Tree House Design – Part II

Having covered a lot of the preparation topics – like tree selection and fixings – in the Part I, let’s pick up where we left off and continue with the rest of the tree house. In this post we’ll look at how to design a tree house pod, as shown Figure 1 below. This includes the basics of platform design, flooring, wall construction, cladding and roofing.

Tree House Design
FIGURE 1 – Finished Tree House

DESIGN INTENT

First and foremost, designing a tree house is all about personal preference. So you should always make yours whatever you want it to be. This particular example is something along the lines I’d probably build for myself. Meaning it’s not everyone’s cup of tea, but that’s kind of the point. Anyway, let me try and explain the idea behind it. This tree house is basically a designers retreat. It could easily suit a writer, an artist, or any profession where you need to fully devote to your craft in an inspiring surroundings without unnecessary interruptions.

DESIGN PRINCIPLES

The design is  based mainly on the principles and techniques of lightweight timber construction. So effectively, it’s like building a shed. The design we have here is made up of two equal size pods. One, which sits higher in the trees, is divided into a bedroom and an office. We can call this the sleeping pod.

Another pod, let’s call this the chill out pod, is located few meters lower than the sleeping pod and features a comfortable seating and table area, as well as a kitchenette, compost toilet and a small outdoor decking area. As you can see by the happy people shown in the image below, this space can be used for pretty much any purpose. Whether it’s a secret poker night, or a family picnic up in the trees, this pod’s got it all.

Tree House Design
FIGURE 2 –  Tree House Interior
MATERIALS

In terms of materials, pretty much everything is made from timber. For the frame, we want to use grade C24 studwork timber. This is one of the two grades you can buy timber in (C16 and C24). What’s the difference you ask? Well, out of the two, C24 is the more premium. It’s stronger with less defects, so we want to use it for the beams as well as all the outside frame construction. There’s really no point penny-pinching and choosing lesser quality timber for a project like this. For fasteners, we want to stick with galvanised screws and bolts (or stainless) instead of mild steel.

BEAMS

Beams will provide the main support for the tree house, so it’s important to make sure that they are strong enough to do this. Here we’re using 3″ x 12″ (75 mm x 300 mm) C24 grade timber beams. Each beam is fixed to two TABs (dynamic triangle assemblies) – one at each end. In case you missed it, we covered the dynamic triangle assembly in our last post. So, in other words, each TAB carries quarter of the total load of the tree house – assuming we have a fairly even weight distribution throughout.

The TABs are capable of carrying a load up to 2.5 metric tons on softer trees like pines and cedars, and up to 4.5 tons on harder trees, such as oaks and maples [1]. That means our tree house can support a total of 10 to 18 tons – so we don’t have to worry about them failing. So the main focus then becomes the beam itself. How do we ensure it’s strong enough? Well, the safest way is to choose beams that support no less than the fixings (TABs). But we can be more technical and work out the weight of the whole pod and then choose a size of timber that is strong enough to support that weight.

This can be a bit tricky and requires bit of guesstimation, especially if doing it by hand. Luckily we’re using design software which does the number crunching for us. It does this by calculating the volume of the solid and multiplying it by the mass density of (pine in this case) of 420 kg per cubic meter. Therefore the weight for the sleeping pod is given as 980 kg – which will also be true for the chill out pod. Keeping in mind that this is the dead weight and doesn’t account for people or any furniture etc. With a span of just over 3 meters, our 3″ x 12″ beams will easily support three times that weight. So we are good to go!

PLATFORM DESIGN

Once the beams are in place we can start building the platform that lays on top of them.  Here we’ve got two separate platform assemblies, one for each pod. Both are made using 2″ x 10″ timber (47 mm x 250 mm). Building a platform is like building decking for your patio – you have your rim joists as the outside border and common joists running perpendicularly in between. Galvanized joist hangers make sure the common joists stay in place. The platform shown below is for the sleeping pod.

