Mold on wood occurs when moisture accumulates in the wood fibers beyond approximately 19–20% for about a week, combined with a deteriorated paint film that can no longer repel water. When wood, which contains cellulose as food and has porous structures that retain moisture, encounters this level of humidity in the hot climate of Thailand, spores naturally floating in the air will begin to germinate and turn into black mold, white mold, or green algae within a few days. Therefore, an effective solution must involve three simultaneous actions: 1. Stop the source of moisture 2. Eliminate the mold down to the wood fibers 3. Apply a protective coating on all six sides. Simply wiping the surface will always result in the mold returning.
What causes mold on wood
Mold on wood is caused by fungal spores, which are already naturally present in the air, landing on wood surfaces that have sufficiently high moisture and available food. These spores are everywhere and cannot be completely eliminated from the air. What we can truly control is only the “moisture.”
Wood is particularly at risk because it combines two properties in one: it has porous structures that absorb water and contains cellulose, which is food for fungi. Unlike concrete, steel, or tiles that have only one of these properties, fungi can adhere to wood more quickly and penetrate deeper.
5 main causes of mold on wood
| Cause | How it occurs | Common areas |
|---|---|---|
| Moisture accumulation in the wood | Wood absorbs moisture from the air or direct contact with water and cannot release it in time. | Wood walls on the rain-exposed side, wood under short eaves |
| Deterioration of the original paint film | Paint has expired, cracked, flaked, or peeled, allowing water to seep into the wood. | Outdoor woodwork that hasn’t been repainted for over 3–5 years |
| Not applying wood preservative before painting | Painting over bare wood without a primer to prevent mold, termites, or insects | New woodwork that is rushed |
| Painting wood not covering all sides completely | Apply only to visible areas, leaving out the back, corners, and ends of the wood. | Lattice, wall panel, wooden doors and windows |
| Poor ventilation | Stagnant air, trapped moisture, no sunlight | Under the stairs, behind cabinets not flush against the wall, bathroom, storage room |
What is the difference between black mold, white mold, and algae?
Black mold and white mold are “fungi” that can infiltrate the wood, while algae are “green algae” that cling to the surface and require sunlight as well. Distinguishing them is important because the severity and solutions differ.
| Characteristics | Mold / White Mold (Mold) | Algae (Algae) | Wood Decay from Fungi (Wood Decay) |
|---|---|---|---|
| Color and Appearance | Black, gray, white, or greenish-brown spots are fluffy clusters | Green stains, dark green, slippery when wet | Dark-colored wood, soft, collapses when pressed, or breaks into chunks |
| Requires sunlight | Does not thrive in dark, damp places. | Requires sunlight along with moisture. | Does not require. |
| Moisture content in wood required. | Approximately 16–19% or higher. | Repeated wet surfaces. | More than 25–28%. |
| Affects the wood structure. | Does not affect load-bearing capacity yet (affects surface and aesthetics). | Does not directly affect but retains moisture. | Directly affects, causing wood to soften and decay. |
| Can it be cleaned off? | Yes, if it is still on the paint film. | Yes. | No, the affected wood section must be replaced. |
The key point is that surface fungi feed on the wood surface, not on the internal fibers, so it does not reduce the load-bearing capacity of the structural wood. This means that if detected at the surface fungal stage, it can still be cleaned and the wood can be kept for further use. However, if left until it reaches the decayed wood stage, the wood must be replaced.
What level is the fungus on the wood? Assess 3 levels before proceeding.
Do not use the same method for every case. Test simply first by using a damp cloth to gently wipe the area and see if the stain comes off, and use a sharp metal object or screwdriver to gently poke to check if the wood is still solid.
| Level | Visible Symptoms | Testing | Solution |
|---|---|---|---|
| Level 1: Surface Mold | Small black spots, light green stains that fade when wiped | Wipes off easily, wood remains hard | Clean, control humidity |
| Level 2: Mold Embedded in Wood | Stains are firmly embedded, cannot be wiped off, color fades unevenly | Cannot be wiped off, wood remains hard | Sand the surface of the wood, use anti-fungal solution and apply protective coating |
| Level 3: Rotting Wood | Wood is soft, collapses, dents when pressed, strong musty odor | Punctures easily or collapses | Replace that section of wood and protect the new wood |
If the mold-affected area exceeds approximately 1 square meter or if there are individuals in the home with asthma, allergies, small children, or elderly people, it is advisable to consider hiring a professional rather than doing it yourself.
Mold Removal Method for Wood Level 1: Surface Mold
If mold is still on the surface of the paint film, wipe it off with a damp cloth; do not use a dry cloth under any circumstances. Because wiping dry will cause spores to spread and attach to other areas, creating a new problem.
