DIY - Houses - Wood species

13 simple rules for outdoor resistant timber construction

Regarding the protection of wood exposed to the outside I wrote an article with most frequently asked questions received from you. Being a good time for activities and small constructions outside, the questions keep coming and diversifying. We're glad you trust us and when we see that the advice we've given you has been helpful. So, I thought it would be much better to make a sort of "rule book" of outdoor woodworking. Because it's not just the products and treatments applied to the wood that are important. Sustainable work starts with the choice of wood type. Below are a number of important rules to make wood constructions as resistant as possible to humidity, temperature variations, various types of rainfall, insects, moulds and the burning rays of the sun.

1. Choose wood that has a better outdoor resistance due to its structure

Not all wood species have the same outdoor resistance. Density, porosity, the presence of resins or tannins influence the resistance of wood to water or to attack by insects and moulds.

Soft, tanned wood, but also hard woods with very large pores and many pores are not very resistant to moisture. They absorb water easily which can lead to wood rot.

Very good outdoor resistance of wood containing resins (resinous in general, larch and spruce in particular), oils (teak) or tannins (acaciaSpecies with very good resistance to rotting are chestnut and ulmul.

timber construction
camphor fence
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2. Do not use very dry or very wet wood

Wood moisture used outdoors is different from that for indoors. The moisture balance of the wood depends on the humidity of the area where the wood is used. By area is meant a large geographical area. In Romania the moisture content is 8-12% (6% for resonance musical instruments) for wood used indoors and 14-18% for wood used outdoors.

Wood that is too dry when used outdoors will absorb moisture from the environment and dimensional variations and even cracks will occur due to internal stresses. If it is too wet, it will quickly lose moisture and shrink in size, resulting in the so-called 'play' of the fitted boards. In addition, warping and cracking can occur.

3. Use only D3 or D4 adhesive, specially formulated for outdoor conditions

If you do adhesive joints or solid wood panel construction (hives) always uses adhesive specially formulated for outdoor use. However, you need to be careful when choosing adhesives. There are adhesives that can be used outdoors, but for bonding protected objects. That means they stay under awnings and are not directly under the sun or in the rain. These are D3 outdoor adhesives. To avoid problems it is best to use D4 adhesives.

Never use D1 or D2 adhesives outdoors because they are not moisture resistant and the joints will come apart after a very short time.

4. If possible, do not put the wood directly on the ground

The area most exposed to decay for a wood is the area immediately above the ground. If you pull out a stake that has been in the ground for a long time, you will see that the area that is thinnest and most affected is that area, not the one that has been in the ground. This is the oxidation that takes place due to oxygen in the air, oxidation greatly aided by moisture (water with salts works like an electrolyte).

If the work doesn't require you to put the wood in or on the ground leave a small gap and use other materials (metal, plastic) to connect to the ground. If you are using stakes or wooden posts that have to be put into the ground, protect them with metal or plastic up to just above the surface of the ground.

timber construction
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5. Protect wood against insects, fungi and moulds

Wood protected in this way will resist better without blackening over time (from mould). Sometimes the protective substances are contained in lacquer (2 in 1 or 3 in 1 products). But there are also separate products for treatment. In general, they are concentrated products that are diluted with water. Dilution is different depending on the type of wood, harder or softer. For hardwood (chestnut) the dilution is lower because the wood absorbs less and needs to be protected. Softwoods (resinous) absorb more and the solution can be diluted more. Dilution ratios are specified by the manufacturer.

Wooden objects that have been attacked by mould and have blackened can be restored to their original condition using laundry bleach (sodium hypochlorite bleach). It is a simple, cheap and handy solution.

6. Do not use colourless transparent materials. Colouring is not only aesthetic in this case

There are many who want to keep the natural colour of the wood and turn to colourless transparent materials. These materials do not protect against UV radiation and the wood will darken over time. The varnish will also suffer because there will be nothing to reflect the sun's rays, the temperature it can withstand will be much higher and it will deteriorate faster.

