Thursday, June 9, 2011

Building an Ecological Treehouse Hotel: Cabanes als Arbres


Cabanes als Arbres is a family-owned hotel, located way up in the foothills of the Catalan Pyrenees, near the town of Sant Hilary Sacalm. Up there the mountains are covered with thick forests, especially chestnut and firs, and inhabited by wild boar. From the ridges and high places, you can look out over green valleys far below, and to the north the well-known mountain Montseny is always visible, among the dark peaks of the Pyrenees.

In 2010, I had the privilege of working on a team of 4 carpenters up there in the mountains, building ecological treehouses for Cabanes als Arbres. Each treehouse was designed to be a single large hotel room, complete with king sized bed, wash basin, compost toilet, couches, table and chairs, and a large balcony with a great mountain view.

When building a treehouse, the tree you choose is very important. This is the foundation of the house. If you choose a weak tree, and it blows around in the wind, the treehouse becomes like a boat in the sea, rocking about and upsetting the inhabitants. We would carefully select our trees to ensure we would have a strong, straight base for the treehouse – and an excellent view, of course!

The way that the tree and the house interact is also very important. Two big topics to think about are: how is the frame of the treehouse connected to the tree, and how is the roofline interacting with the tree.

When framing the treehouse, you need to allow the tree some space to grow. By framing tight up against the trunk, you're going to create a conflict between the treehouse and the tree – and one of them is going to get damaged. A good couple of inches needs to be left between the frame and the trunk.

The next step is attaching the frame to the trunk. The life of the tree is focused in the outermost layer of the trunk, so even small, superficial punctures into the trunk are going to affect the tree, causing its sap to flow out. Instead of causing many small punctures, the key to a good connection is to use a minimal number of large bolts, bored straight through the unliving wood in the center of the trunk. In the next photos, you can see the large galvanized brackets that are mounted in the center of the treehouse's frame. These brackets are connected with a single large bolt that passes through the trunk of the tree. This hole was sealed with silicone to prevent the tree from dripping out its sap while it healed afterwards. A second large bolt passes through the tree above the treehouse, and four cables connect this bolt to the four principle corners of the frame. These two holes are the only holes we put in the tree.

The other major consideration is how the roof interacts with the tree. There needs to be a space so that the tree can grow, and also to allow the tree to bend in the wind. Treehouses that are built with the roof framed tight against the tree can be heard complaining constantly through windstorms, as the frame shudders and pops under the strain of the moving tree. But on the other hand, if a space is left against the tree, then rain can come right in. At Cabanes als Arbres, we handled this part of the design with two features: first, the tree passes through the peak of the roof, so that the roofline is always sloping away from the tree. Second, a space is left around the trunk while building the roof, and afterwards a small tent of wood or canvas was erected, tight against the tree. The tent would shed rainwater while still allowing the trunk room for movement. 

The treehouses at Cabanes als Arbres were framed with fir and pine, and sided with chestnut. The roof is done with chestnut shakes. Chestnut grows wild in the forests of northern Catalunya, and was traditionally used for roofs because it's naturally water and rot resistant. This is due to the high level of tannins in the wood, like its cousin, the oak.

The ceiling and walls were insulated with batts of wood fiber, an environmentally friendly wood-based insulation alternative. Then the floor, ceiling and walls were finished with pine tongue and groove. We added built-in features like the dry compost toilet, shelves, wash basin, and banister. The windows and ladders were made for us by a nearby craftsman.

Bridges and staircases were fun extras, too. We would always be excited when we found a tree that we could build a nice long bridge for!

Working at Cabanes als Arbres was a real pleasure for me. The design and building of treehouses is a true art, and it's not very common to find a forest that's home to so many impressive specimens. From the high level of detail in the plans, to the selection of all-natural, high-quality building materials, the treehouses at Cabanes als Arbres were probably as much fun to build as they are fun to stay in.

- Ben

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