Question About Sealing Leaks Around a Tree Trunk

Periodically I receive e-mail from people with questions about tree houses. Often I respond directly, however, I think this one from Jim Winkle is worth posting. If anyone has any ideas beyond the scope of our discussion please send an e-mail.

The neighborhood kids and I built sort-of-a treehouse last year, and I’m having trouble with rain coming in. It’s carpeted, which means the carpet gets soaked.

Rain comes in where the tree trunk goes through the roof (see photo). I’m looking for recommendations about how to prevent this.

I have a car tire inner-tube wrapped around the tree and attached to the roof (between two layers of shingles). Bunge cords hold it tight to the tree trunk. As you can see, I also have plastic wrapped around the tree above the inner-tube.

I’ve been inside it during a downpour, and the trunk is soaking wet and water drips down from the inner-tube. I wouldn’t mind if the water simply ran down the trunk. It does, but it also drips down onto the floor from the inner-tube on the ceiling. As you can see, the tree is slanted, which contributes to the problem. I made a water collection trough (not pictured) mounted inside at the roof-line, but it dripped, too.

“Drips” doesn’t sound like a lot, but during a heavy rain, it is.

Before a rain, I try to remember to roll up the carpet and put some pieces of hard plastic on the floor, which channel the dripping water towards the tree trunk. This is getting to be a hassle.

If you have any ideas, you’ll find my email address near the top of my home page. Any information anyone can provide would be greatly appreciated! This must be a fairly common problem, and I’m hoping someone has solved it.

P.S. The treehouse has lights, a radio, and a fan inside, and is entirely solar powered!

My response to Jim’s Question:

I’ve had this conversation with many treehouse builders: How do you seal a tree trunk that passes through a roof or wall? There are many flexible-seal type solutions attempted, many similar to the one you have utilized Jim, but none are successful at completely stopping moisture from tracking down the tree trunk and into the treehouse. Simple treehouse builders often just live with the problem — it is not a big deal except that it can lead to a musty smell inside, moisture can encourage growth of mildew, and potential for premature wood rot. However, if you have plans to make the treehouse a comfortable year-round structure to live in moisture leaks are not acceptable.

The problem stems from the morphology of most trees: The shape and orientation of branches relative to the trunk encourage the collection of water channelling, via surface tension, down toward the centre of the tree and to the ground. Some speculate this is to concentrate rain water close to the trunk so the tree roots can absorb it more easily — especially in dryer conditions. Other ideas are that the rain water flowing down the branches and trunk provides moisture to the bark exterior, washes dirt and other material away, and helps to prevent the growth of parasites and other detrimental organisms. Incidentally, moss can benefit from the moist conditions of rain washing as it grow on the shaded side of the tree.

When designing a treehouse in a single tree it is often necessary to pass the trunk, and/or major branches, through the walls or roof of the structure to maximize space and achieve weight balance. The down side, as you point out, is that it is nearly impossible to seal the water from entering through these exit points. I have seen attempts to seal around the tree with flexible membranes (such as rubber inner tubes and plastic) but these never completely work. The problem is that the bark is irregular and porous, and it changes shape over time as the tree grows, so it is impossible to seal anything to it. Attempts to smooth or otherwise modify the bark to achieve a good seal can lead to serious tree damage. The bark is the most vulnerable part of the tree and damaging it, especially in a ring around the circumference, can prevent the flow of nutrients up to the leaves and lead to it’s death.

There are two solutions I’ve seen that work to completely isolate the treehouse interior from the moisture running down the tree. The first is a bit dramatic: top off the tree just below the roof line and build a solid roof above. I have seen a treehouse built in four cedars designed this way and it works remarkably well — however it is not practical for most applications. The second solution is to build a wall/vapor barrier around the trunk on the inside of the treehouse. In essence, this means building the treehouse like a donut with the tree trunk passing through the hole in the middle. This allows you to leave the top and bottom holes for the trunk open to the flow of air and rain water, yet the inside stays dry. The downside to this is a slight loss of interior floor space and the inability to see the tree trunk while inside.

I hope this answers your question and that you can use the information to implement a solution for your project.