What You Need to Know About Sauna Rocks

I get questions all the time about sauna rocks. Whether a wood burning sauna stove or an electric sauna stove, sauna rocks play an important role.

What Kind of Sauna Rocks are Best?

Some swear by igneous rocks, or volcanic rocks. Though lava rocks have a high heat capacity, we find that they don’t hold thermal mass as well. Why is thermal mass so important? We find that an integral part of the sauna experience is the thermal heat, or dense heat, that is only created by heating mass (think heavy water heated radiators vs. tinny electric baseboards). There is a theory that sauna manufacturers sell and promote volcanic rocks for sauna stoves because they are lighter weight so more reasonably priced to ship.

I Am a Big Fan of Granite Rocks

Specifically, rounded aged granite rocks from the icy shores of Lake Superior. Rocks without cracks or crevices. There is a stigma and fear that sauna rocks can explode. Sure, if you’re using rocks that allow for water to get in cracks, they will explode! If you’re worked up about that, you can read about Hydrofraction. Bottom line: use non porous granite rocks on top of your sauna stove.

Take two proposed sauna rocks. Smash them together. If they don’t crack or break, chances are, you are holding two good sauna rocks. This is one way to ensure that the rocks you are using for your sauna are strong and stable.

What Size Sauna Rocks are Best?

Softball to golf ball. This size range works well. Why? Different size sauna rocks hold heat (thermal mass) differently and release steam (löyly) differently from water being tossed on hot sauna rocks. A nice hot wood burning sauna stove or electric sauna stove will heat your sauna rocks, then as water is tossed on the sauna rocks, the water turns to steam, and that process will temporarily cool your sauna rocks. Softball size sauna rocks hold more heat and will maintain thermal mass better. The golf ball size sauna rocks react quicker to the water, turning it to vapor but at the expense of thermal mass.

You want different size sauna rocks.

Why Sauna Rocks?

Consider that savusaunas, the earliest form of saunas, are “simply a room containing a pile of rocks, but without a chimney.” The rocks are heated by fire, with lots of wood and for a long time. Then, the fire goes out and all that heat is contained within the rocks. This same principle, heated rocks, is what differentiates a sauna from a lake. (here’s where we throw infrared light bulb closets under the bus).

Consider building your collection of sauna rocks as a journey, not a destination. Hiking, walking along the beach, snooping around your neighbors backyard are all times to have your granite sauna rock radar detector going. Oh, and how about how much water to toss on your sauna rocks?

How Much Water Should I Toss on the Sauna Rocks?

Just as Minnesota Fats, the famous pool player, would advise that there are two ways to hit a pool shot – soft and softer, there are two ways to toss water on sauna rocks: start with a little water, then add a little more. You can always toss more water as you go.

As Clint points out, all sauna stoves are made to take water. Rocks on sauna stoves are a thermal mass of heat energy. That heat is transferred to steam, as water gets tossed on rocks. That steam then gets transferred to your body as it comes in contact with your skin. You can toss a liter of water on the rocks and try to “ride it out” but this macho ploy is best reserved for the art of reverse cycling.

Steam from water being tossed on sauna rocks, Löyly, is a spiritual thing, involving negative ions. Fire (sauna stove), Earth (sauna rocks), and Water (via löyly) create an aura akin to waterfalls and rainbows, something beyond this writer’s ability to put into words without mind altering assistance.

Bonuses

Starting with a little water, then adding more water won’t shock your sauna stove, so in theory, your stove will suffer less fatigue. Rocks will be less depleted of thermal mass and respond quicker to more water with a smile and a sizzle.

Misnomer

The temperature in a sauna does not go up when you toss water on sauna rocks. It just feels hotter because heat is transferred via water vapor onto your skin.

7 Things You May Want to Know about Your Sauna Rocks

9/07/2017

Welcome Dick from Scandia, Minnesota. Dick works in Research and Development at the Andersen Corporation (Windows). He has worked in design, project management and materials research for 33 years.