This doesn't mean you have to use paints that completely cover the wood. Although, if we establish a hierarchy, paints are the most resistant. There is no varnish on the market that claims to resist outdoors and that does not contain a small amount of pigment. Transparent does not mean colourless. There are clear lacquers or varnishes that are honey-coloured, a very light colour that changes the colour of the natural wood a little, but protects much better.

Wood changes colour over time anyway. You will not be able to keep the same colour as it is when you do the work (cutting, gelling). It is the transformation of some of the wood's inner compounds under the action of light and in contact with oxygen in the air. Even furniture in the house, protected with varnish, changes colour over time. We don't notice because it is coloured or because it changes completely. But if we put a book on a piece of resin furniture finished with clear varnish and leave it for a few months, we can easily see the colour change in the end. So, rather than having it change as it pleases, it's better to control the change yourself by using lightly coloured materials for protection that don't colour very much but prolong the life of the finish.

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7. Do not apply thick layers for better protection. You will get the opposite effect

There is an idea that if you apply several coats of varnish the wood will be better protected and water will not get to it. Not the best idea. Thick layers become stiffer and tend to crack over time. In addition, solid wood has natural dimensional variation with moisture variation. A stiff coating will crack much more quickly and instead of a more durable protection over time you will get one with a shorter lifespan.

Indeed there are some species, such as oak or chestnut, that have larger pores and need more layers to cover the pores and reduce moisture absorption. More layers means 3 and not very thick layers. It's good to know that 2-3 thin coats of varnish applied with drying in between protects better than a single coat of the same thickness with 3.

8. Protect the wood on all sides of the object, inside and out

For the best protection apply varnish or oil to all sides of the piece of wood. Through unprotected areas, even if not visible, the wood absorbs moisture. You will be surprised if the wood warps or swells. If you are curious to see what has happened you will discover that water from the rain has found its way into the hidden areas. So can mould.

9. Use for coverage elastic materials made specifically for outdoor use or non-film forming products - oil or wax

All of these products are good for outdoor use, only some are more suitable for certain jobs. It's good to use finishing systems for windows and doors that include tinted, mould-resistant impregnants and elastic primers and varnishes that protect against weathering and UV radiation.

For fences, sagebrush, pergolas, it is simpler and more useful to use oils or wax impregnations. The oil goes deep into the wood and protects it from moisture and the wax makes a thin film on the surface on which water runs off. In these cases varnish protection does not seem to me a good choice. Such protection needs maintenance to last as the manufacturers say, 10-12 years. If maintenance is not done - and let's face it, we don't think much about fence maintenance methods - the pond will crack, water will get under it and scour it. It will look bad and we will want to redo it. To redo it we'll have to remove everything left on the wood and redo the finish from the beginning. It's not very simple!

With oil or wax the resistance over time is less than with varnish. Resistance depends on how rain washed the wood will be. Because that is what makes the finish less resistant. Oil and wax are rain washed. Nothing is flaking, nothing needs to be removed to refinish. When you find that the wood no longer has the sheen given by the oil or wax and is dry looking, apply another coat of oil or wax. These are the reasons why I highly recommend oil protection: good moisture protection and ease of refinishing.

Exterior varnishes and primers are elastic because wood varies in size with moisture variation and the varnish must follow this variation without cracking. That's why you shouldn't use indoor products, even if you've seen that they're hard and tough. They are not elastic and crack very quickly.

timber construction
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10. Do not compromise on the quality of the materials and follow the instructions when applying the materials. Even good materials don't work wonders

I'm not saying you have to buy the most expensive products. I'm saying you should get products that are clearly made for outdoors, where the manufacturer writes as much information as possible on the label, says how they should be used, whether they are in a system with other products (you need other products to get the best result).

And since I brought up the subject of how to use it: if you want good results you have to follow the directions for use. No matter how good a product is and how long the manufacturer tells you it will last, if you don't respect the dilution, drying time, application method, sanding between coats, quantity and number of coats applied and other such recommendations, the protection will certainly not last the maximum period the manufacturer talks about. They will last less, you will be disappointed and you will also lose trust in the brand through no fault of your own. There are no miracle materials but good or very good quality materials applied correctly.