Tree House Platform Detail
FIGURE 3 –  Platform Dimensions

This is where we need to remember the principles we talked about in the previous post. As the trees need to be allowed to move freely and independently, we cannot fix the platform to both beams. What we want is for both beams to be independent from the platform. Also, bear in mind that the beams will end up moving over time due to tree growth [1]. Therefore fixing to both beams would mean it’s all just one rigid unit and something would have to give – and it’s usually the platform.

So how can we make sure the platform is fixed securely, but not restricting the beam movement? Well, the trick is to fix down only one end of the platform and let the other end rest freely on the other beam. Under the ‘free end’ we can add some UHMWPE pads that allow the beam to slide under the platform as the trees move in the wind. This way we are allowing for the independent movement of the trees, instead of working against them. By adding a length of timber under the platform edge we can make sure that platform never slides far enough to fall off.

Tree House Platform Detail With Beams
FIGURE 4 – Platform Assembly
WALLS

Next step is to start putting up the walls for our tree house. Lightweight timber frame (studwork) construction is ideal because it’s – as the name suggests – lightweight. These walls can be knocked up on the ground, or in a workshop where it’s safer and more practical to work. The finished walls would then be lifted up and fixed to the platform. All the wall frames are made from 63 mm x 38 mm CLS studwork timber.

1) STUDWORK FRAME

The basic wall frame has vertical studs between the top and bottom plates with noggings  (the short horizontal bits) in between. Traditionally the verticals have a 14″ spacing but with a single storey tree house we can use larger spacing – in this case it’s 583 mm centres. Some of the walls do have windows, so at these locations we need to put a horizontal sill and a header to create the rough opening. For the doors, there’s only a header along the top.

Tree House Studwork Wall Detail
FIGURE 5 –  Studwork Detail

When building a conventional house, the standard ceiling height is 2400 mm. But since there are no standards for tree houses this could be anything we want it to be. Here we have a slanted roof which means one end of the pod will have a higher ceiling than the other end. At the lower end the ceiling height is two meters, and at the front of the pod it’s just over two and a half meters.

2) WALL CLADDING

Wall cladding (or siding) is what we use to cover the studwork with. Here we’ve used  some basic plywood sheets which are easy to install and also add bit of extra strength to the walls. On the interior side of the walls, the plywood finish can be left as it is (as we’ve done here) or painted over. You could always use some nice textured veneer MDF or plywood sheets. Or, you might want to try applying a varnish or some type of furniture wax to bring out a nice texture. Plenty of options available here.

On the exterior side we want to use plywood that’s fit for outdoor use, which would be marine plywood. Although they are intended for outdoor use, it’s still a good idea to finish the panels to prevent decay and discoloration [3]. The basic sheet size is 2440 mm x 1220 mm (8′ x 4′) so we need to cut the panels in a way that the joints line up with the vertical stud centres. This gives us something to screw/nail the panels onto.

Having plywood panels on either side of the studwork leaves a void (empty space) in the stud wall. This is normally where the insulation goes, as shown in Figure 6 below. Depending on where the tree house is located, insulation may or may not be necessary. The type of insulation can be anything from sheet material to spray foam insulation – preferably recycled and Eco-friendly.

Tree House Assembly Detail
FIGURE 6 –  Pod Assembly
FLOORING

For the basic tree house, we could just lay down decking boards over the platform. But since we’re building a slightly more sophisticated tree house with dry and warm interiors, we’ll be using plywood instead. That is, we’ll lay a subfloor from plywood and cover that with veneer sheets for the final floor. Again, these veneer sheets come in all sorts of colours and textures for you to choose from.

ROOF

Next task is to build a roof over our heads. A simple, light-gauge roof framing will do the job. Once the frame has been built, we’ll cover it with marine plywood like we did for the outside face of the walls. On top of the plywood we then lay down a rubber membrane to make it waterproof, and finally fix down the roof boards. So let’s start with the first step –  putting up the rafters.

