When water gets into the wood of a treehouse, it gets trapped by poor ventilation and cool temperatures. Wood will survive getting a soaking if it can dry out in a breeze or sunshine, but mould and rot will set in very quickly if the wood stays damp. This happens in shaded areas underneath the treehouse and even inside the house if water is allowed to soak into the floor and walls. Obviously rot is incredibly bad news if it attacks the main supports of your treehouse and it can go unnoticed for a long time.
Keeping as much water as possible off your treehouse is therefore a very good idea. An unheated treehouse will benefit from a flow of air, provided by some form of ventilation. Ventilation is the enemy of the damp conditions that result in mould and rot infestations. Four major areas of attention are treated in different ways to protect them from the weather. In order of importance they are the roof, supports, walls and parts of the tree passing through the structure.
The roof's primary role is of protection and so it is a good place to get right. The ideal roof is impervious to water but also needs to rapidly shed water so there is no chance of pools or damp patches building up. You should never use a flat roof for a treehouse. If your design calls for a flat roof, try to introduce even a small slope of 3-5° so water cannot sit there. A properly sloped (or 'pitched') roof is much superior. Aim for a slope of at least 30°, but preferably 45° or more. If you get particularly harsh winters, a steep pitch is critical to avoid a build-up of snow which could easily spell disaster for a treehouse of limited support.
A good roof protects the whole of the treehouse beneath it. It diverts any rain away so the walls, floor and supports will stay dry in all but storm conditions. For this reason you need to allow a generous overhang on all sides of a roof. The more the better, but 12-18" will be of great benefit in most treehouses. This allows the rain to run off well away from the walls. If you have a large deck, use guttering to keep run-off water away from the floor.
The following are the main three types of roofing materials used in treehouse building.
Rectangular strips of wood, usually cedar, which are used extensively in some parts of the US and look very good on a treehouse. Don't go for less than a 40° pitch with shingles and learn how to fit them correctly before you begin. Shingles are designed to get wet in the rain, and will swell as they do and shrink as they dry. Allow gaps between each shingle to compensate for this effect, and do not use any backing material under the shingles. This would cause a moisture trap, leading quickly to rot.
An excellent roof covering. It is very waterproof, durable and lightweight in comparison with wooden roofs. Usually corrugated, but different profiles are available. Pay particular attention to joins in adjacent sheets, and follow the manufacturer's recommendation about overlaps. It is very waterproof, durable and lightweight in comparison with wooden roofs.
Can be used as a roofing base, with a covering of felt, or tarred paper. While being cheap and good for awkward spaces, this solution is not as hard wearing as shingles or metal. When covering with any waterproof sheet material, start at the bottom with a strip, then the next layer with a good overlap, all the way to the top. A 'cap' layer should go over the apex of the roof, covering the tops of the sheets on either side of the ridge. Use as few nails as possible because the punctures let water in. Seal over nails with thick paint or glued plastic patches.
With a good roof overhead, the supports shouldn't get much water coming their way. However, most supports are quite wide and extend further than the overhang of the roof. If possible adjust your design to prevent this happening, or use guttering to divert the rainwater. To help keep supports from developing rot, you need to thoroughly treat them with wood preserver before construction begins. Wood preserver must be applied correctly to work effectively. All your supports should be cut to their final lengths and allowed to stand in a bucket of preserver overnight. The preserver will be drawn up through the pores of the wood. Liberally paint preserver over the flat surfaces of the wood and allow to dry out in the sun before assembly. You need two or three days for the full process of soak, paint and dry.
If you have an uncovered deck it will get wet, of course. As mentioned above, well ventilated wood will dry quicker and therefore last longer. Build the deck from slats that can be spaced ½" apart to aid air flow and treat all the wood before installation with wood preserver. Another danger with decks is the risk of water running off them and under the main walls of the house. Avoid this by having a small gap all the way round any adjoining walls, or extend the roof to protect more of the deck from falling water.
It's usually not easy to incorporate a drainage slope in a floor, but this is another way to direct water away from the walls. At the design phase you will need to factor in the space needed for the house (which will need to be flat and level) and an adjoining area of sloping deck. A slope of around 1 in 12 (5°) will help drainage and still be level enough to stand on safely.
Keep a check on the state of the internal floor of the treehouse. Coverings like carpet can hide damp and even make it worse by stopping the wood from drying quickly. Coverings should only be used where you are sure a floor is always going to be dry.
The roof should keep the majority of rain off the walls. There is not a very high risk of rot in walls because they drain quickly and usually have good ventilation. Problem areas are around the base of the wall and around window and door frames, both of which can allow water to accumulate. If possible, design the walls so they drip water out past the supports underneath. Water draining onto supports is a bad thing. You can achieve this drainage using a thick plastic skirt fixed behind the wall and flaring out over any supports at the bottom to guide rain away from the wood. Don't forget that any nails or screws used will create puncture points that can let in water, so allow the skirt to hang down without using fixtures.
It is advisable to allow some air to flow through the walls to keep the inside of the treehouse well ventilated and remove humid air. Try not to compromise security, but a system of small gaps or holes can work well. Inch-wide holes drilled at intervals near the top of each wall and faced with mesh to keep out insects will help a lot to keep internal humidity levels down.
Branches and trunks passing through the treehouse
Along with a flat roof, this is one of the things that seems a good idea at the time of design but once in place can be a real headache. Allowing a trunk or branch to flow through the house looks great but brings with it two problems—water flowing down the branch and movement of the tree. There are several solutions but to get a satisfactory result may require some compromise. It is unwise to fix any part of the roof or wall to a through-branch because it will move in different directions to the treehouse's floor, twisting the frame and potentially causing structural damage over time. Branches and trunks tend to channel water in the rain. The simplest way to tackle both movement and water problems (for roughly vertical trunks/branches) is to cut an oversized hole in the wall or roof and another oversized hole where the branch or trunk exits. The size of the holes needs to be determined carefully so the walls/roof are not touched even in strong winds. This way the branch has space to move and water can flow through and out the other side.
A logical upgrade to this system is to use a rubber collar system around the point where the tree enters the house. This method sometimes works very well, but is notorious for small leaks because the rough texture of most trees does not allow a perfect seal. You also need to adjust the collar each year as the tree grows. You may decide to accept a small amount of water coming in as a trade-off for the beauty of a part of the tree contained in your treehouse.