You Can Collect Your Own “Rocks”

If you are still with me I know you probably have a wood burning sauna. Someone who would wonder about collecting their own sauna rocks would probably burn wood. There is a certain person who cuts wood and dries it, carefully splitting and stacking it in preparation for taking sauna- it’s the “Gestalt Principle”, in action! The preparation is just as important as the Sauna and when combined together give us an exhilarating experience. That same person is likely also to search out and find their own “Sauna Stones” enriching your experience further.

If you are wondering why I switched back and forth from rocks to stones I think of them as rocks when they are in the wild and they become “Stones” once they are placed in my sauna heater.

A Word of Caution!

Gathering stones for your sauna is not the same as Rock collecting. Everyone grabs a pretty agate from the restaurant landscaping or even at a city park. Taking an entire 5 gallon pail of rocks from public beaches, parks and private places is not alright. Take care and only collect rocks with permission or from land you own. You might want to try a local landscape company or nursery supply business. Mine lets me even sort through the pile and pick just the “Perfect” ones.

Importance of Stones

The stones deliver the steam. Stone size density, porosity, and surface all play a role in transferring heat from the fire to you. If we use too small of stone it doesn’t hold enough heat to last very long. Too large and it takes longer to heat up. A stone with a nice rough fracture is nice because it has a little more surface area to hold on to the water and gives off more heat. Some stones might stack too close together and prevent the free flow of heat and water through the pile. Too loose and too much water passes through too quickly.

I use stones about the size of my fist with a few larger and smaller to fit just right when I lay them on my stove. Note that folks with those new fangled electric stoves need to be careful about their stones because stone size and placement can shorten the life of the elements.

Black Rocks Matter

I had to shamelessly say it. Darker rocks give off heat quicker. It’s a physics thing. The “Finnish” stones for sale are all dark. Now that I’ve got that out of the way let’s move on.

Types of Stove Stones

I noticed some companies tend to say things like “the only correct stones”, “the only real sauna rock” or “the correct type”…These are written by “Ad men” and are simply trying to convince you that they have the very best rock for your money. Now be assured, proper sauna rocks are a safety concern. But any igneous, structurally sound rock will work in a sauna and you will be hard pressed to appreciate the differences unless you are 100% Finnish.

Without getting into a geology class let’s just say if you can scratch material off the rock with a nail, DO NOT USE IT!.

Sedimentary rocks (sandstone, shale, limestone, etc) and Metamorphic rocks (slate, marble, Quartzite, etc…) all are too soft, have faults in them or have moisture in them and should not be used. These rocks with moisture in them can “POP” when subject to hea

We want an igneous rock (Vulcanite, gabro, peridotite, Basalt, Olivine, Granite, etc…) Note I did not include Obsidian and Pumice, both bonafide igneous rocks but they have issues. Obsidian is so smooth that the water will run off so quickly it will not have time to evaporate much. Pumice is so light and porous that it will cool too quickly and not have enough mass to give you the sustained heat a sauna needs.

If you are having trouble identifying your rocks. Do an internet search for igneous rocks and that should help you. You can also search for a map of your area showing what the predominant rock type is in the area. Some folks might have to wait until vacation to find suitable rocks and depending on who’s with you. You might have to only bring a bucket full at a time home.

Loading of Sauna Rocks- Stone Can Be Dirty

Before loading your stones in the Rock Chamber, wash and rinse them to get off anything that might soil your floor or impart an off odor when heated. Next depending on your heater, start with the largest stones first and then use smaller and smaller, stack them in and around the Rock Chamber until it’s full and heaping. You do not want them so tight that air has a hard time getting through but you want them close enough that when you pour water on, it doesn’t run quickly off and doesn’t have a chance to turn to steam.

Rock Replacement

Some rocks will last longer than others. I’ve heard that some only last less than a year. Whoa! Those are not rocks we want to use. But after a while rocks will crack and get smaller and smaller. Usually the hottest rocks next to the fire pot degrade first. They are subject to more thermal expansion due to the heat / cold cycle. You might want to use room temperature water rather than cold water to limit this stress on the rocks and also use only a cup of water at a time to limit the stress also. It’s better to dump a cup or so each a few minutes apart than it is to dump 2 cups all at once.

When your rocks are cracked and getting small, some are ending up on the floor. It’s time to replace them. Simply remove them all and combine the old usable good ones with your new rocks. See #3 above.

