The decisive factor for treehouse screws is the compressive strength along the wood grain:
Hardwood (> 2.5 KN/cm²) – very suitable for screws:
- Deciduous tree: maple, beech, oak, ash, walnut, sycamore, robinia.
Medium hardwood (2 – 2.5 KN/cm²) – well suited for screws:
- Deciduous tree: birch, chestnut, alder, cherry, lime
- Conifer: Douglas fir, pine, larch, spruce, fir
Softwood (< 2 KN/cm²) – moderately suitable for high load on screws:
- Deciduous tree: aspen, poplar, pear, horse chestnut, elm, willow
- Conifer: cedar
These are only rough guide values – wood strength varies from tree to tree, from place to place and even in the same tree! More about this here…
For the final suitability for tree house construction, apart from the compressive strength, the sealing capacity, the wound healing also play an important role.