The strongest wood species for outdoor projects

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

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

Soil contact resistance of wood. Resistance classes

The most important criterion for classifying exterior wood is its resistance to ground contact. In the contact zone, the wood is highly stressed, the aggression of environmental factors being at its 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", 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 have been established. These can be 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 - low-strength wood. Species in this class last 5 to 10 years. [ pine , molid ]
• Class 3 - moderately resistant wood. Time interval 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 damaging to wood

The outdoor resistance of wood is not only determined by how well it resists ground contact. There are works which do not even take this criterion into account (cladding of wooden houses, roofs, structures positioned on pillars). In this case, it is necessary to see how well the wood behaves in terms of 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's appearance over time, it has been concluded that wood containing resins or tannins is much more resistant to water. This is the case for resinous or that of acacia. That is why species with low resistance to ground contact, such as pine or spruce, are still used to build houses. But they are built on stone or concrete foundations, avoiding direct contact with the ground.

Species that do very well outdoors or in water are oak, chestnut and ulmul. Oak was used extensively in the past to build steamboats, and chestnut (not the edible-fruited kind), widespread in Italy, was used, along with oak, to build the pillars on which the houses in Venice are still supported today.

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

More information on the behavior of different species to the action of wood-boring fungi, wood 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 at here.

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

The external strength of wood is also different inside the same trunk. There is a very big difference between sapwood and heartwood. In the section perpendicular to the axis of the trunk, two zones are visible in most species: a lighter colored zone on the outside (just behind the bark) - alburn - and a central, darker - heartwood.

The sapwood is the area through which food is transmitted when the tree is standing and is therefore more loose and absorbs water easily. The heartwood is the 'old' part of the tree, the area of strength, which is no longer used for food transportation. It's tougher, more compact, with no tendency to absorb food. As cut wood, in the fiber-head zone, sapwood absorbs much more water than heartwood. This is why the sapwood of all hardwoods is classified as Class 1 for resistance to soil contact. The lumber of a wood that is considered to be externally resistant will not be as resistant if it also contains sapwood.

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

I only said about the sapwood of the leaf because things are different with resinous plants. In them it is very hard to distinguish the sapwood from the heartwood (in spruce it is impossible), and absorption is not so different because the transportation of food in the resinous is done by a different system. Softwoods don't have pores, so water absorption is much slower. The hygroscopy (ability to absorb water) is much lower, which is why spruce is a successful outdoor wood.

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 choosing wood for outdoor use.

Increasing the outdoor resistance of wood

To have a timber construction that is resistant to the outside, we need to make sure that:

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

About materials for exterior wood protection we've talked about on other occasions. Resilient or non-film-forming materials are preferable. As you have heard time and time again, wood works, that is to say, it varies dimensionally depending on the outside humidity values. A rigid film will crack and the protection will disappear. That's why I frequently recommend oil or wood protection wax.

Two things I want to emphasize about finishing the wood to make it as resistant as possible to the outside:

• without proper maintenance no material will last perfectly for years
• transparent products are not outdoor resistant. Even if they protect against water they do not protect against UV radiation. Wood will still change color over time. The most resistant are colored paints (not white), and in second place are semi-transparent colored lacquers.

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