The best base for our sauna building: a pour, some piers, or just a bunch of gravel?

In my ebook Sauna Build Start to Finnish, I detail the pros and cons of different options for UNDER your sauna building.

Building a sauna in the basement or inside an existing structure?  None of this applies.

Building a sauna as a free standing structure at the cabin or in the backyard?  All of this applies.

Let’s digest the three different options for the base of your sauna building.

OPTION 1: The cement pour

Pouring a slab base is the way to go when building a garage.  Most cement pours include a thicker perimeter, up to one feet.  This is important as the load from the garage will be carried along the perimeter of the garage.  The cement pour for our backyard garage (including sauna) was under poured.  The contractor who built our garage sub contracted for the cement pour and it was lame.  If I were to do it again, i’d insist on a thicker perimeter, rebar throughout, and fiber cement.

But back to a pour for your sauna.  This is the most expensive and involves the most product.  We will need to get serious equipment on site to excavate the area, ideally lay out a bed of gravel for drainage and a base, then call in the cement contractor for pouring and leveling.  If choosing this option, the clever adaptation for a sauna drain is a “French Drain” system.  One can hand dig, and lay in white drainage pipe either as a “T”, below frost line ideally, or rent a post hole digger and go down 4′, backfilling with drainage tile and rock.

Building inspectors get quite worked up about the concept of a French Drain.  Especially in Minneapolis, MN where order and rules like to be administered.  But drains in garages are generally not a good thing.  Meatheads pour motor oil down these drains.  This is why laws like this are in place.

Advantage of a slab is that there is no way rodents are going to burrow underneath and come up and say hi.

The disadvantage of a cement pour for our saunas is that, in addition to having to deal with your hot room drain, we will have to account for sloping our slab to the drain.  A cement contractor can do this, but it must be done right.  If building inspectors won’t allow a drain in your secondary structure, you’ll have to get creative about a little trick of illegally laying out your drain JUST below your pour line, mark the location well, slip an extra couple $20 bills to your cement contractor to do the pour, get inspection approved, come back and tap out and lay down your out of code drain.  Sounds janky, doesn’t it?  (i’ve done it!).

The other disadvantage of a cement pour for our saunas is that this cement slab is our winter iceberg.  Yes, we will be laying duck board on top of our pour in our hot room, but underneath is a cold slab.  Not really a big deal but just saying.

OPTION 2: Pads

Digging and setting pads is a reasonable option if you want to get your structure level and above ground.  Allowing for air flow under your building and having good drainage off your roof is about the best way to avoid water damage and rotting, etc.

OPTION 2 A is renting a post hole digger, laying and pouring sonnet tubes.  This is brutal work, best done with a friend who lettered in football or wrestling, and can be bought cheap with a couple beers.  With this option, your sauna building will (in theory) not heave or settle, something quite settling to know.

OPTION 2 B is hand digging down below the topsoil, and laying down cinder block, or what I prefer, concrete deck piers.  I love these piers as they are dense.  They are designed as inverted trapezoids, so when we dig and backfill these piers, they lock into the ground.  Concrete deck piers have a profile built in to accept a 4×4 post or 2x framing.  I am a big fan of concrete deck piers.

OPTION 3: Gravel pour

If building a sauna in an area with good drainage, and fairly level, often the easiest way to create a base for our saunas is to simply pull off the top layer of topsoil and get a load of class 5 gravel delivered, shoveling and raking it around with a 4′ level.  If choosing this option, I recommend extending the class 5 gravel beyond the perimeter of your building, for good drainage.  Further, we want to be sure to have our gravel bed above grade a couple inches.  You will need more gravel than you think you’ll need.  The good news is that it’s not too expensive.  You can think about trying to save some money by loading the trunk of your car with class 5, instead of getting a load delivered, but don’t think too hard about this.  You’ll drive yourself crazy doing this.  You need more class 5 gravel than you think you’ll need.

What do I recommend?

It depends on your set up, but generally, I most prefer OPTION 2 B.

Where to start?  A roll of carpenter string.  Prep the area.  Go to town.