Treehouse Weight Calculation

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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

Our aim:

We want to know how much load is acting on our individual connections so that we can then find the perfect treehouse attachment.

 1. The total weight of the treehouse – surface load per m²

The weight of your treehouse is the sum 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 treehouse has to withstand.

Wind load

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.

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 supporting structure 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.

These are orientation values for the dead load of three treehouse types.

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

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!

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 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!!!

Details:

• 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

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 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!

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!

Another disadvantage:

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 hold?

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.

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