The Spiritual Side

“A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral sauna.”

Where to Get Sauna Rocks

So, we are approaching the “Finnish line” of our authentic sauna build. We’ve followed along with every step of the way of the DIY ebook. We’ve cured our sauna stove with a good long burn, burning off the factory paint. We’ve put some finishing touches in place, like attaching our towel hooks and our hot room door hardware. We’ve checked the calendar and have now invited our friends over for the inaugural Friday sauna.

We’re all set to go, but wait! “I need sauna rocks!”

Well, a quick trip to the shores of Lake Superior is out of the question. And despite the metaphor of a million grains of sand, there’s a guilt that comes with dragging a 5 gallon pickle bucket out to the beach shoreline to collect free range organic sauna rocks.

What about railroad tracks? There’s a fringe line of thinking that “there’s asbestos from train brakes in dem der rocks.” And then we can get caught up with the idea that if everybody got their sauna rocks from train tracks, would anybody hear a train whistle blow if the train ran off the tracks?

What’s a morally conscious sauna enthusiast to do?

Landscape center!

Landscape center!

Landscape center!

For a few bucks, at a place like Landscape Love in South Minneapolis, a sauna enthusiast can bring a milk crate or 5 gallon sauna bucket, and hand pick through the pile of landscape rocks to get a great assortment (golf ball to softball size) of sauna rocks.

43 thoughts on “What You Need to Know About Sauna Rocks”

  1. I always equate Sauna with Fire and Ice.
    Game of Thrones – NO
    The Finnish winter war-MAYBE
    Coming out of the sauna to jump in a cold northern lake or roll in the snow-YES!!!!

  2. Hi Guys, at Glenn’s suggestion I’m posting this as a comment rather than a separate story.

    ******Be Careful when Scrounging Sauna Rocks – A Cautionary Tale******

    This is a grea post by Dick. Rocks are one of the least understood aspects of good sauna gear, and Dick’s article is really helpful. I definitely support sourcing your own rocks if you know what to look for, following Dick’s guidelines.

    Dick also mentioned cleaning, saying:

    “Before loading your stones in the Rock Chamber, wash and rinse them to get off anything that might soil your floor or impart an off Oder when heated.”

    But friends – this is a point I want to push further. Much further, in fact.

    Here’s a story I heard recently, which should stand as a lesson to all about the importance of sauna safety as a holistic concept.

    Last year, I met the head of one of Australia’s leading sauna companies. We spoke for a few hours about sauna culture and sauna technology, and at one point we started talking about rocks. We were standing next to a few ton of sauna rocks at the time (sitting on wooden pallets).

    I asked why he imported rocks – could he not source them locally? He explained that in the end it was much easier to from his supplier (one of the major Finnish sauna companies), due to the large quantity he traded in, as well as the need for consistent quality. And safety.

    He explained to me that several years ago, he wanted to see what other sauna suppliers in Australia were selling for rocks.

    Some were fine, but some of the rocks he received were clearly harvested locally, and were poorly selected for size.

    He went a step further by sending everything for chemical sampling. This is where things get scary.

    One day he got a call from the lab doing the test, and they said is was real lucky they were wearing safety suits, because one of the samples was covered in asbestos.

    Let me repeat that: ONE OF THE SUPPLIERS HAD PROVIDED ROCKS FOR USE IN THE SAUNA THAT WERE COVERED IN ASBESTOS.

    WTF? How did this absurd situation come about?

    Investigation revealed that it was a shonky dealer who was known for other dubious standards and even more dubious practices. It turns out that apparently, this idiot had been picking up rocks from the train tracks. I’m not sure what the rocks on train tracks look like in other parts of the world, but in Sydney, they generally look like they would be excellent sauna rocks – hard, dark, and not too large.

    See here for some examples:
    * http://bit.ly/2fevuke
    * http://bit.ly/2w4gHU4
    * http://bit.ly/2y1f8TS

    Looks great, right?

    But this idiot – this cheapskate, tight-ass, irresponsible idiot – decided he would use these rocks to sell with his other sauna gear.

