What you need to know about sauna rocks but didn’t know who to ask.

We at saunatimes get questions about sauna rocks.  Until a geologist or thermal heat inspector checks in, we’re going to wing a few points for review.  Whether a wood burning sauna stove or an electric sauna stove, sauna rocks play an important role.

What size sauna rocks? Softball to golf ball.  This size range works well.  Why?  Different size sauna rocks hold heat (thermal mass) differently and release steam (loyly) 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.

What kind of sauna rocks? 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.  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 baseboard).  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.  (What did you get for Christmas?  “A box of rocks!”).

We are 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 surely 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.

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 fake.  (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?

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13 thoughts on “What you need to know about sauna rocks but didn’t know who to ask.”

  1. 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?

  2. 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.

  3. 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?

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

  5. 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.

  6. 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?

  7. 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.

  8. 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?


  9. 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.

  10. 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 =)

  11. 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!

  12. 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.

  13. 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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