How to choose wood, construction solution and protective materials for outdoor resistant projects

For a novice woodworker or beginner, there is not much difference between an interior and exterior project, at most the lacquer used for final protection. Between a gate or an interior door the only differences they can think of are the pattern and the finesse of the workmanship. And yet, from the wood to the construction solution and from the type of adhesive to the type of varnish, everything is different. The choice of materials and solutions is very important because it depends on them whether the gate will last 3 months or just need to be repainted after 3 years.

Why it is important to choose wood for outdoors

Wood is sensitive to moisture, fungi, micro-organisms, insects, and all of them are outdoors. But some species are hardier than others and that's why it's best to choose those with more outdoor resistance. The outdoor resistance of wood is called sustainability. In general, durability refers to resistance to all external aggressors, but there are species that, for example, are resistant to moisture but have poor resistance to insects. (douglas).

In terms of humidity, both wood and external humidity are important. The moisture content of wood used outdoors is different from that of indoor wood. For wood used in the manufacture of indoor furniture the moisture content is 8-12% and for outdoor wood 14-16%, sometimes even higher. The moisture content of wood is in equilibrium with the humidity of the environment. Too low a moisture content of wood used outdoors means more water absorption to reach equilibrium, and the result will be greater dimensional variation. Too much moisture creates the conditions for fungi and micro-organisms to grow, which will lead to decay more quickly.

It is the high humidity that creates the conditions for insects to thrive. Very wet wood is easier to eaten and will be easily attacked by insects. That's why wood in contact with wet soil, standing in puddling water, will not last long.

Species with good outdoor resistance are acacia, oak, chestnut, larch, douglas fir, teak, iroko. Durability is not related to hardness, a good example being beech, a hard species with strength but no durability. It is not recommended for outdoor projects, like birch or poplar. More information on outdoor hardy species can be found at here.

The right construction solution will make the difference between a resilient project and one that will fall apart after the first rain

The elements to be considered when choosing the construction solution are the dimensional variation of the wood according to the external humidity, water drainage and air circulation.

Wood varies dimensionally with external humidity, i.e. swells when humidity rises or when sitting directly in water and shrinks when the outside humidity is very low. Being a non-homogeneous material, the dimensional variation is different depending on several factors: species, tree maturity, felling method. At the same variation in outdoor humidity, some species moves more and others less. This is another factor to consider when choosing the type of wood. A good choice is laminated wood. Lamination reduces movement making the element more stable. In this case too, the species chosen for lamination must be resistant to the outside.

Very important is the dimensional variation depending on the cut. Unlike metals, wood does not vary significantly in length. It does, however, vary noticeably and almost equally in percentage terms in radial and tangential direction (tangential to the annual ring). That is, a piece of wood can become loose enough to force a clamp and unclamp it. The construction solution must be chosen to allow such movements of the timber without endangering the construction.

It is water that can destroy wood. Do not do immediately but only if it sits for a longer period, if it puddles on its surface. This is why the construction solution must allow water to drain and not accumulate. It is preferable that the chosen solutions protect the wood from the raindrops bouncing off the ground and the contact area between wood and ground.

Another important rule when choosing the construction solution is to let the air circulate. Water does not destroy wood immediately, proof that we can clean wood with a water jet without anything bad happening. But if it stagnates, it gets into the wood and with it micro-organisms, fungi and insects that destroy it. Air has to circulate for the water to dry. Construction solutions should not include very narrow spaces or wood laid directly against the wall. Air must circulate and ventilate all areas wet from rain, fog or snow.

Find here the most important rules for outdoor resistant construction.

Fastening systems/adhesives

The fastening systems are also very important. If metal systems are used, it goes without saying that they must not rust in contact with water, but neither must they be attacked by the tannin in the wood and thus be weakened. Oak wood, which is very strong on the outside, is known for its tannin which attacks the metal. That is why stainless steel systems are recommended for fastening various oak elements.

There is a lot of use, even outdoors, of gluing wooden elements. Like the wood chosen, the adhesive must be suitable for the exterior. Depending on moisture resistance, adhesives are classified from 1 to 4 (D1-D4), D1 being the least resistant and D4 the most resistant. For exterior use D4 adhesives are recommended, D3 only if the construction is protected by awnings or other such systems. There are different types of adhesives that can be used outdoors, the strongest being D4 polyurethane or aracite (PVA polyvinyl adhesive).

Without a strong coating on the outside, the wood will not be protected and the construction will deteriorate in a short time

To prolong the life of wood used outdoors, it is recommended to protect it with oil systems, varnishes or paints resistant to moisture and UV rays. As mentioned above, there are very hardy species that can stay outside for quite a long time without protection without rotting. There are also very hardy types of wood such as Kebony or Accoyaobtained by a special industrial impregnation with various substances. Ordinary impregnation, without structural modification of the wood, is quite widely used, as wood treated in this way is much more resistant than wood coated with varnish or paint. But even these types of impregnated wood need surface protection.

Wood can be protected with oils, lacquers, varnishes or paints. Of the natural oils, the most resistant is tung oil. Oil of Flax is not a very good choice because it is quickly washed away. Depending on how much rain falls in the area, it may need to be replanted every 3 months. A good choice is blends of the two oils because they provide protection for the wood, both inside and out.

Lacquers, varnishes and paints must be specially formulated for exterior use. They must be elastic to allow dimensional changes in the wood without cracking and contain a small percentage of pigment to ensure resistance to UV radiation. The content of a wetting agent also protects against the sun's rays. In terms of sun resistance, the most resistant are matt paints and the least resistant are transparent gloss varnishes.

Find out more about the most suitable outdoor systems here.

Maintenance

Any building or outdoor object needs maintenance. Even if we are told that the applied varnish will last 10 years, this does not mean that 10 years means we should forget about the existence of that construction. Outdoor durability depends on too many factors to take any such information at face value. To prolong the life of finished objects they should be inspected regularly. There are, for example, maintenance products that can be used for exterior wooden windows or doors. These are easy to apply and have the effect of prolonging the life of the finish.

The regular inspection of the exterior construction is also aimed at identifying possible faults in the construction so that they can be rectified. In this way we can discover early on if water is puddling, accumulating in certain areas, if parts are failing to dry out. The appearance of black spots in those places shows that dampness is persistent and mould has already set in. Left like this it will get into the wood and destroy it from the inside.

I hope you find the information useful. As usual, additions are welcome. And if you have any questions or queries, please leave them in the space below. I'm sure I'll reply.