How to select the right type of foundations for your sauna.
So right here. This blog post. Is actually where it all began.
I’ve not been totally honest with you up until now about the reasons for writing this blog. It was not (as I hoped it has appeared), a long well thought out plan where I came up with the idea of building a sauna and then started documenting the process for prosperity from day one.
Few good things in life are planned so easily and executed upon and this, is no exception. The actual process for me starting this blog, was this : I started building a sauna at my new house. I spent a long time researching different kinds of foundations to use. I eventually decided on one and started building it. Half way through building my foundations, in the car on the drive home – I came up with this AMAZING blog title “Foundations, foundations, foundations”.
“That would be a great title for a blog post!” I thought, “I could summarise all my learnings about foundations for anyone else about to embark on the same journey!” but one blog post on it’s own? That makes no sense, especially as my other blog is about technology and politics. So. I need a new blog. What should it be about? How about documenting the whole sauna building process?
And thus: https://buildyourownsauna.wordpress.com was born. It’s not pretty, but that’s actually how it came to be. Everything up until now, has been me going back over all the steps I took up until this point. It’s taken a lot of extra effort just to get to the point where I can write this one blog post – with a clever name, I had to construct a whole blog and write a whole lot of extra content. In fact I am now thinking that I can use this material as the base for a book (there are not many good sauna buildings books out there).
So anyway. Now I will begin. For real. On foundations.
There are several different types of foundations and there are not many websites that provide clear and straightforward guidelines on which to choose and where to get material and know-how to be able to build them, I will try below to summarise the different types and point out their pro’s and cons.
Slab/pad (concrete) foundations
This is probably the most typical recommendation you’ll get from a builder. It basically involves : digging a big square hole, putting down some hardcore (basically cheap rocks) and sharp sand (don’t worry it’s not made from Sean Bean) to level the area, then a moisture barrier and some insulating foam around it, creating some concrete forms around the edges (a wooden frame) and dumping a whole load of concrete on top. If you want to do the fancy version, you can add some bent steel bars into the big hole to give the concrete extra strength (I believe this is called rebar – in the trade). Here is a friendly Australian fellow explaining it in more detail :
Look’s easy right? It’s a bit harder than it looks and there are some potential drawbacks :
- It uses a lot of concrete. Concrete is the single most carbon intensive product in the world today, producing concrete creates 5% of global carbon emissions on the planet. So using it sparingly is probably not a bad idea, until we find a good less intensive replacement. So I know your sauna is important and all, but really – is it worth ruining the ecosphere for?
- You need to hire a compactor. Yeah, you can get them from diy rental places, but it’s a hassle, you need a stong car that you don’t mind ruining the boot of and you might accidentally flatten one of your feet.
- If you change your mind in a few years and want to get rid of the sauna, it’s a pain in the ass to move a concrete slab. Concrete is for life, not just for christmas.
- In this video he only digs a hole around 30 cm, which is probably fine in the land down under, but if you have a colder climate, then you need to go down at least 80cm to prevent frost heave (more on this later).
On the plus side,
- It’s very stable
- It’s easy to build on with other building materials (e.g. brick)
- Materials are generally easy to get hold of
- Most builders/ diy folk can learn to do it
Ok, these are probably the second most common foundation type your friendly ‘ello guvnor type builder will throw at you. Fairly straight forward premise, the name sort of gives it away. This one involves digging a trench (surprise surprise) round the outline of where you want your walls to go. Then, once you have the trench, you fill it up with concrete like a sort of builders version of a castle moat. Only in this moat, after 48 hours it turns solid as rock and you can lay a couple of courses of brick work on top. So best not to fall in it. Drawbacks of this style of foundation are :
- You still use a fair amount of concrete
- You have quite a lot of digging to do, although less than the pad
- you need to buy bricks
- You need a brickie to come along to create you small stable skirt of bricks for you to build on (or you try it yourself, this is harder than it looks)
- You still have to dig deep below the frost line to avoid the ground heave
plusses for this style :
- also very stable
- less concrete use than a pad
- less digging than a concrete pad
- plenty of tradesman about who will gladly put you one of these together
Ever been to Brighton Pier? – (the working one not the burned one that looks like it should be in the final scene of Planet of the apes.)