    Why is this a problem? Because the old generation of trains had brake pads made of asbestos. So every time they creaked on the brakes (generally near a station in exactly the type of place someone would sneak on to), they would rain down asbestos dust onto the rocks.

    Imagine you haven’t heard this story, you find these rocks (maybe someone even sells them on ebay after never using them), but you don’t wash them properly, so instead of a beautiful cloud of löyly, you also get a big blast of asbestos cascading around the air.

    Imagine how many people he actually sold these rocks to.

    Honestly, it makes me shudder.

    Moral of the story – dirt is not the only problem. If you scavenge rocks, always wash them really carefully unless you are quarrying them yourself. You never know where they have been.

    Stay safe, saunafolk!

  3. Before reading this I had never thought about collecting my own rocks fearing that the rocks would crack or shatter, or, worse, that I could irreparably damage my sauna. Thank you for helping me get passed my fears!

  4. So here is a question – how many / how high should the Sauna rocks be in a wood heat stove – should they all be touching the metal, or should they be piled high?

  5. Peter:

    Too many rocks: the Loyly (steam from water being tossed on sauna rocks) can be compromised: not as “ahhhhhh.” Too few rocks: not enough thermal mass and too “spicy” loyly. I am a fan of golf ball to soft ball sized sauna rocks so the thermal mass and loyly are well blended. As far as how high and all that, I say just keep adding and taking saunas until you’ve found the right blend. I welcome others to chime in on this nuance. It’s worth the thought.

  6. I found someone who dug up granite boulders, they were free to take, however, some of the rocks crumbled when dropped; I thought granite was supposed to be hard, they clearly appear to be granite and I did read something about decomposing granite, so does that mean they are not safe to use in a sauna?

  7. Rosie: Not safe. You want to look for and use hard rocks with no fissures. Rocks that bounce when you drop them. Super dense.

  8. what you are saying is igneous rocks, not sedimentarhy. I get mine from a gravel pit a mile from where I grew up, and if unsure, knock them together. Bouncing may not work all the time. The other thing is you are not really supposed to take rocks from BWCAW, although it is done. Lots of good choices outside of area, and most people are happy to have you take them.

  9. Jack Tsonis
    Thank you for that article. I was wondering what the difference between train track rocks, basalt and vulcanite rocks were = they are none.

    However, now I’m equally concerned as to how much that shoddy dealer has sold, that’s still waiting in storage somewhere when I buy it from a supplier.

  10. Hello, is there a lot of Olivine stone in Minnesota and other northern states? It is by far the best stone for smoke saunas.
    If you have a lot of it around let me know!

    A new portable smoke sauna (stove) is coming to Minnesota next year! (prototype is almost ready)

    It will bring happiness to all for those who enter in….

  11. Hi there
    I am based in Dublin and myself and my friends go sea swimming a lot. It has just started to get cold. In one of our sea swimming places we have discovered two abandoned fishermen huts. They are basically stacked rocks to shelter from the wind and rain.
    To get to them you have to do a bit of cross country walking and transporting materials is not that easy. they are about 4mtr x 6 mtr.
    We want to turn them into saunas what advice would you have.

  12. Gareth: My advice is to get it going. Unlike a canoe trip, you’ll only have to transport your materials in one direction, so remember that as your trudging it along.

  13. A roof. You always need a roof. And fuel, so a wood shed. Fuel nearby or it makes no sense. You can get by without a metal stove if you have a smoke hole, and so on. Rock walls will be hard to heat. Good luck!

  14. Rose. I think you need a bigger hammer.

    Bigger rocks and bigger fire.

    It’s all about thermal mass. As you know, in Native American sweat lodge and Temescal traditions, rocks – often big rocks – are heated in a fire then brought into the structure, then door closed and ceremony begins. But these rocks need to have a lot of thermal mass, and these rocks need to heat for a long time.

    Can’t vouch for the safety question. You’ll want to be using hard solid granite rocks with no cracks or fissures, for sure. Good luck! More on Temescal here

    We built a sweat lodge structure in the yard. Tried some alternatives to heating the rocks by fire. We dumpster dove some granite and broke them down to small pieces. We fired up a small hibachi grill, laid the stones in and cooked them for about an hour and a half. While it gave off good steam, it only lasted about 7 minutes total. Did we get the rocks hot enough? Did we use enough rocks? Maybe we should have let the rocks heat the tent before we splash water? Probably trail and error? My husband thinks we could use the house oven instead of an outdoor grill. Safe?

