Mould on wood, what are the effective remedies?

Many people call or write to us to find out how to eliminate mould that appears on wood. The cases are several: some people find mould on wooden beams, others on the decking in their garden, or others on furniture in their homes.

If you have a similar problem and are trying to solve it, you are in the right place.
Let's try to understand together why mould can form on wood and what the most effective remedies are.

To talk about development factors, one must first understand what moulds are.

Moulds are organisms belonging to the Kingdom Fungi. They are generally made up of thin branched filaments called hyphae, which are derived from the germination of spores. The set of hyphae constitutes the so-called mycelium.

Moulds can usually be recognised by their characteristic stinging odour and 'spongy' appearance.

If you want to learn more about mould, we recommend reading our article 'Understanding mould. Causes, problems and solutions'. Here, we just take up some fundamentally important concepts, which allow us to understand when and why mould can proliferate on wood.

Let's see them together.

Fungal spores only germinate in environmental conditions that are suitable for them, and certain factors play a key role in their development:

- temperature.

It is optimal between 20 and 30 °C, but some moulds can also live at higher or lower temperatures. Outside the optimal range the hyphae do not grow but retain viability and may resume growth if conditions become favourable again.

- pH.

Acid pH is optimal (between 4 and 6) but there are species that can also grow in a wider range between 2 and 9.

- relative humidity in the environment.

Spores can grow both inside and outside homes, especially where excess moisture is present.

In addition to these factors we have just listed, spores also need a suitable substrate, i.e. a base that can accommodate them and is adequate to support their growth. Wood, as an organic substance, can be an excellent substrate for moulds! Let us look at some aspects in more detail.

We mentioned earlier that spores only germinate under suitable environmental conditions (which differ from species to species), but you also need a substrate - i.e. a surface - capable of supporting their growth.

In order to grow on wood, a key role is obviously played not only by the ambient humidity but also by the moisture content of the wood: most fungal species require a value of around 30% for spores to germinate.

Keep in mind that seasoned wood normally has a moisture content of around 12 per cent.

The maximum percentage of moisture compatible with growth varies from species to species, but almost none can develop in wood that is completely saturated with water because fungi are mostly aerobic organisms, so they need the oxygen present in the wood in order to proliferate. As
proof of this, think of the wrecks of sailing ships and vessels sunk in the seas...have you ever thought how they maintain themselves for centuries underwater?

Returning to the concept of wood moisture, you should know that it is possible to measure its content using an electrical hygrometer: the analysis is done by inserting two electrodes into the wood and measuring the electrical resistance between the two. It must be borne in mind that high humidity values in wood are not always correlated to a fungal attack in progress, but they certainly indicate an optimal situation for fungi to take root.

An atmospheric factor such as the wind has a dual action, which can be both negative and positive. It can, in fact, transport spores by favouring their contact with the wood, but at the same time the wind can decrease the ambient humidity and water content in the wood, thus inhibiting the proliferation of spores.

The above applies especially to wood exposed outdoors, used for roofs, gazebos, fences, cladding and flooring. However, mould also forms on furniture at home!

As mentioned earlier, spores can grow both inside and outside the home.

In indoor environments, mould can form due to excessive ambient humidity, which can be caused by:

- poor ventilation;

- insufficient thermal insulation;

- water infiltration.

A classic example of poor ventilation is mould forming behind wardrobes.

What happens in this case? When heat generated in closed rooms comes into contact with cold walls, so-called condensation moisture is formed. If this moisture forms behind furniture leaning against the walls, it remains trapped, because not enough air circulates to ensure proper transpiration.

Et voila! There you have the perfect situation for mould.

In most cases the spores germinate in the first layers of wood cells and the hyphae penetrate less than 2 mm, through natural openings or those caused by other organisms. Moulds do not degrade cellulose or lignin - which are the main constituents of wood - but feed only on sugars and simple proteins.

It must be borne in mind that mould can also develop on other wood-related substrates, such as paper, paints, wood stains or glues.

It can sometimes happen that mould forms between wood and the paint treatment. How is this possible? If the wood used is not sufficiently seasoned or has been subjected to rain and has not been allowed to dry well before treatment, it is likely that, over time, mould may develop under the treatment.

Aspergillus, Penicillium, Cladosporium and Gliocadium are the genera that most frequently attack timber in situ, especially in environments where humidity is high, as we have already mentioned.

Pay attention, because often the areas affected by decay are those in direct contact with the masonry or very close to it.

A special case in point is Aureobasidium whose hyphae are able to pierce the varnished wood, leaking outwards, to give rise to dark-coloured fruit.

Since moulds do not degrade cellulose and/or lignin, infested wood does not normally undergo mechanical changes, only colour changes. These types of fungi are in fact defined as chromogenic. This does not detract from the fact that it is better to eliminate the presence of these organisms, not only for aesthetic reasons, but above all to safeguard your health. Indeed, you should know that the spread of fungal spores in the air can cause allergies or asthma.

Having discussed the causes and damage, let us now see how to eliminate the problem.

As we wrote at the beginning of this article, many people write or call us for information on how to get rid of wood mould. The situations are very various.

In the following, we will try to group them by type, but it is good to make a small premise about cleaning systems.

Information on various methods and remedies can be found on the web, but not all of them are really useful. Certainly the ones that are most effective against mould are products based on sodium hypochlorite and those based on hydrogen peroxide.

Although both compounds have specific hazards, if you want to do a do-it-yourself treatment, sodium hypochlorite is certainly more suitable because it is easier to use and more stable, even when used diluted.

It should be remembered that these compounds should be used on wood as little as possible and only when strictly necessary.

Also, remember that you must understand the causes of mould growth: acting on those will prevent it from reforming in the future!

