How Hot Tubs and Spas Are Built


Most manufacturers use the fiberglass lay-up method of spa construction. This manufacturing method combines layers of polyester resin and glass fiber. A heated acrylic sheet is placed over a spa-shaped vacuum mold. Air is drawn out through hundreds of small holes. When the acrylic cools, they remove it from the mold and reinforce the underside of the spa with resin and chopped fiberglass mixture.

These Blisters Don’t Heal

This polyester resin and glass combination can lead to a common problem – acrylic blistering. Blisters occur because of a chemical reaction between moisture from the tub and the polyester resins used in the reinforcing process. Many spas have been ruined because of blisters.

A Half Truth

Spas made from polyester resins are blister resistant, but their finish and structure warranties are separate. The salesman points out the structure warranty because it is usually 10-15 years. Unfortunately, it’s too tempting to neglect to mention that the finish warranty is only 1-3 years. You could have a blister problem in as little as one year!

Does Wall Thickness Affect Spa Durability?

The acrylic provides color, not structure. We measure durability by weight and the thickness of the fiberglass, not the thickness of the acrylic. The acrylic covering is at most 1/8 inch thick and tapers to 15 or 20 thousandths of an inch in the footwell of the spa.

Water is a Big Deal

Water is heavy. Water in a spa can weigh one to two tons. What happens when you fill a thin-walled spa with water? The water pushes down and gradually bends or breaks the spa. To compensate for the lack of strength in a thin-walled spa, some manufacturers fill the underside (between the bottom and the ground) with urethane foam. Others require sand or a wood structure underneath the spa. Sand or foam provides support under the benches or seats, lounge area and step. Portable spas often depend on the wooden skirt to support the water. Without the skirt, many of these cheap spas would collapse.

Revealing Comparison

Let’s compare two well-known spas. One shell weighs just 95-100 pounds and has 1/8 inch walls. Another weighs 120 pounds and has 3/16 inch walls. Spas with flimsy walls like these require additional support. Great Northern® spa shells weigh 225-260 pounds and have walls up to 5/8 inch thick.

The Support Is Literally Built In

Our spa shells don’t need additional support. They’re built to last! How can you tell how thick the wall is? It’s a little tricky. Try flexing flat wall areas. If you can, try to examine a spa that doesn’t have the jets installed yet. The holes will show the spa thickness. Lift the spa shell only. Does it seem too light? If the manufacturer uses foam fills or requires sand filling or wood framing, you know the vessel walls are too thin.

How Can I Recognize A Good Quality Spa?

Quality spas have the following characteristics. They…

  • Are constructed from blister-proof materials;
  • Are thick enough to hold water without sand, foam, or wood support;
  • Are deep enough for over-the-shoulder soaking;
  • Have high vessel weight (indicates good wall thickness);
  • Have the same warranty for structure and finish;
  • Don’t rely on a wood skirt to support the water weight;
  • Are well-molded with straight sides, no warping, and flat across the top.

Hot Tubs

The art of wood tank building goes back to the fifteenth century. People built barrels and tanks to hold liquids. We build hot tubs the same way that medieval craftsmen built their tanks and barrels. Barrels, wooden tanks, and hot tubs are built from curved pieces of wood called staves. Craftsmen assemble the staves and place metal hoops around them to hold the staves in place. At Great Northern®, we make hot tubs out of redwood or cedar. We coat our hoops with plastic to eliminate rust.

The Good Old Sixties

Back in the 60s, the popularity of wooden hot tubs really took off. These tubs were made from old wine vats or 500-gallon water tanks. They became watertight when the wood saturates, swells, and seals the joints. Wooden vessels have always been popular with people who love depth and leg room.

No More Old-Growth Wood…But That’s Okay

All-wood tubs are no longer as popular as they once were since all-wood construction requires clean, knot free heartwood from old growth trees. Nowadays, no one wants to sacrifice these beautiful old trees to make hot tubs. But, not to worry because you can get the benefits of a traditional, all-wood tub (leg room, depth, portability and good looks) with a modern, plastic-lined tub.


The best-known lined tub is the Rubadub Tub®. Bill Jaworski, a pioneer of the hot tub and spa industry, created it in 1978. These tubs are easy to clean and have the same hydrotherapy jets and equipment as spas.

Is The Liner Vinyl?

No. The interior finish on the Rubadub Tub® is made from PVDF, an easy-to-clean industrial tank lining material designed for high-temperature service. Its semi-gloss finish resists chemical and weather damage just like acrylic. Many people mistake PVDF for pool vinyl. Pool vinyl doesn’t hold up to the high water temperature you have in hot tubs. BEWARE! Some manufacturers use vinyl liners in their hot tubs. The life expectancy of these liners is only 1-2 years.

Are There Any Advantages Of A Hot Tub Over A Spa?

Yes! There are several reasons you might want a hot tub instead of a spa. Here they are:

  • You can install a hot tub anywhere. Because hot tubs aren’t molded in one piece like a spa, you can have a large, deep hot tub anywhere you want! Hot tubs come in a package that fits down the stairs, through the door, and even through the windows!
  • The Rubadub Tub®, once assembled, can be moved indoors or out as the season requires. Its flexible, lightweight construction allows it to be moved anywhere easily!
  • You can create your own hydrotherapy system. Where does your back hurt the most? How about that neck strain? You can position the jets in your tub at the best height for your aches and pains. The 5- foot diameter Rubadub Tub® allows you to place jets where you need them. If you want one jet on the small of your back and another between your shoulders, just install two jets vertically. If you want more jets, you can have them. You can have as many jets as you want; it just takes a larger pump to power them.
  • You can have any size tub you want. If you want a custom-size vessel, a Rubadub Tub® is the easiest, most economical way to get it.
  • All Rubadub Tubs® are made one at a time, so ordering an extra large or extra small tub is easy. Almost any size is possible. We have built tubs as small as 30 inches around by 30 inches high for one person, and as large as 14 feet in diameter for ski resorts.

How Can I Recognize A Good Quality Hot Tub?

Quality hot tubs have the following characteristics. They…

  • Are deep enough for soaking (35 to 44 inches);
  • Are lined with PVDF for long life and easy cleaning;
  • Are built from high-quality redwood or cedar;
  • Have poly-coated hoops to prevent rust;
  • Look like high-quality furniture.