar under a fiberglass shower pan?

Based on industry best practices, here are some recommendations on when to use mortar under a fiberglass shower pan:

• For three-wall alcove showers, always use mortar – This provides essential support within framing.
• With clawfoot bathtub conversions – Maintains stability when converting a tub to a shower.
• On uneven or damaged subfloors – Helps level and fill when the subsurface is not smooth.
• For heavy tile surrounds – Supports the weight of heavy tile around the shower pan.
• If recommended by manufacturer – Follow specific product instructions.
• In problematic deflection areas – Minimizes bounce or instability in the floor.
• For second story installations – Reduces movement for showers installed above ground level.

In most remodels, you will want to take the extra time to create a mortar base. This provides maximum durability and prevents the pan from cracking or flexing over time.

When can you skip the mortar under a fiberglass shower pan?

There are some instances when it is acceptable to install a fiberglass shower pan directly on the subfloor without a mortar base:

• For drop-in shower pans – Most drop-ins do not make contact with the mortar bed below.
• On engineered subfloors – Materials like backerboard provide sufficient support without mortar.
• For temporary shower setups – Mortar may be skipped if the shower pan is not permanent.
• Over a concrete subfloor – Concrete provides a level, stable surface.
• For accessibility and curbless showers – Mortar bed would interfere with zero-step entry.
• If the pan manufacturer says it is not required – Follow product specifications.

The subflooring and type of installation are the key factors determining if mortar under the shower pan can be skipped or not. Consult codes and manufacturer instructions.

How to install mortar bed under fiberglass shower pan

Installing a mortar bed under a fiberglass shower pan involves a few key steps:

1. Frame shower walls and dam area – Construct 2×4 framing to required size and height per rough opening. Install blocking and framing below shower pan area.
2. Install shower drain – Place drain assembly according to drain location on pan and secure firmly.
3. Build containment – Construct temporary wooden dam around shower pan footprint to contain mortar pour. Seal with caulk.
4. Pour sloped mortar bed – Mix and pour mortar to a minimum 1/4″ per foot slope. Smooth and level surface.
5. Prepare for pan – Once mortar has cured, waterproof pan surface and install drain coupling.
6. Install fiberglass pan – Carefully lower pan into framed area and connect to drain. Embed pan flanges in thinset.
7. Seal edges – Caulk all around pan edges and seams to prevent leakage.
8. Water test – Fill pan and drain to check for leaks before continuing with surround install.

The mortar base must cure completely (typically 1-2 days) before setting the pan to avoid cracking. Be sure to follow all preparation and installation steps from the pan manufacturer.

Conclusion

Installing a sturdy, long-lasting fiberglass shower pan starts below the surface. In many cases, a mortar base provides critical support, stability, and drainage under the pan within the framing and subfloor. This prevents the pan from shifting or flexing over time. However, some drop-in style fiberglass pans do not require mortar.

Always check the manufacturer’s recommendations to see if mortar is required or recommended. When in doubt, it is generally best practice to take the time to create a proper mortar bed before carefully lowering the fiberglass shower pan into place. This provides maximum strength, durability, and performance.