Treehouse size and weight

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

If you have little experience in wood construction, it can be difficult to estimate the possible size of a treehouse. Despite all the treehouse fun, family, friends and children should feel safe and the tree should not be overloaded! 

As a civil engineer, trained carpenter and treehouse builder, I can help you with my practical experience. Using simple examples and rules of thumb, I will show you how to better understand the statics of trees and houses. The aim is to find a suitable supporting structure and the right attachment.

1. Load-bearing capacity of tree – will my tree hold?
2. Treehouse weight – rule of thumb calculation
3. Basic rules of load distribution
4. Load distribution – who carries how much?
5. Choice of right fasteners
6. Summary

Introduction – How the 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 the weight and specifically builds up muscles.

It is similar with a tree: it senses the treehouse and builds extra strong wood in the important places. Properly dimensioned, tree and treehouse will form a lifelong symbiosis!

1. Load-bearing capacity of tree – will my tree hold the weight?

The load-bearing capacity of the tree is obviously sufficient. Limiting factors for the size of the treehouse are:

• the individual fasteners and their maximum load-bearing capacity
• the type of tree (i.e. the strength of the wood in the area of the fastening; hardwood or softwood)
• the diameter of the tree, which determines how many fasteners can be mounted

2. Treehouse weight – area load per m²

In order to know how much load the individual fastenings are exposed to, we first need to calculate the weight of the treehouse. The total weight of the treehouse is made up of:

Dead weight

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

Live load

The sum of all moving parts: People and animals. Imagine 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 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, and at the same time the bigger the lever arm – 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, build your treehouse as evenly as possible around the trunk.

Snow load

Pay attention to the snow load in alpine regions, it can be over 500 kg/m²! Platform size and roof pitch should be adjusted accordingly.

These loads must be supported by the platform structure and transferred to the ground via the tree and its roots. The loads must be distributed as evenly as possible to the individual fastening points.

In the following example, the house and thus the main load is on one side of the tree. In order to safely absorb this load, the supporting structure is constructed from triangles with professional Treehouse Screws:

The largest and most interesting load for us is the dead load of the house:

Rule of thumb – dead load of three treehouse types:

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

Example – Calculating the dead weight of a treehouse

According to the table above, a “habitable treehouse” with 10 m² (108 sq ft) weighs:

10 m² × 125 kg/m² =  1250 kg (2756 lbs)

3. Basic rules of load distribution

After estimating the tree weight (dead load & traffic load) and the load situation (effect of wind & snow load), it is necessary to distribute the loads as sensibly as possible among the trees.

When building in several trees of the same species, the following applies:

• Large tree – large load – many fasteners (e.g. GTS Treehouse Screws).
• Small tree – small load – few fasteners
• The strongest tree is the fixed point because it moves the least.

Different tree species – hardwood vs. softwood:

If you build your treehouse between several trees of different species, it is not only the diameter of the trunk that is important, but also the load-bearing capacity of the individual fasteners! GTS Treehouse Screws, for example, can bear about twice as much in hardwood as in softwood!

4. Load distribution – who carries how much?

Because trees grow wildly and their branches bend in all directions, it is rarely possible to place the house exactly in the centre of the tree in a balanced position. Often, one also wants to avoid branches penetrating the shell of the house and the house weighs on one side next to the tree trunk.

We are interested: Where does the load go? Which tree is subjected to the greatest load? Which supporting structure and which fastening method are best suited to take this load?

The following example shows the construction of a treehouse between two trees. With this simple example you will learn step by step how to calculate the loads and find the perfect attachment.

Step 2: Finding 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 middle line of this line (yellow line: a perpendicular dividing line at the middle point).
• What you see now are the areas that each tree has to carry …

Step 3: Load areas in m²

• The area of our platform is 4 m × 4 m = 16 m² (172 sq ft)
• The load area of the left, small tree is approx. 6 m² (65 sq ft)
• The load area of the large tree on the right is approx. 10 m² (107 sq ft)

Step 4: Place the treehouse on the platform

Now we position our treehouse on the platform – in this example in the middle for simplicity’s sake.

Here it becomes clear:

• If the house were in the green area, the “small tree” would have to bear the main load.
• If the house were to stand in the orange area, the “big tree” would have to bear the load.

Step 5: Calculating the total load

Now we calculate the maximum load to be expected on our treehouse using our rules of thumb (see above).

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

Data from example treehouse:

• Total area = 16 m²
• The treehouse stands in the middle of the platform (type “habitable treehouse”; 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

Example – Calculate treehouse weight:

Formula for the total weight of the treehouse in [kg]:

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 of treehouse [kg] = 10 m² × 125 kg/m² = 1250 kg

• Dead load of the remaining platform area [kg] = 6 m² × 30 kg/m² = 180 kg

• Live load “Funny People” [kg] = 6 × 71 kg = 426 kg

• Snow [kg] = 16 m² × 100 kg/m² = 1600 kg

Total weight treehouse [kg] = 1250 kg + 180 kg + 426 kg + 1600 kg = 3456 kg (7619 lbs)

Step 6: Distribution of the load among the trees

With the total weight and the load areas, the load on the individual trees can be calculated.

• The load on the large tree is:   (10 m² / 16 m²) x 3456 kg = 2160 kg (4761 lbs)
  The small tree is loaded with:   (6 m² / 16 m²) x 3456 kg = 1296 kg (2858 lbs)

SAFETY FACTOR (SF >2)!

In the next step, we choose the appropriate fastening method.

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

Instead of 2160 kg, we assume twice the load of 4320 kg, so our calculations are always on the safe side. Then even hurricanes and wild party nights can’t harm your treehouse.

Now it’s a matter of finding a fastening method that will safely support your platform. There are often several good solutions and combinations. For example, our Treehouse Screw GTS Allstar is perfect for our example project.

If we use two GTS Screws per tree, the maximum load on one Screw is:

“Big tree”:

2160 kg / 2 GTS = 1080 kg

1080 kg x SF2 = max. 2160 kg / GTS Screw   (4762 lbs / GTS)

The load capacity of a single GTS Allstar is approx. 2 tonnes in spruce (softwood). In combination, the Screws stiffen so that two screws working together can take up approx. 5 tons of load in spruce – our example treehouse is therefore safely supported!

Now we will show you two ways to anchor the platform with our GTS Allstar:

Variant 1: Parallel beams (Dynamic-Set)

Analysis:

This is the classic treehouse construction. However, the whole thing would be a bit wobbly here, as the platform and house protrude far beyond the beams at the sides.

If, for example, all 6 beer-friends were to pose for a photo in the lower right corner, the platform would start to sway. The construction would certainly hold up, but too much wobbling is unpleasant and puts strain on the lanyards in the long run!

Another disadvantage:

With a diameter of ø 40 cm (small tree), two opposing GTS Allstar 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 screws could touch in the middle – we want to avoid that at all costs!

Variant 2: Tribeam Construction (V-Bracket-Set)

Analysis:

In our example, a triangular construction is perfect! The bolts lie neatly under each other (see pictures below), so that even a small trunk diameter is no problem. Thanks to the V-Bracket, the triangles can be opened towards the cantilevered platform side. This guarantees that nothing wobbles and the loads are perfectly transferred into the tree.

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 window areas with triple glazing, heavy tropical woods, steel sculptures and dairy cows right from the start.

Choose a smaller “backpack” – you can always expand the treehouse later when the tree has become accustomed to the new load!

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

If you are unsure about statics, loads and construction, or are planning a public project, be sure to get help from an engineer or carpenter.

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