Thursday, March 3, 2011

Pool Bottoms for Vinyl Liner Pools—What’s best? Cement or vermiculite?

There seems to be a lot of confusion about what is the best bottom for a vinyl liner pool. 

Gererally, a vinyl liner pool consists of steel or polymer walls stabilized by a concrete footer.   A poured bottom is then added to complete the structure.  The two most common used bottom materials are 1) cement or “grout” and 2) vermiculite.  The walls and bottom are then covered by a vinyl liner which is the final membrane that makes the structure water tight.  (Note-a third type of bottom that most all installers have discarded is a mere sand bottom.  However, sand moves around, leaves imprints in the liner and is clearly an inferior choice.)

While costing about $1,500 more, and being more difficult to install, the advantages of a vermiculite bottom are significant to the consumer.  Vermiculite was specifically designed to be the ideal bottom for vinyl liner pools.  First, vermiculite is pourous and allows water to permeate it and to recede through the material.  In contrast, a grout/cement bottom is non porous and groundwater pressure will crack it possibly resulting in expensive repairs at some point (especially at the time of a liner’s replacement).  This is the reason that vermiculite is generally considered to be the better structure for a vinyl liner pool. Vermiculite is also a softer material that can add to the life of a liner.  Concrete is rougher on the liner and cracks or pocking can create sharp edges that may damage the liner.  (See also,; for two manufacturer’s discussion of vermiculite).

The advantages of a grout/cement bottom are primarily cost and ease of installation.  A grout/cement bottom generally costs about $500 less for the materials and there are significant savings on labor—as much as $1,000.  The savings on labor generally arise because the cement can be poured out of a truck by 3 men in a couple of hours.  The vermiculite must be mixed on site and carried down into the pool to apply.  This process takes 5 men about 8 hours to complete.  The grout/cement bottom can also be poured in more varied weather conditions than vermiculite.  Vermiculite requires a fairly dry hole and up to 2 days to cure during which it can be damaged by rain and is thus an annoyance to installers. 

So is the $500-1,000 added cost of a vermiculite bottom worth the cost?  The answer is YES!  A repair of a cracked concrete bottom can be significant and requires draining the pool and removing the liner and jack hammering out the bad area and re-pouring.  The risk of such a costly repair is greatly mitigated by the use of vermiculite.  Add in the fact that your liner will probably last longer and it is clearly the better financial decision.  So why do some installers still use cement?  The lower cost and ease of installation appear to be the primary attraction as opposed to the higher quality of a vermiculite bottom. 

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  6. What is the ratio of vermiculite to Portland cement and water?

  7. What is the ratio of vermiculite to Portland cement and water?

  8. Vermiculite is sold in a common size of 4 cubic feet and Portland cement comes in 94 lb bags.The ratio is 2 bags of Vermiculite to 1 bag of cement and 4-6 Gal of water.