Seen how it’s held up basically by columns coming out of the sea? Well that’s a pier foundation, true to it’s name. The essential run down of pier foundations are, dig a hole deep enough and stick a big pole coming out of it. Repeat. You want to separate them at regular intervals to the same height – (usually a distance or around 1.5-2 meters is good enough to keep the load distributed).
The pole can be concrete, steel, wood, brick, whatever you fancy. Then when you’re done, drop your house frame on top of it.
Things to remember are to go at least 80cm deep (in Northern Europe), depending on your ground frost depth to avoid ground heave. Also dig a bit deeper and add some gravel to the bottom of the hole this serves to help you level the post but also allow water to run off down into the soil and not pool at the bottom of the post. And also If you are using concrete, you need a form and depending on the height of the piers – to add some rebar (over 50cm out of the ground I would recommend it). There are some great companies that do cardboard forms that you can remove after the concrete has dried. Or if you’re on a budget, nip down to your local hardware store and pick up some cheap wide diameter sewage pipes that also can be used as a form. Here’s a surely American explaining it all with moving pictures:
Drawbacks of Pier foundations are :
- You have to dig a deep hole, but only a few.
- You have to wait at least 24 hours for the concrete to dry.
- It’s hard to move piers if you want to move.
Positives are :
- Much less concrete use overall
- Very stable and strong structure
- Easy & can be done by one person
OK, so just to clear up any misconceptions. Screw foundations, have nothing whatsoever to do with fornication. They are infact called so because they use the principle of fastening posts into the ground using a helical ridge. So. Onwards to process. Take a giant metal screw, position it where you would like a foundation post and twist it. Repeat until it is at desired height. Simple. No concrete. No mess. The only tools you need are a big screw driver (or a screw attachment to a JCB). Job done. The big plus of screw foundations is when you’re finished with your building or if you want to move it. It’s a piece of cake. Unscrew and reuse at no extra cost.
- No wet trades
- Easily removable
Wow, you made it all the way down to Plinth foundations! I’m impressed! You must be serious about your sauna and it’s foundations. Well the great thing is our last stop on the foundation train, plinth foundations, might just be where it’s at in terms, of cost, installation effort, reusability, low carbon footprint. They’ve got it all.
A plinth foundation is s pyramid shaped concrete block, you simply dig a hole a 10 or so centimetres into the ground, put down a moisture barrier, drop in some gravel to bring it back to ground height (you may also need a plastic tray to keep the gravel to stop it spreading) and place your plinth. Hey presto, after laying one of these blocks every 50cm or so under your building, you have a sturdy base with which to position your subfloor. If you live in windy climbs, you may also have to ‘peg’ your building into the floor (that’s right I said, peg, like a tent) to stop if flying away in a strong gust.
Here’s another example of someone laying some plinth foundations (sorry for some reason again it’s an Australian!)
So is it really that easy, no downsides? Well, everything has a downside :
- As of writing, these are expensive! Expect each one to set you back at least 100 euro in europe but handi blocks seem to be much cheaper in the antipodes.
- Distribution – where can you get them? A company called swift claims to deliver within the EU but I have no direct experience.
- Building pegging is sometimes required.
But on the plus side you have :
- Least amount of work
- Least amount of experience required
- Totally removeable and reuseable once finished
So all in all, Plinths seem like a pretty wise choice for the casual diy’er.
There was a topic I mentioned a couple of times, that I said I would get back to. That topic is ground frost. If you live in northern europe, of the US or anywhere where the temperature drops down to 0 degrees centigrade and below, then you are going to have to take into account ground frost. You see the ground, it has water in it, sometimes in little pools, sometimes distributed in little drops, sometimes just randomly dotted around. This water, when it gets cold, it freezes. What do we know from pre GCSE physics? When water freezes it expands. So when the ground water freezes, that too expands, and it creates force. If your foundations are not anchored in lower that the level of ground to which the frost penetrates, then it will put force on your foundations. That can mean nothing, it can mean your whole concrete pad lifting out of the ground. It can mean after a few years of force, your pier foundations start popping out of the ground like someone hit the eject button. Trust me, it’s bad, so make sure you know the front line for your area (in Berlin here it’s about 80cm) and make sure your foundations go below it, or you use plinth foundations.
Well, that’s it on foundations. I hope that gives you enough information to make the best decision for your sauna and property location. Just incase you’re wondering, we went with self made pier foundations and so that will be our base for the rest of the blog. Good luck!