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FOR TREEHOUSE BUILDERS
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You want to build a treehouse and are not sure how big and how heavy it should be? We show you the basics of tree(house) statics.
In the following page we will look at structural design and treehouse statics. This is important if you want to build a large treehouse or if you are planning a public project.
As a base, you should already have a rough idea of:
- The tree you want to build your treehouse in
- Total size and orientation of your platform
- Total size of your treehouse m²
- The different attachment methods
We want to know how much load is acting on our individual connections so that we can then find the perfect treehouse attachment.
Introduction – How a tree deals with a house
A treehouse is to a tree as a large backpack is to a human being – health, stature and environment determine how large and how heavy the backpack may be so that the person can still move freely. If you carry the backpack for a while, your body gets used to its weight and specifically builds up muscles.
It is very similar with a tree: it will feel the treehouse and build extra strong wood in the important places.
Properly dimensioned, the tree and your treehouse will form a lifelong symbiosis!
1. The total weight of the treehouse – surface load per m²
The weight of your treehouse is the sum of:
The sum of all fixed, static parts of your house: wooden beams, boards, roofing, windows, furniture, screws, …
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 treehouse has to withstand.
The walls of your treehouse 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 treehouse as evenly as possible around the trunk.
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.
These are orientation values for the dead load of three treehouse types.
(the values include substructure, platform, railing and house):
Simple children's playhouse
(< 15 m² incl. terrace)
- Very light construction
[kg / m²]
(10 - 25 m² incl. terrace)
- Lightly insulated
- high-quality roofing
- Glass window
[kg / m²]
(15 - 40 m² incl. terrace)
- Thick wall construction
- Excellently insulated
- Sanitary facilities
- Heavy windows
[kg / m²]
The dead weight of a “habitable treehouse” with 10 m² is therefore approximately:
10 m² x 125 kg/m² = 1250 kg
2. Basic rules for load distribution
For a project in several trees:
- The strongest tree is the fixed point (e.g. with “Static Support”), as this moves the least.
- Large tree – large load – many fixings (e.g. tree screws).
- Small tree – small load – few fixings
Exception rule hardwood vs. softwood:
Note that it is not the diameter of the trunk that is primarily decisive in absorbing the loads, but the load-bearing capacity of the individual fasteners! This is about twice as high for tree screws in hardwood as in softwood!
Here is an example to explain:
We build a treehouse between two trees:
- Beech: diameter ø 50 cm
- Spruce: diameter ø 70 cm
We use 2 x of our treehouse screw GTS Allstar for fastening; it carries approx. 4 tons in beech wood, only 2 tons in spruce wood.
Consequently, 8 tons of load can be carried by the “smaller” beech, the “larger” spruce should carry a maximum of 4 tons.
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 attachment method.
Step 1: The floor plan
We assume that you have already thought about your tree house, that you have measured your trees, and that you already have a plan of the location of your platform.
In our example it looks like this:
Two spruces 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: a perpendicular dividing line at the centre point)
- what you see now are the areas that the respective tree has to bear…
Step 3: Load bearing areas in m²
- The area of our example platform is 4 m x 4 m = 16 m².
- The load bearing area from the left, small tree is approx. 6 m²
- The one on the right, big tree approx. 10 m²
Step 4: Place the treehouse 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 over the load
- if the house were in the orange zone, the “big tree” would bear its load.
Step 5: Calculate the 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 treehouse construction must withstand!!!
- Total area = 16 m²
- The treehouse 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
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 catchment 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. for the large tree, for example:
Instead of 2160 kg, we assume double the load 4320 kg, thus our calculations are always on the safe side. Then even hurricanes, earthquakes and tsunamis won’t be able to harm your treehouse!
4. Choice of suitable attachments
The next step is to find a mounting method that will safely support your platform. There are often several good solutions and combinations.
The “GTS Allstar” is perfectly suitable for our example project, the maximum load of a screw is according to the calculation:
1080 kg x SF2 = max. 2160 kg / screw
The load capacity of a single GTS Allstar is about 2 tons in spruce (softwood). In combination the screws stiffen, so two screws working together can take about 5 tons load in spruce.
Now we will show you two variants for anchoring the supporting structure with our GTS Allstar:
Variant 1: Parallel support beams
Analysis: This is the classic tree house construction. 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 attachments!
With a diameter of ø 40 cm (small tree), two GTS Allstar opposite each other may not have enough space. If the diameter is somewhat smaller at this point, e.g. because the trunk is oval instead of round, the tubes could touch 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 small trunk diameter does not matter.
Thanks to the V-Bracket, the triangles can open up to the cantilevered 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 carry the treehouse?
Trees can take 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 (more details on the individual fastening types can be found 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 very unsure about statics, loads and construction, or if you are planning a public project, then definitely get help from an engineer or master carpenter.