Using Spray Foam Under A Shower Pan: Pros, Cons, & Cautions

If you are considering using spray foam under a shower pan, there are few important points that you need to consider.

Using spray foam under a shower pan: Spray foam has become popular in recent years as a solution to reducing flexing of a shower pan. But the following guidelines should be followed:

• Ensure proper and accurate leveling
• Only use minimal-expanding spray foam to prevent damage
• Apply weight to the pan until the foam has cured

It’s important to understand that this approach is not considered best-practice and in conversations with several contractors, most would not recommend this. Still, it’s a popular DIY solution that’s worth understanding.

What Causes Shower Pan Flexing?

The design of most shower pans is similar to that of modern bathtubs in that the bottom of the area where you actually stand often does not touch the ground. It’s designed to this way because the pan has to facilitate water movement toward the drain.

Some pans come manufactured with supports underneath but often they are not enough to completely eliminate flex. When you stand in the shower, you feel the pan (or bottom) of the shower give way a little underneath your feet.

This usually doesn’t hurt anything as the pan is designed to hold weight but many people are bothered by it. It just doesn’t feel solid.

What Type Of Shower Pans Are Prone To Flexing?

If you have a cultured granite shower pan, flexing isn’t usually going to be an issue. It’s more common with less expensive acrylic designs that are commonly found in the big home improvement stores.

Where the spray foam approach is commonly talked about is rental property owners and house flippers who are looking for easy, inexpensive solutions. I’ve come up with my share of creative ideas when rehabbing houses over the years so I can appreciate the appeal of this.

Type Of Spray Foam To Use

This is important. Only a minimal-expanding foam should be used under a shower pan. The reason for this is that when foam expands it exerts pressure against opposing forces. And so, you could end up with bowing or even cracking of your shower pan.

A product like Great Stuff Window and Door (link to Amazon) is the only type you would want to consider. It’s designed to not expand aggressively.

But I keep feeling compelled to emphasize that this is not an intended use for spray foam. So, before you start a project like this, there’s one consideration that deserves discussion…

Do You Even Need To Worry About Your Shower Pan Flexing?

In all honesty, this is often less of an issue than people make it out to be. Most shower pans are perfectly capable of withstanding the weight of a person standing in them. They are designed for this.

If any additional support is needed, you can probably accomplish the same end result by sliding a couple of pieces of shimmed down treated wood under the pan.

That would reduce the flexing and you wouldn’t have to deal with cleaning up foam spray waste. Have you ever tried to get that stuff out of your clothes?

In fact, there are a number of alternative ways that you can solve the flexing issue besides spray foam.

Comparing Solutions To Shower Pan Flex

Below are the three most common DIY solutions to solving flex in a shower pan:

• Treated wood shims
• Spray foam
• Manual grout pump

Each of these has their own benefits and ideal applications so which one you use will depend heavily on your situation.

First, let’s compare the pros and cons of each:

Solution	Pros	Cons
Treated Wood Shims	Completely reduces flex	Can be difficult to determine precise thickness needed.
Spray Foam	Reduces most flex while allowing reasonable “give”	Difficult to clean up, especially from clothing
Manual Grout Pump	In line with professional practices.	More labor intensive than other options.

I’ve already gone over spray foam and treated wood shims but let’s have a look at the manual grout pump approach.

The reason I want you to at least consider this option is that it is relatively inexpensive, easy for the DIYer, and in line with professional practices.

The way these pumps work is you fill a bucket with grout, mixed according to manufacturer’s guidelines, and with the nozzle-end submerged, you pull on the hand pump.

This creates a suction that pulls the grout mixture into the housing of the device. Then, you place the nozzle where you need the grout applied and press the handle in. This pushes the grout into crevices where it will dry and harden.

Here’s a short video demonstration of how these work:

Hand Mortar Gun (Grout Pump)

The advantage of using a hand mortar pump is that you have the ability to wedge into tight areas like spray foam but you don’t have to worry about expanding issues.

And there’s one other point worth considering; longevity. Spray foam degrades over time (source). Mortar is pretty much a permanent solution.

Best of all, you are using a product that is intended for this type of application. It has stood the test of time unlike trendier solutions such as spray foam.

You can buy a hand mortar pump at your local hard store or online. Here’s one example (link to Amazon) but look around and find the one that best suits your needs.

Conclusion

Spray foam under a shower pan is an option for reducing flex. It’s not the only solution and it may not be the best choice depending on your situation but if you do decide to use it, make sure that you opt for a non-expanding foam.

Remember also that a little flex in acrylic shower pans is normal and may not be an issue. Unless you are concerned about cracking or other physical damage from flexing, you may be better off just leaving it alone.

Related Reading: Can Expanding Foam Be Used Under a Bathtub? Uses & Concerns