Step-by-Step Guide: How to Prepare Base For a Spa in NZ

09 February, 2024

Spa pools and swim spas are great for relaxation and fitness, but achieving the best experience requires a solid foundation. It might appear daunting, especially if you're undertaking this task for the first time. But no worries; this guide covers everything you need to expect from building a base for a spa pool or swim spa.

But first, let's answer an important question...


Do you need a foundation for a hot tub? Yes! Here's why

Having a sturdy base for your spa or swim spa isn't just about the initial setup. The spa base plays a big role in maintaining the performance and durability of your investment. Plus, it significantly eliminates the risk of potential damage. Here are two reasons why a solid base for your spa pool or swim spa is crucial:


1. Spa bases prevent your spa from cracking and distorting:

The average spa presents a significant load for any foundation to support. Even our smaller spas can weigh up to 2000kg when filled with water and occupants; some of our larger spa pools can reach 3000kg, and our swim spas up to a whopping 7000kg.

So, if you put your spa on a soft surface such as grass, sand, pebbles or dirt, without a spa pad or base, it will place excessive stress on your spa, causing it to distort and/or crack.

Refer to your spa brochure or our website for dimensions and approximate filled weights of Sapphire Spas models. For example, the specifications on myChillout spa page show that it weighs 1535kg when it's full of water, plus the weight of the people in the spa. Remember to take the weight of the occupants as well.


2. Your warranty can be voided without a proper spa base

Your spa's warranty becomes invalid if it's damaged due to an improper foundation. So, it's your responsibility as the owner to make sure the spa is correctly installed. So, whether you're choosing a concrete slab or timber decking to support your spa pool, we strongly recommend using a licensed contractor.
For more details about warranties, check out this helpful guide to the ins and outs of spa warranties.


What is the best base to put under a hot tub?

A concrete slab is the best way to provide a solid foundation for a spa pool or swim spa as it provides an even, level surface free from cracks or hollows. To be more specific, a reinforced concrete slab that's at least 100mm thick is the most suitable foundation for your spa. The best base for a swim spa is a concrete slab that's increased to a minimum of 150mm thickness.

In both cases, you must ensure this base is an even, level surface free from cracks, doming or hollows. No fall/gradient is to be present within the spa footprint; the concrete surface should be finished with a perfectly flat screed. That's why having a certified contractor on board is important: they would be familiar with all these requirements.


How thick does a slab need to be for a spa?

  • For spa pools (up to 3500mm in length) - a minimum 100mm thick concrete slab base/ 1 layer of steel reinforcing mesh
  • For swim spas (4000mm+ in length) - a minimum 150mm thick concrete slab / 2 layers of steel reinforcing mesh


How big should the base be for a hot tub?

Make sure your hot tub base extends at least 200mm beyond the spa size for good stability. It helps keep everything steady and secure for your hot tub setup. Leave about 600mm of clearance around the spa for easy access and maintenance.


How to build a base for spa pool or swim spa: the complete process from prep to decking

Here are the steps needed to prepare a solid, level concrete foundation that will remain stable for the life of your spa.


Step 1: Preparation

Your tradesman will prepare a level surface large enough to extend approximately 200mm on all sides of your spa cabinet dimensions. This will help to provide stability and prevent cracking of the concrete.

They'll level the space by removing any uneven grass, debris and clumps of dirt and adding rocks for stability before pouring concrete.


Step 2: Reinforcement

Always check with local conditions but these are the minimum suggested reinforcement specifications for concrete spa and swim spa pads:

  • Reinforcement for a base of a spa pool should include:
    - a minimum 100mm thick concrete slab base
    - 1 layer of SL72 steel reinforcement mesh positioned as per the diagram
    - suggested 25mpa concrete mix
  • Reinforcement for a base of a swim spa:
    - a minimum 150mm thick concrete slab base
    - 2 layers of SL82 steel reinforcement mesh positioned as per diagram
    - suggested 25mpa concrete mix


Step 3: Creating a level spa footprint

The contractors or spa installers will ensure the spa area is perfectly level, without any slopes. They'll need to finish the concrete surface smoothly using a flat screed. If there's a conduit for electrical connections, the installers will clean the concrete around it, preventing any raised areas that could interfere with the spa base.

dirt level for spa installation

Caption: See how smooth and level the surface is once the spa base has been properly installed. This level of smoothness and rigidity ensures the spa's longevity and resilience against potential damage from uneven surfaces and excessive wear and tear. 


Step 4: Drainage

A gentle slope or gradient can be designed across the concrete slab outside the spa area to facilitate proper drainage.

However, the entire area on which the spa cabinet rests needs to be flat and level; no slope or gradient is acceptable here, as this may cause the spa frame to warp and twist, thus voiding the warranty. Any drainage channels need to be installed outside of the spa footprint (see diagram).

Step 5: Spa Installation

Now that the base is prepared to perfection, you are ready to take the next plunge - the spa installation. This is indeed an exciting step, as it brings you one stride closer to your refreshing aquatic experience. Follow the steps in this guide to spa installation in New Zealand, which covers everything from delivery to electricals, fencing and council approvals.

Find local spa installers here to help you navigate selecting and fitting an impeccable base for your spa pool or swim spa.


Step 6: Timber decking

If you plan to put your spa on a deck, it's a good idea to check with a structural engineer. They can make sure your deck can handle the spa's weight. If you decide to go for it, we recommend putting the spa on a concrete slab first and then building the deck around it.


Final thoughts on building a base for a spa pool or swim spa

  • Your spa needs a sturdy base to support its weight and last longer.
  • Soft surfaces like grass or sand aren't spa-friendly. They put too much stress on your spa, making it crack and distort.
  • If any such damage happens due to a weak foundation, your spa warranty can become void because Sapphire Spas will take no responsibility for the installation of your spa.
  • A concrete slab is the best base for a swim spa or spa pool– it's strong and stable. Extend it 200mm beyond your spa and leave 600mm around for a stress-free setup.
  • Building a concrete base for a swim spa or spa pool involves groundwork, laying strong reinforcement using steel mesh, creating a level spa footprint, providing efficient drainage and finally, the actual installation of the spa or swim spa.
  • You can choose to have timber decking to complete the installation. This is decorative yet functional.

And that's all there is to it! Once your timber decking is complete, you will now have a resilient, aesthetically appealing, well-drained base for your hot tub/spa pool, swim spa or plunge pool. This base will not only safeguard your investment from unnecessary damage but also greatly enhance the lifespan and functionality of your spa. However, remember that the building process might vary to some extent depending on the specific model and size of your hot tub or spa.

Explore our spa pool, swim spa and plunge pool collection and book a test dip at your local Sapphire Spas retailer in New Zealand. Give us a call at 0800 477 276 if you have questions, or go through our spa installation planning guide for more details.