The strongest wood species for outdoor projects

Most frequently asked questions received from readers are related to the behaviour of wood outdoors and why it should be made as resistant as possible. The resistance of wood to the exterior depends not only on the quality of the materials used to finish it or the way they are applied, but also on how well natural wood behaves to the action of external factors. Outdoor resistance is often confused with durability. Durability is, however, a broader property that shows how resistant wood is to physical, chemical and biological stress factors. It is the property that engineers and architects need to take into account when using wood in construction or interior design projects, especially when they are structural.

Wood does not have the same strength on the outside regardless of species, which was discovered long before sophisticated laboratory determinations were made. In using it, people have discovered that some species are more resistant than others. From experience they have learned which are most resistant to rain and snow, in soil or water. That's how they knew how to choose the best wood for the strength of houses and fences, the one for ships or the pillars of lake dwellings.

Resistance of wood to contact with the ground. Strength classes

The most important criterion for grading outdoor timber is its resistance to ground contact. In the contact zone, wood is highly stressed, the aggression of environmental factors being maximum. If you are curious to see what a stake that has been in the ground for a longer period of time looks like, you will notice that the most "eaten", the most thinned area is exactly the area in contact with the ground. And the part that has been completely in the ground has held up much better.

Based on this criterion, resistance classes were established. These are found in the European Community standard EN 350-1, which is also valid in Romania. Here are the 5 resistance classes:

• Class 5 - the wood is perishable, with a durability of less than 5 years (less than 1 year in some EC countries). This category includes poplar, fagul, birch
• Class 4 - wood of low strength. Species in this class last between 5 and 10 years. [ pine , molid ]
• Class 3 - wood with moderate strength. Time range 10-15 years. [ dolaglas, larch ]
• Class 2 - durable wood. Life span: 15-25 years. [ oak, chestnut, acacia, red cedar ]
• Class 1 - very durable wood. Over 25 years. [ tec, iroko ]

External factors that are an aggression to wood

The strength of wood outdoors is not only determined by how well it resists contact with the ground. There are works that do not even take this criterion into account (cladding of houses with wood, roofs, structures positioned on pillars). In this case it is necessary to see how well the wood behaves to expansion and contraction, how much it cracks due to seasonal changes in humidity or temperature, how resistant it is to the action of the sun's rays or how quickly it is attacked by fungi, mould or insects.

From strength tests, measurements of certain parameters and simple observation of the wood compartment over time, it was concluded that wood containing resins or tannins is much more resistant to the action of water. This is the case for resinous or that of acacia. This is why species with low resistance to soil contact, such as pine or spruce, are still used in house construction. However, the buildings are built on stone or concrete foundations, avoiding direct contact with the ground.

Species that do very well outdoors or in the water are oak, chestnut and ulmul. Oak was used extensively in the past to build ships, and chestnut (not the edible fruit), which is widespread in Italy, was used, along with oak, to build the pillars on which the houses of Venice still stand today.

Of all the species, the one that performs very well in all outdoor tests is tecul. It is wood that can be used outdoors without protection and its properties remain unchanged for a long time.

More information on the behaviour of different species to the action of wood-boring fungi, wood-boring beetles or termites can be found in EN 350-2. A paper with many determinations, figures and technical information on the durability of different wood species in the country can be found in here.

Is the wood of the same species equally strong regardless of age, cutting method or internal structure?

The strength of the wood on the outside is also different within the same trunk. There is a big difference between sapwood and heartwood. In the section perpendicular to the trunk axis, two areas are visible in most species: a lighter coloured area towards the outside (just behind the bark) - alburn - and a central, darker one - heartwood.

The sapwood is the area through which food is transmitted when the tree is standing and is therefore more loosely woven and absorbs water very easily. The heartwood is the 'old' area of the tree, the area of resistance, which is no longer used for transporting food. It is tougher, more compact and does not tend to absorb food. As sawn wood, the sapwood absorbs much more water in the fibre end zone than heartwood. This is why the sapwood of all hardwoods is classified as Class 1 for resistance to soil contact. The timber of a wood that is considered to be strong on the outside will not be as strong if it also contains sapwood.

Absorption is also different with age because the ratio of albumen to heartwood changes with age. The proportion of sapwood is higher in youth, when the need for food is high. Over time, some of the wood hardens, making the tree stronger and harder to fell. This is why young trees are not chosen for houses and load-bearing structures.

I only said about the sapwood of the broad-leaved trees because with softwoods things are different. It is very difficult to distinguish sapwood from heartwood (in spruce it is impossible), and absorption is not so different because the transport of food in softwoods is through a different system. Softwoods have no pores, so water absorption is much slower. Hygroscopy (ability to absorb water) is much lower and that is why spruce is a successful wood for outdoor use.

The water absorption capacity is also determined by the way the wood is cut. Water is more difficult to absorb in radially or tangentially cut elements, which are preferable when wood is chosen for exterior use.

Increasing the strength of wood outdoors

In order to have a timber construction that is resistant to the outside, we must ensure that:

• the wood has a moisture content of less than 20% but more than 14%
• is as well protected as possible to avoid absorbing water. Wood that does not come into contact with water or water vapour under normal atmospheric conditions will not exceed 15-16% moisture content.
• has not been attacked by insects, fungi or moulds and is protected against possible attack by them
• the materials with which it is protected are resistant and the protection is applied to all sides or edges of the wood.

About materials for exterior wood protection we've spoken on other occasions. Elastic or non filmy materials are preferable. As you have heard many times, wood works, i.e. it varies dimensionally according to the external humidity values. A rigid film will crack and the protection will disappear. This is why I frequently recommend oil or wood protection wax.

Two things I want to point out about finishing the wood to make it as strong as possible on the outside:

• without proper maintenance no material will last perfectly for years
• transparent products are not resistant to the outside. Even if they provide water protection, they do not provide UV protection. Wood will still change colour over time. The most resistant are coloured paints (not white) and the second most resistant are semi-transparent coloured lacquers.

I hope you find the above information useful. If you find the article of interest to others, please feel free to share it. And if you have any queries or questions, please leave them below in the dedicated area. You will certainly get an answer.