Let us now look at some case studies.

If mould appears on furniture or windows, you can use common bleach containing sodium hypochlorite, available in supermarkets.

Since we are talking about delicate surfaces, it is better to proceed step by step.

We advise you to make an initial test by diluting the bleach you have purchased by 50%.

Then dab the mould stains with a cotton ball or sponge soaked in diluted product. Wait a few minutes and check if the stains have disappeared.

If they have only faded, you can go over the surface a second or third time until you no longer notice the stains. At this point, we recommend rinsing the surface with a dampened sponge to remove hypochlorite residues.

If the mould stains are on the plywood that makes up the back of your furniture, you can give bleach directly pure a try.

Usually when mould stains appear on furniture, they are also present on the walls. So don't forget to remove mould wherever you see it, otherwise you will have only half solved the problem.

In most cases, mould forming on rafters and beams occurs in structures placed outside. There can be various reasons for this: the roof has been exposed to the weather during construction work, or there is water seepage. Or the structure has been poorly designed and water stagnates in some places, or the structure has been placed in a very humid environment.

In general, if the attack is mild and not too extensive, the use of classic laundry bleach may suffice. You can dab the surface with a cotton ball or sponge soaked in the product and after a few minutes the mould stains should disappear. It is better not to rub the surface to avoid spreading any spores that may still be alive.

In the case of more extensive attacks, it is best to use more specific and professional products, such as Lignum Biosel. In this case it is better to entrust the work to a professional.

Remember to be careful when using sodium hypochlorite on tannin-rich wood (e.g. chestnut or oak): it is better to carry out preliminary cleaning tests, otherwise you risk making the situation worse rather than better. This is due to the fact that tannin can surface and stain the wood further, so you may find yourself having to deal with both mould and tannin stains.

At this point you may be asking yourself: "Does this also apply to beams and beams already treated with wood stains or varnishes? "

In cases like this it is always a good idea to carry out small tests: start with a diluted version of the product you have chosen to use. If possible, it is better to do the test on an inconspicuous portion of the surface. This way you can check the degree of resistance of the treatment, which may dull or lighten on contact with the sodium hypochlorite.

We always recommend contacting the person who supplied you with the treated wood or the manufacturer of the paint product, because they are better able than anyone else (or at least should be able!) to give you the best technical assistance.

Beam attacked by mould and cleaned with Lignum Biosel

In the case of impregnated or painted wood, remember that mould can form both on the treatment and between the wood and the treatment. In the latter case, it may not be easy to remove the fungi as the applied protective coating acts as a barrier between the wood and the external environment, inhibiting the effectiveness of a chemical treatment. If you find yourself in such a situation, it may be necessary to give the surface a light sanding to remove the paint product and only carry out the removal of the mould afterwards.

If you have a wooden floor exposed outdoors and it has not been treated with protective agents, you can use pure or diluted bleach depending on the extent of the fungal attack. The point made earlier about not rubbing the surface still applies: it is better to dab to avoid the spread of spores as much as possible.

If the floor is painted, oiled or impregnated, it is a good idea to carry out small tests to check the reaction of the treatment with the chosen product.

If mould appears on interior wooden floors, great care must be taken when using bleach. It is best to do a few small initial tests with a very diluted amount.

Parquet stained by sodium hypochlorite: the oval halo was caused by a bucket that contained bleach and was wet at the bottom

This is probably the simplest case. If mould appears on the raw wood, just use bleach (if the problem is very small) or a professional product - in the case of a more extensive attack.
After cleaning as described above, you may also decide to give it a light sanding to remove any residual (dead) hyphae that may have remained in the wood.

If you are planning to give the wood a protective treatment at a later date, be sure that the wood is well dried and seasoned and no suspicious stains remain on the surface.

Now that we have given you guidance on what you can do, it is good to also tell you what not to do when mould is present.

People who write to us tell us about their attempts at mould removal, made before they contacted us. In most cases they used the wrong products and techniques.

Let us see together what not to do:

• The surface must not be brushed. Brushing only spreads the fungal spores, without killing them, increasing the possibility of their proliferation.

• It should not be sanded. The reason is the same as in the previous point: this also spreads the spores. This case is perhaps even worse than the first, because sanding the surface abrades the fibre, helping the spores to penetrate into the wood.

• Vinegar (or acetic acid) should not be used. Vinegar has no fungicidal function and above all, since it has an acidic pH between 2.3 and 2.9, it changes the pH of the surface in favour of moulds: as we said before, fungi are predominantly acidophilic, since they prefer acidic environments.

• Simple detergents should not be used. A detergent has the function of cleaning a surface, but if it is not specifically effective against mould, its use is almost useless.

These and other DIY remedies often found on the web are in most cases useless and sometimes even counterproductive. At first glance it may seem that the moulds are gone, but they will return as soon as the environmental situation is suitable for them.

Speaking of the environmental situation... we will never tire of reminding ourselves that the root causes must be eliminated.

Here are some useful hints for avoiding the formation of mould:

• ensure that external walls, foundations and attics are insulated and well ventilated;

• maintain a humidity inside the home of less than 50%;

• do not leave clothes hanging out to dry for a long time in closed, poorly ventilated rooms;

• Try to eliminate condensation and, if condensation is present, ventilate the room frequently;

• properly maintain humidifiers, air conditioners and mechanical ventilation systems;

• as far as possible, keep furniture away from walls;

• protect wood exposed to the weather with suitable products.

Bibliography

• Degrado biotico del legno, M. Dalprà, Daniela Piazza Editore

• La biologia vegetale per i beni culturali - Biodeterioramento e conservazione, G. Caneva, M.P. Nugari, O. Salvadori, Nardini Editore

Did you find this article interesting? Share it on social media!