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Vinyl liner pools regardless if it is steel or polymer walled, needs a liner to go in and sit on a perfectly flat and smooth pool floor.
Pretty simple to understand that you want your pool bottom flat and smooth, but what is it made out of?

Different Pool Bottom Options

• Sand... if your pool contractor is using sand to put in your pool bottom, fire him on the spot.
• Vermiculite... a combination of Portland cement, sand, and vermiculite.
• Concrete... a special mix and blend of concrete made at the redi-mix plant used specifically for inground pool bottoms.

Are Pool Bottoms Region Specific?

NO.
The pool bottom you put isn’t based on the region you live in.  The substrate (ground under the pool bottom) that you are building on needs to be prepped to make sure that anything you put down will adhere correctly. Clay, sand, stone, rock, dirt... doesn’t matter.

Yes.
Many regions pour pool bottoms a certain way based on advertising, marketing, tradition & availability. For example, what is common in the Midwest isn’t common in the North East, even though they have the same climate conditions.

1. Midwest – Vermiculite pool bottoms are very common due to multiple vermiculite sites and widespread acceptance through distribution.
2. Northeast – Concrete pool bottoms are much more prevalent.

What is Better or Worse?

Pros for each type of pool bottom.

• Vermiculite
  • Easier to install as it a uniform product and can be worked for long periods of time.
  • Slightly softer feel when walking on.
• Concrete Pool Floor
  • Smoother and cleaner finish.
  • Structural and allows for in-floor radiant heat.
  • Faster install and hardens quickly.
  • Non-porous so it will hold back ground water; helping prevent floating liners.
  • Sturdy, allows you to replace a liner in the rain with no worry to the pool bottom.

Drawbacks of each vinyl liner pool bottom.

• Vermiculite
  • Messy and dusty when mixing on site.
  • Long install time (~8 hours) and must be rain free for 8+ hours so product can harden.
  • Porous which leads to ground water coming up and aiding in the floating of liners.
  • Hard to finish without leaving trowel marks, often times must be sanded for perfect finish. Pool lights show every imperfection.
• Concrete Pool Floor
  • Specific skill set needed to pour a concrete pool floor, above average pool contractor can’t do it.
    • Not forgiving, it is either done right or done wrong.
  • Harder surface when walking on, but only if you have walked on something softer previously.

Vermiculite vs Concrete Pool Bottom, What is Better?

Vermiculite came to be to fill a need. It was adapted to get away from a sand pool bottom and be used by those that couldn’t or didn’t want to pour concrete.
Concrete is a strong and sturdy base that when done is very smooth... think of your garage floor the day after it was poured, that is what your pool bottom is.

Many say that having a porous pool bottom is good so it doesn’t crack. If you have that much ground water or hydrostatic pressure, then your pool was built wrong and you need or should have a sump system installed on your pool. A concrete pool bottom will not allow water to pass through it, which helps prevent floating liners.

Concrete pool bottoms are going to be a harder more skilled install, which then leads me to think that you are going to have a better pool contractor, but that definitely isn’t always the case. For my money, I will always opt for a concrete pool bottom.