1) RAFTERS
Tree House Roof Detail
FIGURE 7 –  Rafter Detail

Depending on the location of the tree house, you might have to check the average snowfall in your area which will then determine the recommended rafter span. Here we’ve used 100 x 50 mm (4″ x 2″) timber at 420 mm centres. These sections usually come in 4.8 meter lengths, which isn’t long enough to cover the whole length of our tree house. So the solution is to create a joint on top of the middle partitioning wall and connect the sections with flat jointing plates. The ends can be fixed with some angle brackets.

2) WATERPROOFING

Once the rafters are in place, we can fix down the marine plywood over them. Then, to make sure the roof is waterproof, we want to cover the whole roof with a rubber membrane. This is basically a rubber sheet that you roll out and screw, or stable, down to the roof. Along the edges of the roof we want to use some sort of edge trims to make sure the water is directed away from joints vulnerable for leaks.

These can be found under labels such as drip edges, eaves protectors or wall flashing – all more or less the same thing. Remember that all the outside wall panel joints also need to be waterproofed with a flexible adhesive sealant, flashing tape, or something similar. It’s also recommended to varnish the exterior walls, to offer better protection against the elements and therefore extending their lifespan.

Tree House Roof Waterproofing Detail
FIGURE 8 –  Roof Detail
EXTERIOR CLADDING

Effectively, the tree house is finished now and we could leave it as it is. But having a plywood box up in the trees would look a bit weird, to say the least. Instead, we want to camouflage it, to make it look like it’s part of the forest. To do this, we’ll be using some old reclaimed timber we’ve managed to scavenge from uncle Billy’s old barn roof – or salvaged from the demolition site down the street.

These old bits of timber have that nice aged “patina” we’re looking for. We’ll fix them on all walls and on the roof in a way that is supposed to mimic the texture of a tree bark. So we can forget about perfect lines and smooth finishes here. Instead, we want different width planks and unequal joint gaps and even creating different depths.

Tree House Roof Cladding Detail
FIGURE 9 –  Exterior Cladding Detail

The process itself is simple – just like putting up roof shingles. First we start by fixing some battens on top of the rubber sheet. Here we want to leave a gap under the battens to allow rain water to drain down the roof. This also allows air to flow through to avoid problems with condensation. You can either use counter battens, or just stick some timber packers under the battens like we’ve done here. Where the timber plank sections butt up to each other, we just double up on the battens to provide that screw base.

CONCLUSION

As you can see, there isn’t anything overly complicated about building a tree house. Once you understand the basic principles, you can then go crazy with your creativity and make it your own. This particular example follows the same principles you’d use to build a timber shed. Meaning that you could actually live in this tree house for longer periods if you wanted to.

Having said that, to make a tree house more habitable you probably want to install some services – mainly water & electricity. Unless you’re planning to build in the backyard of your house, the tree house will end up being off grid. So just like remote cabins, you’d end up relying on using natural resources. For instance, you could install few solar panels to the roof/walls to provide electricity for some basic needs. But it might be worth having a backup generator as well – just in case. It all depends on how much electricity is needed, really.

Also rain water can be harvested and used in the tree house if you want to install a sink tap in the kitchen, or even a shower. This can be a little tricky though, as you’d need to install the tank higher than the tree house to allow for gravity to do it’s job. Otherwise you need to install a water pump (and possible an electric water heater), which obviously would just be another appliance requiring electricity to run. Also you might have noticed that there is a toilet into the chill out pod – so this would be some sort of compost toilet. Meaning there would need to be a way to empty this compost as well.

Hopefully you are now inspired and excited to start designing you own tree house. The point of this post was to familiarize you with the basic tree house design principles and building techniques so you can get an idea where and how to start. In terms of building techniques, the Black & Decker Complete Guide to Treehouses is a good starting point for basic tree houses. And as I mentioned in the previous post – for a little more advanced techniques you definitely want to read Pete Nelson’s Be in a Treehouse. If you happen to know any other good tree house books, then just let me know in the comments below.

REFERENCES & FURTHER READING

1. 2. 3. 4.

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