  15. I have a question I see that some gyms use stones that look like Rain Forest Gray Large Mexican Beach Pebbles, and I wanted to know when they turn black or look burnt and mark up the walls is that suppose to happen? Or should they be replaced?

  16. Serious sauna folks in Finland believe that sauna rocks should be replaced regularly, like every 100 saunas. I can’t get my arms around that. I am replacing my sauna rocks, though, after a few years. Hope to research and write more about sauna rocks. Those stones you talk about in gyms, i’m not familiar with those kind of stones.

  17. Hey Glenn

    Thanks for all the great info! What about quantity of rocks? I’m wood stove shopping and some stoves built for the same size sauna have a wide range of rock capacity, for example Timblerline at 25 lbs and the Harvia M3 at 90 lbs. Any suggestions?

    Thanks!

  18. Trent: Very good question! The larger the capacity for rock, the better the loyly and overall heat. Searching “thermal mass” on saunatimes will advise further.

  19. Hi Glenn
    Looking to buy a Narvi AK57.
    What about the rocks to put inside?
    Ceramic?
    River stone from our site? ( Nicely rounded)
    What about the temp they will hopefully reach.
    Confused about which safety wise way to go!
    Like the idea of our site rocks, btw in unlimited supply! Is idealistic.
    Great book, finally getting all the pieces in place, lead time for the stove is the start time.
    Thanks for a great informative site.
    John

  20. Hi John:

    Glad the book is helping you. As you know, I wrote it for people just like you and me: amateurs committed to building really good saunas.

    AK57: Fabulous. Did you hear my Sauna Talk podcast episode with Jessi from Narvi, by chance?

    Rocks: Your email tells me that you’re in the UK. I’m familiar with your superior beer and music, but not with your rocks. If you’re collecting rocks from around your site, a rule of thumb is to see how they react to a good wack from a hammer. If solid, you’re in good shape. Now the Finns may take issue with diy rock collecting, as the “sauna pros” in the Mother land are hell bent on buying rocks from a box, and replacing them every 100 saunas. Either they are smarter than we are, or they have been (over) marketed to from the active sauna rock industry in Finland.

    The ceramic stones: same story. I submit to the fact that they may hold more heat, yet it reminds me of 30 years ago, telling my Uncle about wanting to sell bottled water and he thought I was crazy. Now he tells the story at family gatherings.

    So, like good capitalists, saunatimes is starting to sell sauna stones, and we’re going to start selling bottled “sauna air.” (true on the stones, joking about the air).

    Wishing you good heat, John, and send along a couple pics and stay in tune. Great project you have going!

  21. Glenn; Great info for new sauna user.
    We bought a house with a electric Finnish sauna. Seems to work fine but the rocks do not generate any steam when water is put on them with a ladle. The temp was 180 degrees.
    Are the rocks worn out? Can they be? Should I replace?
    It seems a pretty simple process to generate steam over the rocks…but mine are “dead”
    Also..the water falls onto the floor under the heater…is there suppose to be a water catch basin? or is the presumption that the water evaporates so fast that it is unnecessary?
    Any help appreciated. Thanks

    PS We live in AZ are of course have a very dry climate.

  22. Hi Katie: Well, it’s great that you have a Finnish sauna, and yet totally weird how your rocks aren’t producing steam. I have experienced electro saunas the ship with lava rocks (probably because they are light) but those kind of rocks are lame. I am a tad perplexed, however, how your hot room can get up to 180f but those rocks don’t emit any steam. A crazy thing.

    Yes, for sure, replace your rocks. I have really good sauna rocks i’m going to start selling, but don’t have them built yet on the shopping cart. If you want to email me, glenn@saunatimes.com, I can help you out.

  23. Sounds like something wrong with Katie’s thermometer or thermostat. I’d bring in a different thermometer and see if it reads 180. My guess is that the room is not really 180F. Maybe the rocks are, though, and that explains no steam

  24. She may be confused by the term “steam”. Sauna loyly, as we know, does not produce steam like a steam room. On the upper bench in my sauna you feel the loyly but you don’t see it!