One more thing to know. Outdoor resistance depends on many factors and they can be different on the same construction. For example, windows that are on the side of the house where the sun is more present during the day or where the wind blows and snow deposits in winter will resist less than those in more protected areas, even if they are protected with the same materials and the same technology has been applied. This is also true for other timber constructions.

11. Don't start from the premise that you have painted for a lifetime. Without proper maintenance no product will last 10-15 years.

There is no 10-15 years resistance without maintenance. Using protective materials such as oils or maintenance waxes 2-4 years after varnish application can make it last even longer. In the case of windows, if you find the time when you wash your windows for Christmas and Easter cleaning, to run a cloth soaked in maintenance oil over the outer window frames at the end you will find that they will last very well for years without cracking or peeling.

timber construction
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12. Works such as doors, window, shutters, terase they last much better if protected by awnings

UV radiation is the enemy of finishes. And when accompanied by high temperatures it can significantly reduce the durability of some varnishes. So if you can, protect doors, windows and patios with awnings or roofs. Keeping the sun's rays away from the finished surface as much as possible prolongs the life of that finish. That's why I always say that the resistance in fences is much reduced. They are continuously in the sun and this becomes very noticeable over time.

13. The golden rule of timber construction - let the air circulate

It is the rule that Christof de la Techniques considers it the most important. He says that when working with wood, the cladding or construction should be done so that air can circulate. If for some reason moisture gets into the wood, the air circulation will dry it out and prevent mould, insects or even rot. That's why wood cladding in buildings is done in such a way that there is a small space between the cladding and the front of the house. Air will circulate continuously and the wood will not rot.

I'm not saying I've touched all the sensitive points, and perhaps more rules will be added over time. But if you follow these you will certainly be happy with the protection you get for the wood you have used outside.

timber construction
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About the author

Mihaela Radu

Mihaela Radu is a chemical engineer but has a great passion for wood. She has been working in the field for more than 20 years, wood finishing being what defined her during this period. She gained experience working in a research institute, in her own company, as well as in a multinational. She wants to continuously share her experience with those who have the same passion - and more.


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  • Good evening,

    I read your article on the "guide" of rules on outdoor wood maintenance and I can say it is the best I have read on the internet. However I would have a few questions, I have an exterior oak and fir wood gate that I varnished last year. Although I kept looking to get the best varnish I can say I am very disappointed, after only a year the varnish has mostly faded from the wood. Now I'm wondering how to remove the old varnish! Mechanically, by sanding or is there a substance that will remove this varnish? After this step, I was thinking to give the wood with oil or wax, but I don't know if it's good because the wood has been varnished before? What do you recommend? I don't want to give the porch any colour, I want to keep the natural colour of the wood as much as possible.

    Dan vlad

    • Hello.
      Thanks for your appreciation.
      I would pickle the remaining lacquer. I used Deconol some time ago, I think it was from Policolor. It did the job very well on a difficult surface (with sculpting). But it's not the only stripper out there. Apply with a brush, let it work for a while then remove the soaked varnish with a spackle. Finally "wash" the surface with thinner (wipe with a rag with thinner) and sand. In this way you remove all traces of varnish and can apply oil. If the varnish is gone so quickly, it is clear that the varnish did not penetrate the wood very well, so there is no risk of varnish remaining after the above interventions.
      For finishing use oil-based varnish or wax impregnation. The products should be lightly coloured because otherwise they do not withstand UV radiation. But there are also light colours that change the natural colour of the wood very little.
      Good luck!

      • Hello,

        I have also used Owatrol stripper with great success, you can try it. I don't want to advertise it - that's what I used, that's what I recommend 🙂

  • To protect a wooden countertop in the bathroom, what would be the best solution?It would be a countertop on which the sink is placed.And what kind of wood would be better?Thank you.

    • Good evening!
      There are 2 variants. Oil, which makes it very resistant to moisture, but has low shock resistance (depending on how hard the wood is).
      The second option is a moisture-resistant varnish - polyurethane, water-repellent varnish.
      In both cases the wood must be protected on all sides and in the area cut out for the washbasin.
      Use a hardwood (oak, beech, ash) because it absorbs less moisture.
      All the best!

