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.
Hi, I would like to build a small children's cottage at height.What kind of wood do you recommend to make it and what maintenance materials.Thanks.with respect.
Hello!
Softwoods, spruce or pine (or larch, but it's harder to find and more expensive). They're affordable, tough outdoors and have a lower specific weight. Oak and acacia are also good for outdoors, but they are harder species - harder to work with, harder to machine - heavier, and more expensive.
In my opinion, for such a house, the best finish is the oil finish. Use oil-based coloured varnish because it is also resistant to the sun (UV radiation), not just moisture. If you don't want the wood to show (to paint it) use oil-based paint or alkyd paint (the latter is cheaper).
Below are some links that may help you.
All the best!
https://revistadinlemn.ro/2018/08/03/12-reguli-simple-pentru-a-avea-constructii-din-lemn-rezistente-la-exterior/
https://revistadinlemn.ro/2018/06/25/8-cele-mai-frecvente-intrebari-despre-protejarea-lemnului-la-exterior/
https://revistadinlemn.ro/2019/04/17/7-reguli-de-respectat-la-finisarea-cu-ulei-a-lemnului/
https://revistadinlemn.ro/2017/09/20/lemnul-de-larice-stejarul-coniferelor/
Hello, I would like to know what wood to use for a countertop of 8 cm maximum 10 cm that will support a small natural stone sink located in the bathroom. Thank you
Hello!
You can use acacia, oak, resinous. The construction solution is very important because water must not puddle if it gets on the wood, and air must circulate to allow drying. If these conditions are not respected, the wood will deteriorate over time, no matter how strong it is. I recommend finishing it with oil. It is a very good protection against water.
All the best!
Hello! First of all please allow me to congratulate you for the very interesting materials. The fact that you take your time and answer those who ask you, for that you have all my esteem and admiration. I have a garage made of rosewood, it was once varnished, but I would like to protect it as I have seen wasps nesting and some of the wooden bars have cracked and I would like to know if it is enough to apply a thick translucent varnish, Sadolin Extra, or is something else needed? Also, as dust has settled on the wooden bars over time, what could I clean them with? Is it enough to wipe them with a damp cloth, sprinkle them with a jet of water and then leave them for a few days to dry? Thank you very much for your kindness!
Hello!
Thank you for your appreciation!
If the previously used varnish was not water-based, you can use Sadolin Extra. Sadolin is an alkyd varnish, similar to oil-based paints and is not compatible with water-based products. Do not apply a single thick coat because thick coats crack over time due to stresses in the film. The rule when varnishing/painting is: thinner coats and more coats. In this case apply 2-3 coats of varnish. If the varnish is very thick you can dilute it with white spirit (petrosin, gas) or another petroleum derivative (petrol). There is no need to apply anything else over the varnish.
For cleaning, water jetting would be more effective. Being outside, the dirt accumulated over time is more consistent and yields harder just wiping with a wet cloth. After washing thoroughly with a water jet, wipe with a cloth to remove stubborn dirt. Then leave to soak thoroughly before applying varnish.
Good luck!
I discovered this site as a result of my woodworking related research. I've been experimenting and building stuff for about 6 years, since I moved from my apartment to a house with a yard. I've made a wooden deck connected to the house, I've done the attic, with extra insulation, I've also made furniture. As I've progressed, I've also acquired more powerful power tools, a stationary circular saw, in addition to hand tools, bormasini, etc. Now I plan to purchase a hearse. The reason is as follows. I discovered in my parents' farm, behind a building, a pile of willows 2-3 m long and 10-15 cm diameter, which are about 10 years old. Also, this spring I cut down a plum orchard (100) that were no longer producing, planted in 1990. I saved a few whole ones, they are about 15-20cm diameter and I intend to make something out of them. For now I'm letting them dry out.
My question is about how long would it take to let them dry before cutting splinters from them. The intention is to make shingles and some small furniture, coffee table, shelf, bookcase body. Out of the sails I don't know what to do. They are thin but would work cut into shingles and then glued, still some table top. What advice do you have on how to use them? These are dry, minimum 10 years. Also, I would have to decommission a pig sty made of willows, about 60-70 years old, I think I can salvage some of the 10-12 cm logs, pretty shrinky.
Fine!
Good evening!
Congratulations courage and determination!
As for the plums, the sooner you cut them into planks, the better. It's a high-density wood that's at risk of splitting. It's best to insulate the ends with a solution of aracite, wax, paraffin or diluted primer. The bark should be removed quickly to avoid the risk of insects underneath entering the wood. Plum wood is very beautiful, a real gift for enthusiasts. Before you process it the humidity should be in the range 8-12% if you use it indoors. Otherwise there is a risk of twisting or cracking. I leave below the article about plum wood, maybe it will inspire you.
Acacia wood is highly valued for the strength structure of houses and for outdoor projects (gazebos, pergolas, fences). It is very resistant. Salvaged acacia from pigsties will be very hard and durable. Vapors from the animal waste make the wood so strong that sometimes the axe will jump when you hit it.
A few articles at the end, I hope you find them useful.
Good luck in your new projects!
https://revistadinlemn.ro/2021/11/17/irizatiile-roz-violet-olive-sau-gri-fac-din-lemnul-de-prun-unul-remarcabil/
https://revistadinlemn.ro/2016/11/02/salcamul-cel-puternic-rezistent-si-frumos/
https://revistadinlemn.ro/2017/07/29/cum-construit-foisorul-din-salcam-recuperat-dintr-un-vechi-hambar/
https://revistadinlemn.ro/2022/06/23/ce-solutii-au-tamplarii-pentru-reducerea-craparii-lemnului-la-capetele-bustenilor-grinzilor-si-scandurilor/
https://revistadinlemn.ro/2017/07/04/uscarea-lemnului/
https://revistadinlemn.ro/2021/09/02/am-taiat-nucul-cum-il-usuc-sa-nu-crape/