Building a fireplace in a treehouse
Add warmth and comfort to your treehouse with a small wood-burning fireplace and hearth.
Won't the treehouse burn down?
Not if you build carefully and pay attention to common sense safety issues. Having a fireplace in a treehouse is like having one in any normal house. There will always be a risk of sparks flying out of an unwatched, unguarded fireplace. Keep your fire attended and use a metal mesh guard to keep sparks under control. Keep a container of water nearby at all times just in case. A fire in a treehouse could be as dangerous as one on a ship - there are very few places you can escape to.
My fireplace
I installed a fully functional fireplace in my treehouse. It was originally just for novelty value, but it's actually great when the air gets a bit chilly later in the day, and it makes heat out of all the off-cuts that I otherwise have to carry home to get rid of them. At times I have been able to use it, because there were some squirrels living in the chimney box, then some birds. The chimney was accessible by a small opening from the outside.
Building the fireplace
There were a number of problems I experienced with getting a fireplace to work in a treehouse. Firstly, my original ideas had the fireplace itself inside the treehouse, with a chimney carrying the smoke out via a few 90 degree turns. This failed for two reasons; the chimney was too small and there wasn't enough updraft so smoke billowed into the treehouse and choked me out.
It was also hard to seal the fireplace so no smoke leaked out before entering the chimney. The whole system also looked pretty messy, sitting in the middle of my (limited) floor space. I solved all these problems by moving the whole system outside the treehouse. I built a special platform bolted onto a main support (with extra bracing for the weight of bricks and concrete). On top of this I built the main surround from bricks and concrete. The concrete was carried up in bags and mixed in the treehouse. This was very dusty, and everything got covered in wet cement. I suggest you mix your concrete in a large bucket on the ground. You will probably also need a lot more concrete than it looks, so be warned!
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The chimney I got was a proper gas boiler chimney, 1.2m in length. It fixes onto surfaces with brackets that hold it about 20cm from the fixing surface. This suited me fine because the chimney is placed nicely over the centre of the burning area. When the chimney was fixed, I wrapped thin metal mesh around the base to form a funnel shape to guide the rising smoke up the chimney efficiently. The mesh was basically a guide for the concrete that was to form the actual funnel. I used wooden 'forms' to contain the concrete in the areas I wanted it. I built the funnel in two layers to prevent the mesh collapsing under the weight. The finished mass was about 7-10cm thick and formed a smokeproof seal between the bricks and the chimney.
It turned out that this basic design was not quite right, and I had to add an extra piece of metal to stop the smoke flooding into the room. This gave the effect of lowering the funnel, giving it a higher capacity to hold smoke as it was drawn up. Finally, concrete was built up round my wooden fireplace surround, to form a smoothly integrated look. A touch of paint and the whole thing looked the part.
Mixing concrete
Concrete for a fireplace doesn't need to be particularly strong, but it does need to resist heat. I made mine without any additives or special cement, and quite a lot of small cracks developed because of the heat. This isn't too serious but you're better doing it right! So you will need cement, sand and water. Fine aggregate (concretese for 'small stones') is good but not essential.
Composition of concrete
- 3 parts sand (or sand/aggregate mix)
- 3 parts cement
- 1 part water
Concrete works best if mixed in a 3:1 ratio with water, but in practice you will probably need more water to obtain a workable mix. Start with 3:1 and if necessary increase it, a little at a time. Concrete hardening is a chemical process so only a certain amount of water is essential. Excess water has to evaporate out of the set concrete, leaving tiny voids in the material which reduce strength. Again, this is more important in a more life-critical situation, but you want this concrete to be the best it can, don't you!
This mix will harden in a couple of hours in a warm environment. You can remove the forms but it'll be pretty crumbly for a while longer. Now is a good time to score the surface with patterns, the year, etc. It takes around 24 hours to properly set. Colder/wetter environments will take longer.
Improvements on my design
Better supports to take weight
Concrete and bricks are very heavy and should only be used where there is ample support for them. I would estimate my fireplace at about 80kg (the weight of an adult), so bear this in mind when planning supports. It is essential that the supports don't move after they are fitted, as the bricks will be all skewed and may develop cracks. I recommend fitting supports onto existing treehouse supports so the fireplace moves (in wind) the same way as the wall you're trying to fit it into.
Firebricks and fireproof cement
As mentioned above, these will stop gaps and cracks appearing. In theory, ordinary bricks may explode if heated while wet, so firebricks are a good bet. If you want to risk it, ensure all concrete and bricks have had at least 2 weeks of dry conditions before lighting your first fire.