You want to build a treehouse and are not sure how big and how heavy it can get? We will show you the basics of tree(house) statics.

In the following article, we will dive a deeper into the topic of supporting structure and statics. This is important if you plan a public project or if your treehouse is going to be very large compared to the tree.

In order for this article to help you, you should already have a rough idea of:

• The tree you want to build your treehouse in
• Overall size and orientation of your platform
• The total size of your treehouse (m²)
• The different mounting options

Our goal here:

We want to know how much load is on our individual treehouse anchors so we can size them correctly.

1. Rule of thumb for calculating the total weight of a treehouse

The load / weight of your tree house is made up of:

Dead load

The sum of all fixed, static parts of your house: wooden beams, boards, roofing, windows, furniture, screws, … 

Live load

The sum of all moving parts: People & animals. Consider the following scenario: at a party, all your friends go up to the tree house and all pose on one side for a photo…. “and on the count of 3, please all jump!” That’s live load – that’s what your tree house has to withstand 😉

Wind load

The walls of your tree house act like a sail. The higher your house is in the tree, the stronger the wind load becomes, and at the same time the larger the lever arm becomes – so the higher your house is in the tree, the smaller it should be!

The tree and the fasteners do not like torsion: To avoid twisting movements, build your tree house as evenly as possible on the trunk.

Snow load

Pay attention to the snow load in alpine regions, this is sometimes estimated at over 500 kg/m²! Platform size and roof pitch should be adjusted accordingly.

All these loads must be absorbed by your platform construction and transferred into the ground. This is done either directly via the tree and its roots, or via an artificial support set in concrete.

The load should be distributed as evenly as possible over the individual fasteners.

Here you will find orientation values for the dead load of three different treehouse types

(the values include the substructure, platform, railing and house):

The dead load of an “inhabitable tree house” with 10 m² therefore is approximately:

10 m² x 125 kg/m² = 1250 kg

2. Important basic rules for load distribution

Important basic rules for a project in several trees:

• The most massive, thickest tree is the fixed point (e.g. with “Static Support”), as this moves the least.
• In general, the larger trees should carry more load – larger trees sway less and can take more fixings (e.g. multiple bolts).

Exception rule – hardwood vs softwood:

Note that it is not primarily the trunk diameter that is decisive when taking up loads, but the load capacity of the individual fasteners! This is approx. twice as high for tree screws in hardwood as in softwood!

3. Load distribution – find the perfect position of platform and house

Using a classic 2-tree treehouse as an example, we show you how to calculate your loads step by step and then choose the perfect fixing method.

Step 1: The setting

We assume that you have already thought about your tree house, that you have mapped your trees, and already have a rough idea of the orientation of your platform.

In our example it looks like this:

Two spruce trees ø 40 and ø 60 cm; and a square platform of 4 x 4 m which slightly overhangs to the right.

We now want to check how much weight is acting on the individual trees.

Step 2: Find the centre line

The centre line is the dividing line of the load bearing surfaces. It shows us what proportion of the area of the platform acts on which tree; this is determined graphically:

• connect the tree trunks with a straight line
• draw the centre line of this line (yellow line: at the centre a perpendicular dividing line)
• what you see now are the areas that the respective tree has to bear…

Step 3: Load bearing areas in m²

• The total area of the platform is 4 m x 4 m = 16 m².
• The load bearing area of the small tree on the left is approx. 6 m².
• The load bearing area of the large tree on the right is approx. 10 m².

Step 4: Place the tree house on the platform

Now we position our house on the platform – in this example, for simplicity’s sake, centrally.

Here it becomes obvious:

• if the house would be in the green zone, the “small tree” would have to take the load;
• if the house were in the orange zone, the “big tree” would bear its load.

Step 5: Calculate the maximum total load

Now we calculate the expected maximum load of our treehouse with the help of our rules of thumb.

This extreme peak load probably only occurs every few years – but then the tree house construction must withstand!!!

Details:

• Total area = 16 m²
• The tree house stands in the middle of the platform (type “habitable tree house”; 10 m²) = dead load
• There are 6 funny adults inside, all drinking a litre of beer (70 kg + 1 kg) = live load
• It is winter and there is a lot of snow (100 kg/m²) = snow load

Calculation:

This is the formula:

Load [kg] = Area [m²] x Area load [kg/m²] (+ Other loads [kg]).

The area load is the sum of the individual loads (dead load + live load + other loads).

• Dead load tree house [kg] = 10 m² x 125 kg/m² = 1250 kg
• Dead load remaining platform area [kg] = 6 m² x 30 kg/m² = 180 kg
• Live load “Funny People” [kg] = 6 x 71 kg = 426 kg
• Snow [kg] = 16m² x 100 kg/m² = 1600 kg

Total load [kg] = 1250 kg + 180 kg + 426 kg + 1600 kg = 3456 kg

Step 6: Load distribution on the trees

From this, the load on the individual trees can be calculated with the help of the load areas:

The load on the large tree is: (10m² / 16 m²) x 3456 kg = 2160 kg

The load on the small tree is (6 m² / 16 m²) x 3456 kg = 1296 kg

The SAFETY FACTOR (SF >2)!!!

Now it is necessary to choose the appropriate fastening method.

It is essential to calculate with the safety factor > 2, i.e.:

Instead of 2160 kg, we assume e.g. 4320 kg load, thus our calculations are always on the safe side. Then even hurricanes, earthquakes and tsunamis will not harm your treehouse!

Variant 1: Parallel support beams

Analysis: This construction is a real classic! However, the whole thing would be a bit wobbly here, as:

• the platform protrudes far to the side
• the house is relatively large and protrudes sideways

If, for example, all 6 beer friends were to pose for a photo in the bottom right corner, the platform would start to wobble. The construction would certainly hold, but too much wobbling gives an uneasy feeling and in the long run strains the fasteners!

Another disadvantage:

With a diameter ø 40 cm (small tree), two opposing GTS Allstars are very tight.

If the diameter is somewhat smaller at this point, e.g. because the trunk is oval instead of round, the tubes could touch each other in the middle – that would be a big bummer!

Variant 2: Triangle construction with V-bracket

Analysis: A triangular construction is the perfect solution here!

The bolts lie neatly under each other, which means that even a smaller trunk diameter does not matter.

Thanks to the V-bracket, the triangles can open up to the projecting platform side. This guarantees that nothing will wobble and the loads are transferred perfectly into the tree.

5. Resilience of the tree – will my tree hold?

Trees can carry incredible loads!

Wood has an average compressive strength of approx. 2 kN/cm² along the fibre, which is the equivalent of 200 kg/cm². A round, straight tree trunk, diameter ø 40 cm, can therefore theoretically take a load of 1256 cm² x 200 kg = 251.2 tonnes.

As mentioned above, the tree will most likely not be decisive – it is the individual fasteners that set the limits – this is 500 – 6000 kg depending on the fastener and type of wood (more details on the individual fastener types here).

6. Summary – What we have learned

• Treehouse construction is lightweight construction – avoid heavy materials such as stone slabs, huge window areas with triple glazing, hardwoods, steel sculptures, dairy cows, …
• Never push the load limits of your fixings completely – always work with a safety factor > 2! This includes possible defects in the wood and exceptional loads such as violent storms or too many fat party guests.

Even if the screw can theoretically take 4 tons, it should only be loaded with 2 tons.

• It is better to choose a smaller “backpack”; you can always add on and extend the tree house at a later date if necessary, when the tree has become accustomed to the new load!

If you are unsure about the topics of statics, loads and construction or are planning a public project, then definitely get help from an engineer or carpenter.

Schlederlukki Mukki Fukki