  25. Dan:

    Stone walls in sauna offer a great attribute: lämpömassa. Yet the price for this is heat up time. (stone being a conductor of heat).

    So, there’s an optimal mix: not too much stone in our saunas, but enough, especially in the surround, for safety (distance to non combustibles) and heat retention.

    I’m not sure about limestone specifically as a material to hold up to heat. I’d suggest contacting a stone company, like kitchen counter guys. Please let us know how you make out. Walls behind a quality sauna stove can get to 280 – 300f.

  26. I understand this article is regarding actual rocks to be placed in the heater..but what about stone on the walls of a sauna? I am thinking about using limestone for two of the walls of the sauna (5×5 sheets bust down to 84″ total height) and then the remainder of the walls and ceiling will be cedar. Any thoughts? I’m just wondering if the limestone can handle the heat up cool down + water, etc… thanks in advance!

  27. Hello fello Juomala worshippers!

    I am currently building a sauna.
    Its going to be a classic smokesauna. Somewhat unorthodox ofcourse. no budget basicaly.
    Im thinking about building the stonemagasine of old gravestones made out of granit.
    maybe around the fire and topp it off whit smaler mor ordinary rocks for throwing the laulu on.

    would love to hear your thoughts.
    sorry for spelling im only a scandinavian =)

  28. Tough to answer without seeing them, Carlos. Mainly you are looking for hard rocks with no fissures or cracks of any kind, as the article describes.

  29. A little different use here. I am making a Solar heater and want to fill the open void areas in the solar box with Sauna rocks to hold heat longer after the sun starts to settle. What type rocks would hold the heat the longer? Igneous, Lava, Granite, Etc.? And what size would you recommend? The void areas will be approximately 6 foot long, 18 in wide, and 6 in deep.

    Thank You!

  30. Hi John:

    The Motherland (Finland) specs olivine diabase for sauna use. For your application, even though you aren’t going to be dousing your stones with water, you want to source round stones with no cracks or fissures.

  31. Hi, I am building my sauna. I found second hand Helo heater, which suits the size of the sauna. However it holds about 30 pounds of rocks maximum. I am wondering if it would be worthwhile to add the wire net with more rocks and mount it on the wall above the heater? Thank you. Igor, Montreal.

  32. Hi Igor:

    You can add a wire mesh atop a stove that holds a small amount of rock, but it may be like trying to haul an Airstream trailer with a moped.

    Your heater oomph may not be powerful enough to be able to support the added rock density. And these additional rocks may never get hot enough to create “aaaaaaahhh” steam.

  33. Hello,
    I have a Finnish electric sauna heater with original stones that I have been using for 10 years. Seems to work ok – good steam & good temperature. The stones by now have a lot of mineral deposits on them from the hard water that we have here. Should I worry about that?

  34. Railroad Civil Engineer/Surveyor here. Asbestos? meh, unlikely. Roundup? definitely. Federal Felony Theft and Trespassing? definitely. Best to stay off the right-of-way unless they want you there.

  35. Derek:

    I’ve used and continue to use lots of “rounded granite” for sauna stove application. This is the core essence of the revered Lake Superior rock that we sauna nuts in Minnesota aspire to – golf ball to softball size. Instead of seasoning the rock in an open campfire, i’d be thinking of adding them to your sauna stove and get it rocking for a couple few hours. Then, while wearing military gear and shields, douse some water on the rocks, with the hot room door open and ready to high tail it out of there if you here a “pop.”

    As this article details, the key with good sauna stones is very hard rock, and no fissures or cracks, as you know.

  36. Hello, I have purchased 3″-6″ rounded granite from my local landscaper. With granite, do I need to ‘season’ the rock by putting it in an open campfire to ensure there isn’t internal moisture that could cause a safety hazard during first use? I am thinking not, but worth an ask.

  37. Can you use petoskey stones on a sauna stove? Very beautiful and from the shores of Lake Michigan. If you don’t know what they are google them and you will see plenty of images.

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