      Don't forget to subscribe to the printed Wood Magazine! For only 58 lei/year you can find out news in the field, discover craft ideas or trade secrets. We remind you that the content in the printed magazine is different from the one on the website. Details in the link below. 
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  • Hello
    Thank you for all the articles written about wood. Many times I have turned to your site to enlighten me in some problem related to our wooden constructions and especially ways to protect them satisfactorily. I have seen that you recommend protecting wood on the outside more with oils or wax impregnations.
    I know about the linseed oil protection (spicy or not?), but a carpenter-sculptor told me about the very good protection that mineral oils give. He told me that he uses Castrol motor oils... and others in the same range, with success. What do you recommend? Linseed oil or mineral, synthetic...
    Now we are building a fence outside made of planks and pine cabinets; but we will also have pine doors outside, pine furniture outside (benches...).

    • Good evening!
      Thank you too for following us!
      Regardless of the oil used, it's all about moisture protection. Oils penetrate the wood and protect it against water from precipitation or moisture from the air.Mineral oils can also be used, but I think they are more expensive because they go through different purification processes. In the past, burnt engine oil was used to protect wooden fences or fittings, or unpurified products (tar, tar) were used. Tar was used to impregnate wooden railway sleepers. Mineral oils are derived from petroleum.
      Linseed oil has been used since ancient times to protect wood, especially interior wood because it was odourless (unlike petroleum variants). Its problem is that it dries very slowly (4-5 days). In order to dry faster it has to be boiled or treated with chemicals (sodium hydroxide).
      In conclusion, mineral oils can be used to protect wood. For outdoor use, for fencing, a little pigment staining is needed. Staining is intended to protect against UV radiation. Colourless oil only protects against moisture.
      Pure mineral oil is used to protect wooden dishes that come into contact with food (you have a link below).
      All the best!

    • Hello!
      In order to have as little moisture as possible the wood should have been cut in late autumn or winter (November-January).
      After it is cleaned, the cutting area is coated with paraffin or wax so that the water comes out slowly to avoid creating tensions in the wood that lead to cracks. It should then be stored in an unheated space with constant temperature and air circulation (cellar, shed or attic). In the house, because of the high temperature and dry air, water tends to escape more quickly and cracks can occur. Depending on the thickness of the stems, drying can take several months.
      Natural drying is recommended for birch. If it is force-dried or treated with various stabilizing products (Pentacryl) the wood and bark change colour and turn yellow.
      All the best!

    • I neglected to mention that once it has reached a humidity of 8-12%, it is best to apply a very thin coat of clear varnish to make maintenance easier. You can use water-based varnish or polyurethane varnish sprays. If the tree is cut in winter and is not very thick, it can be used for decoration in 2-3 months.
      All the best!

  • Hello, thank you so much for the extremely useful articles you post and for the thoughtfulness with which you answer the questions in the comments (very useful answers too). Your passion for wood inspires.

    I have read your recommendations for exterior wood protection with professional products (ICA/Lomilux, Sirca, etc.) or Sadolin Tinova. I asked a Lomilux representative about durability and was surprised to learn that the outdoor products offer approx. 3 years. Given this, I was wondering why a professional product like ICA would be more suitable than any other commercially available lacquer that has similar durability but is perhaps cheaper. Does it offer other benefits or is the durability on average longer?

    Thank you in advance for your answer! I'm in great doubt about what to buy to paint my pine fence.


