If the sauna heater is like a car’s engine, you could say that sauna rocks are like car engine oil. The stones must be changed regularly or the engine will eventually stop working. The sauna stones will eventually break down under the heat load. Changing the sauna rocks is therefore part of the normal sauna maintenance.
You don’t necessarily have to change the stones every year, but you should inspect the sauna rocks, stack the rocks properly, and replace any broken or cracking stones.
This guide will answer all the questions you may have about sauna rocks including, which kind of rocks are the best, and how to stack them properly. Let’s get started!
Why Are There Rocks in a Sauna?
Let’s start with the basics: why are there rocks in the sauna, anyway?
Rocks are used in a sauna to store and release heat.
When the rocks are heated by the sauna heater, they absorb and retain heat. When water is thrown onto the stones, it releases steam which increases the heat and humidity in the sauna. For the sauna rocks to do their job properly, you need to preheat the sauna for 15 to 45 minutes depending on the heater so that the stones have enough time to absorb heat.
In traditional Finnish saunas, the rocks are heated by a wood-burning stove, while in modern saunas, they may be heated by an electric heater. The use of rocks in a sauna dates back to the earliest saunas, which were built by heating rocks in a fireplace and then transferring them to a room with a bench for people to sit on. The rocks were then splashed with water to create steam, which helped to warm the air and increase the humidity in the sauna.
Infrared saunas don’t use rocks at all.
How Many Sauna Rocks Are Needed?
The most important feature of sauna stones is quantity. The amount of rocks is measured in weight and is indicative.
The number of stones must be in proportion to the size of the sauna. The basic idea is that the more spacious the sauna, the more stones you need.
The number of rocks needed for your sauna may vary depending on the size and type of the heater.
Most sauna heaters need between 30 to 90 lbs (15-40kg) of rocks. But some special types of heaters or very large heaters may need several hundred pounds of stones.
Here’s how you can find the weight of the sauna rocks needed:
- Follow the instructions in the manufacturer’s instructions. This is the information you should follow, if you have it available.
- You may find the information also online or from online stores selling the type of heater you have.
- You can take out the old rocks, weigh them, and use them as your guideline.
- Lastly, if none of the above options work, you can search images online and use your common sense. The rocks should be piled slightly higher than the top of the heater.
See the example image below for the number of rocks on a Finnish wood-burning Harvia sauna heater. In the picture, there are about 55 lbs (25kg) of stones.
What Type of Rocks Work Best for a Sauna?
Historically, sauna stones have been dug up from the bottom of the sea and rivers in Finland. Nowadays, this cannot really be recommended.
You may have a lot of stones lying around in your backyard, and it might be tempting to just use whatever stones you find. Sooner or later you will have to replace them with industrially mined ones due to cracking or poor heat storage capacity and thermal conductivity. Some stones can also cause odor problems.
Considering the price of sauna rocks, they are not a big investment in proportion to their importance. (later in this article I will tell you where to buy sauna rocks)
Quality of the Rocks Matters
The quality and size of the stones also matter. Stones have a direct effect on the quality of the sauna experience, which means they must be chosen according to the desired properties of the stove. The quality factors of sauna stones are:
- Heat resistance
- Thermal conductivity
- Heat storage capacity
The most obvious of these is heat resistance. At the end of the day, all stones will crack and corrode, but durable – e.g. the most heat-resistant – stones will last significantly longer.
Best Types of Sauna Rocks
The most important properties of sauna stones are design, thermal conductivity, heat resistance, and size.
The stones are a visible feature in your sauna, so they must fit into the interior of the sauna. Options for appearance are limited but jagged, rounded, and near-round stones are available in black, gray, and white color.
The best type of stones for home saunas
The most common available sauna stone types are:
- Diabase, also known as dolerite
- Lava rocks
Olivine diabase is one of the most popular, inexpensive basic stones with the best properties. Olivine is heavier and has higher heat storage capacity, but it costs a lot more.
Here’s what olivine-diabase rocks look like.
White decorative stones
White, calcareous stones are more porous and release moisture more calmly.
White stones are suitable only for decoration as the top layer. You need to still add diabase, lava rocks, or other suitable rocks as the base layer.
Where to Buy Sauna Rocks?
Sauna rocks are available in hardware stores, such as Home Depot. You can also order sauna stones online from sauna manufacturers, specialty sauna stores, and well-known online stores like Amazon.
Stacking the Sauna Stones Correctly Makes a Big Difference
When you buy new sauna rocks, you should first rinse them with water before stacking them on the sauna heater. This gets rid of any dust and dirt that may cause a bad smell or smoke when heated.
?Tip: Wear gloves when handling the stones to protect your hand from the sharp edges!
Here are some general guidelines for stacking the rocks in the sauna heater:
- If you are using old rocks, inspect them for cracks and discard stones that are crumbling. Read more here about inspecting old sauna rocks.
- Rinse new and old rocks before installing them into the heater.
- Generally, an electric sauna heater should be filled quite tightly with slightly smaller rocks. A wood-burning stove can be filled with a bit bigger rocks leaving more room in between them.
- Stack the bigger rocks at the bottom of the heater and the smaller rocks on top. Place flat stones in a vertical position.
- If you are using decorative stones, add those last at the top.
- The surface rocks can also be more porous or friable, in which case they have a larger surface area for water vaporization releasing more steam.
- In rare cases, new rocks may produce unpleasant odors when heated for the first time, so turn on the sauna after changing the stones to see if they put out a bad smell or smoke. Let the sauna run for an hour to get rid of the smell.
Video Guides about Stacking the Sauna Stones
Watch these videos from a Finnish sauna manufacturer Narvi for correctly stacking the rocks in an electric and wood-burning sauna stove.
Do You Need to Replace Sauna Rocks?
If you notice crumbled rocks under your sauna heater or a decrease in heat and steam, it’s time to inspect your sauna stones.
These stones should be inspected annually and any broken or fragile stones should be replaced.
To do this, remove the cold stones from the stove and place them in a large container. Then, take another container and go through the stones one by one, checking them by gently tapping them together and removing any that are crumbling or cracked.
After removing the old stones, rinse them out to remove any stone dust and place them on the heater, leaving gaps between them so that air and water can flow through. The stones should completely cover the heating elements, but avoid creating a pile that is too high.
When disposing of old sauna stones, do not simply throw them into nature. Individual stones can be disposed of with mixed waste, while a large number of stones should be taken to a recycling station. Old stove stones can also be repurposed in a flower or herb bed or for other decorative purposes in your garden.
Did you find this article helpful? If you have more questions about sauna rocks, I’m happy to answer them in the comments below!