Treehouse size and weight

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You want to build a treehouse and are not sure how big and how heavy it can be?

If you have little experience in timber construction, it can be difficult to estimate the possible size of a treehouse. This is especially important for larger treehouses and public projects. Despite all the treehouse fun, your family, your friends and visitors should feel safe at airy heights and the treehouse must not be overloaded!

This article will help you to better understand the statics of the tree and the house, in order to find a suitable supporting structure and the right fastening.

1. Treehouse weight – rule of thumb for area load
2. Basic rules for load distribution
3. The perfect position of platform and house
4. Load and choice of individual fasteners
5. Load capacity tree – Will my tree hold?
6. Summary

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 the treehouse will form a lifelong symbiosis!

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

The total weight of your treehouse 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 treehouse 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 (102 lb/ft²)! 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 stilts in concrete. The load should be distributed as evenly as possible over the individual fasteners.

Rule of thumb – Dead load of three treehouse types:

(The values are for estimation and include: Substructure, platform, railing and house)

Example: The dead weight of a “habitable treehouse” with 10 m² (107 ft²)  is approximately:

10 m² x 125 kg/m² =  1250 kg (2755 lbs)

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 tree fasteners (e.g. treehouse screws).
• Small tree – small load – few fasteners

Different tree species – hardwood vs. softwood:

Note that it is not the diameter ø of the trunk that is primarily decisive in absorbing the loads, but the load-bearing capacity of the individual fasteners! Tree Screws, for example, bear about twice as much in hardwood as in softwood!

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

Now we will show you an example of how to calculate your loads step by step in order to then find a suitable treehouse position and the perfect attachment method.

Example – “2-tree treehouse [16 m²]”

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 (172 ft²).
• The load bearing area from the left, small tree is approx. 6 m² (64 ft²)
• The one on the right, big tree approx. 10 m² (108 ft²)

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 (172 ft²)
• The treehouse stands in the middle of the platform (type “habitable tree house”; 10 m² = 107 ft²) = dead load
• There are 6 funny adults inside, all drinking a litre of beer (70 kg + 1 kg) = (156 lbs) live load
• It is winter and there is a lot of snow (100 kg/m²) = (20 lb/ft²) snow load

Calculation:

This is the formula:

Load [kg or lbs] = Area [m² or ft²] x Area load [kg/m² or lb/ft²] (+ Other loads [kg or lbs] ).

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

• Dead load treehouse [kg] = 10 m² x 125 kg/m² = 1250 kg (2755 lbs)
• Dead load remaining platform area [kg] = 6 m² x 30 kg/m² = 180 kg (397 lbs)
• Live load “Funny People” [kg] = 6 x 71 kg = 426 kg (939 lbs)
• Snow [kg] = 16m² x 100 kg/m² = 1600 kg (3527 lbs)

Total load [kg or lbs] = 1250 kg + 180 kg + 426 kg + 1600 kg = 3456 kg (7619 lbs)

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 (4762 lbs)
• The load on the small tree is   (6 m² / 16 m²) x 3456 kg = 1296 kg (2857 lbs)

The SAFETY FACTOR (SF >2)!!!

Now it is necessary to choose the appropriate tree attachment 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 an attachment method that will safely support your platform. There are often several good solutions and combinations. Our “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 (4762 lbs / 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 of load in spruce – so our example treehouse is safely supported!

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 treehouse construction. However, the whole thing would be a bit wobbly here, as the platform and the side of the house overhang far beyond the support beams.

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 – we want to avoid that for sure!

Variant 2: Triangle construction with V-Bracket

Analysis:

A triangular construction is the perfect solution here! The screws lie neatly under each other (see pictures below), 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?

As mentioned above, the load-bearing capacity of the tree will not be the determining factor for the size of the treehouse. They are:

• The individual fasteners and their maximum load-bearing capacities
• The type of tree (hardwood or softwood strength)
• The diameter of the tree in question, which determines how many fasteners can be installed

6. Summary – Treehouse and tree in harmony!

Treehouse construction is lightweight construction, not because the trees are too weak, but because the attachment points must not be overloaded! Therefore, avoid heavy materials such as stone slabs, huge windows with triple glazing, heavy tropical woods, steel sculptures and dairy cows.

Choose a smaller “backpack” – you can always add on and extend the treehouse at a later date if necessary, when the tree has become accustomed to the new load!

Never push the load limits of your fasteners completely – always work with a safety factor > 2! This includes possible defects in the wood and exceptional loads such as heavy storms, wet snow or too many party guests.

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

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