    • Hello!
      Thanks for your appreciation!
      The strength of the exterior finish also depends on the position of the object. The fence is most affected, standing directly in the rain and sun. For this reason the resistance of the fence finish is the lowest. Probably this was the reason why the ICA representative will tell you that it will last only 3 years. From my information ICA has oseries of outdoor products with very good resistance, over 10 years. But like I said, because of exposure, the durability of the same product can be different. The finish of the windows of the same house yield differently, if one faces south and others face north.
      As far as the difference between professional rpoduse, such as ICA and DIY, it really exists. However, in this system, the way the wood is prepared and the way it is applied is also important. That's why I recommend specially designed products for DIY projects. Professional products, used incorrectly, can have lower resistance than DIY products.
      As far as fencing is concerned, I recommend oil-based or water-based waxes for fencing (I believe the ICA representative recommended such a product). They are products with lower resistance over time, but refinishing is very easy, just applying another coat, without sanding or other kind of preparation. Re-application should be done every 2-3 years or when the wood no longer has that silky sheen given by oil or wax and gets a dry look.
      I hope this has been helpful.
      All the best!

  • Good evening Mrs Mihaela,
    Please, I am faced with the following situation: I need to extend the top edge of a balcony outside by 12cm in order to reach the level of the railing to close it with a double glazing (the top edge is made of reinforced concrete.The balcony is exposed to strong sun, rain or snow, depending on the season. I was thinking of a wooden cabinet (beam, I don't know the exact term) fixed with studs to the upper part of the balcony and the area above, outside of the wood covered with sheet metal. What kind of wood and what technical and maintenance solutions could you recommend?
    Thank you very much and congratulations for what you do.

    • Hello!
      Thanks for your appreciation!
      In my opinion, the most suitable would be a pine (resinous) beam. For greater stability use a glued beam (several layers of glued wood) for the outside (available in building material centres). Very strong wood for the outside is acacia and oak. But they are more expensive and harder to find.
      If the beam is covered with board, a linseed oil protection is very good. If it is not coated it will be hard to maintain because whatever finish you use needs maintenance and even refinishing after a few years.
      To avoid problems, the technical solution must be such that the wood is slightly inclined to prevent water from accumulating and puddling. The wood must also be mounted in such a way as to allow air to circulate. This way, if it gets wet or condensation occurs, it dries without any consequences.
      All the best!

  • Hello. I would need some advice as quickly as possible if possible please. I'm going to buy wood for roof construction, beams, planks, laths, etc. From where I buy it is stored outside in the sun. The question would be when is it good to buy it? The roof will be done by August at the latest but I would buy it now. What do you recommend?

  • Hello,

    I want to renovate the ground floor of my house, the ground floor dating from 1930. The house is made of load-bearing masonry, on a concrete foundation (about 120 cm), probably without a capillary breaking layer. Soil : sandy clay. On the ground floor, the flooring (in fish scales) is fixed on wooden beams, on beaten earth. In areas where the flooring has been covered with solid furniture for a long time, the beams have rotted and the flooring has sagged.
    I would like to redo exactly the same system, but to have the beams on a 10 cm layer of monogranular gravel.
    Could you please advise me:
    What kind of wood to make the beams from
    Which treatment should be applied (anti humidity, insects)
    To provide local asphalt cardboard under the beams, is that enough? If enough continuous sheeting what type of sheeting lets vapour/moisture circulate ( permeable ) - a geotextile ?
    On the joists I want to put a tri-ply oak parquet (I can't use the old parquet).
    For this I have to beat the floor as a support and then I would like to float the floor on a layer of cork (which is vapour permeable from what I read on the data sheet). Does this new material complex remain permeable enough? The principle of the old floor being that any moisture is evacuated diffusely through this permeable ventilated wooden floor.
    Can I use OSB boards fixed on beams as support instead of the shower, or do I risk creating a vapour barrier that will not let the moisture in the soil ventilate? The floor and the support will be placed at a distance of about 1 cm from the wall, all around the perimeter, which will be covered with skirting. I think this will allow for a circulation of air currents.

    Recap layers:
    1. Pamint
    2. 10 cm monogranular gravel
    3. Asphalted cardboard under beams or permeable foil everywhere
    4. Wooden beams at 40 cm centres
    5. Flooring or OSB tiles
    6. Sound and thermal insulation made of cork (min. 4 mm)
    7. Oak parquet, floating

    Thank you for any advice you can give me!

  • Good evening,
    I have 20 sqm outdoor terrace but covered, made of 50mm thick red pine wood, exposed to the sun and partly rain when it's windy. what treatment would be indicated? Thank you !


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