How to repair broken bathtub or shower floors
...and why injecting foam and gluing on a mat is foolish at best,
and a health nightmare waiting to happen at worst
How is a bathtub or shower floor constructed?
The floor of a fiberglass bathtub is constructed like an Oreo cookie. It consists of a core, usually press-wood, which is sandwiched between two layers of fiberglass. Between the floor of the tub and the subfloor is a hollow area that is important to the installation.
How does a fiberglass bathtub or shower become weak and spongey or just crack and break?
The bathtub does not rest directly on the sub-floor, but usually on integrated skids for installation on uneven surfaces. Without a full support structure the skid acts as a fulcrum.
A fulcrum is a powerful tool that channels great physical forces with very little effort. The same physical forces and properties that cause a living room floor to sag and break are at work on your bathtub floor.
These forces are called "compression" and "tension".
Simply put,
"compression" means squeezing, like with the sponge,
and "tension" means pulling, like with the spring.
Let's look at that floor joist again. The load of the floor is pushing down on the top of the joist; it's under compression. The bottom of the joist is pulling apart; it's under tension. The areas under tension always break first.
Each person using the shower compresses their full weight on the same spot - day after day, year after year. The bathtub floor begins to flex, and one day you see a crack. But remember, the areas under tension always break first.
When we cut off the cracked top layer of fiberglass and remove the damaged area of the core, the major damage to the bottom layer of fiberglass is revealed. The areas under tension always break first.
Where the tub floor meets the tub wall, in the curve, the top layers of fiberglass are in tension as well. If you have a crack in the floor, you probably have cracks here as well. The areas under tension always break first.
Now that we have a good understanding of the construction of the unit and the basic physical principals that govern that construction, we can determine the extent of the damage, and the proper repair solution.
Determining the extent of the damage
We start with cutting out the area surrounding the crack in the floor and examining the condition of the core. If the core is damaged or missing, more area will need to be cut out to determine the full extent of the damage that needs to be addressed.
In this photo, we see that there was no core directly under the crack. Reaching inside the unit, the core was found to be degraded and was delaminating, but the extent was unknown and further exploration was required. After cutting out a larger area, it was discovered that the unit had only two pieces of wood core support for the entire floor (shown in the photo).
The existing core support was discovered to have been rotting, with mold beginning to form. Presswood cores are like cardboard – when they get wet they begin to degrade and do not regain their strength when they dry out.
We see that the core has lost its structural integrity and delaminated, thus becoming less useful than it was before. Technically, the inspection would stop now because all of the information has been gathered and a course of action ascertained.
Since the rest of the bottom had no core or support at all, the proper course of action would now be to either rebuild the entire bottom, or replace the whole unit. Let's now take a look at how it should be repaired.
How we repair a broken bathtub floor
If you recall, the bathtub floor was originally constructed like an Oreo cookie. It has an upper and lower layer of fiberglass (the cookies) and a core (the cream filling). The first step is to remove the damaged core.
It initially broke because of a lack of support, so a solid structure needs to be constructed. Footers in contact with the subfloor, and a full header to distribute weight, like a snowshoe.
Fiberglass is laminated to the header and the bathtub with vinylester resin to recreate the bottom layer, or "cookie". Lamination gives it strength - it's easy to break one stick, but not a bundle.
We install a composite honeycomb core used in boat building. The individual cells not only distribute weight evenly, but if the floor is damaged by impact the water will be contained to the damaged cells and will not rot or wick water.
The core is fiberglassed and laminated to the bathtub as the top layer, or "cookie". It's then faired in (made smooth) to recreate the original contours designed to move the water to the drain.
The final step is the topcoat and non-slip texture. No grit or abrasives that can be picked out of the finish or cut your feet are used. The texture is identical to a factory finish and is very easy to clean.
Now, let's discuss why we won't use the "foam and glued on mat" method.
Injecting foam and then gluing on a mat does not repair a bathtub or shower floor
A popular method for addressing a crack in a bathtub or shower floor is to drill holes through the floor, inject foam into the hollow area, and glue on an inlay or mat to hide the repair work.
The floor of a fiberglass bathtub is constructed like an Oreo cookie. It consists of a core, usually press-wood, which is sandwiched between two layers of fiberglass.
Between the bottom layer and the subfloor is a hollow area. Without support, the area where a person repeatedly stands weakens over time and it breaks - just like breaking an Oreo cookie.
When a crack occurs in a bathtub floor, water can wick (or be drawn) into the core through capillary action. Just like an Oreo cookie that soaks up milk, the core will become soft and squishy, and will never regain its strength or rigidity even when it dries out. Injecting foam under this damaged core will not magically fix it.