- Wear protective gear Rubber gloves, safety goggles, and N95 mask
- Mix cleaning solution Diluted distilled white vinegar or a mixture of water with a mild cleaning solution
- Wipe with a damp cloth Gently wipe each section, changing the cloth frequently. Do not wipe repeatedly in the same area with a dirty cloth.
- Follow up with a clean cloth and allow to dry completely. Open a fan or window to help ventilate the area.
- Recheck in 2–4 weeks. If it returns, it means the source of moisture has not been addressed.
Why is it not recommended to pour bleach on wood?
Bleach (chlorine) can lighten dark stains but does not address the root problem. Because the water content, which makes up about 90% of bleach, will seep into the wood, while chlorine remains on the surface. The result is that the stains appear lighter, but the moisture in the wood increases, and mold often returns stronger than before. Therefore, effectively removing mold from wood is not about trying to kill all the spores but rather controlling the moisture to be below the level at which mold can grow. This should be combined with using products specifically designed to penetrate deeply into the wood.
| Method | Effective Against | Limitations |
|---|---|---|
| Diluted distilled white vinegar | Surface stains in the early stages | Does not penetrate deeply into the wood |
| Bleach | Lightens stains | Increases moisture in the wood, does not address the root cause and may cause discoloration of the wood |
| Fungicide specifically for wood | Mold embedded in wood | You must sand the wood surface first to achieve the best results. |
How to deal with mold on wood, Level 2: Mold embedded deep in the wood (6 steps)
If the stain is so stubborn that it cannot be wiped off, it means the mold has spread into the wood. You need to sand the wood surface and use a complete product system. Painting over without removing the mold first will cause the mold to push the new paint film to bubble and peel within a few months.
- Step 1: Ensure safety and stop the source of water
Find out where the water is coming from, such as a leaking pipe, leaking roof, rain splash, standing water on the floor, or air conditioning drip. If you do not stop the water first, all subsequent steps will only buy time. Then, wear gloves, goggles, and a mask. - Step 2: Sand the wood surface
Sand off the old paint film and the wood surface that shows signs of mold until the wood is clean. While sanding, lightly mist with water to reduce the spread of spores and wood dust. Use coarse sandpaper first, then progress to finer grades. - Step 3: Clean and wait for complete drying
Dust or vacuum away all the powder, then let the surface dry completely. Do not rush this step. If the wood is still damp and you apply a coating, it will trap moisture under the paint film. During the rainy season in Thailand, it may take longer than usual, or you can use a fan to help dry it. - Step 4: Remove any remaining mold
Apply Beger Mold Free fungicide thoroughly to the area and let it dry for at least 3 hours without rinsing off. This step addresses spores that are still hiding in the wood fibers, which cannot be completely removed by sanding alone. - Step 5: Apply termite and mold prevention products on all 6 sides
Apply BegerDrite Termite and Mold Prevention product in 2 coats, allowing a 1–2 hour interval between coats, ensuring to cover all 6 sides of the wood, including the back, corners, and cut ends. -
Step 6: Paint or coat to seal
After prevention, apply wood stain or polyurethane to create an outer waterproof film. The paint film is the first line of defense, while the prevention solution is the last line. Both are necessary for complete protection.
For softwood or woodwork that requires special protection, you can add a base coat to preserve the wood with:
- Beger Wood Preservative B-6900
- BegerDrite Wood Preservative Water Base (water-based formula)
- BegerDrite Wood Preservative (oil-based formula)
Read more: Solve deep-rooted mold and algae problems with deep-penetrating fungicide.
How to prevent algae on wooden walls from returning
Long-term mold prevention on wood is to keep the moisture content in the wood below 16% consistently. This can be achieved through three parallel approaches: controlling water, controlling air, and controlling protective film.
Controlling water and humidity
- Allows wet wood to dry within 24–48 hours which is the time before mold starts to grow.
- Keep the relative humidity in the home below 60%. Use an inexpensive hygrometer to measure; if it exceeds, turn on the air conditioner in Dry mode or use a dehumidifier.
- Fix leaks in roofs, gutters, water pipes, wall joints, and air conditioning drainage pipes.
- Adjust the ground to slope away from the house to prevent water from pooling near wooden bases or wooden posts.
- Elevate wooden work off the ground to avoid direct contact with damp soil or concrete floors.
Control ventilation and light
- Leave a gap of at least 5–10 cm between wooden furniture and walls to allow air to flow behind.
- Trim branches that shade wooden walls as walls that never receive sunlight will always dry more slowly.
- Move potted plants away from wooden walls as daily watering adds moisture to the walls every day.
- Install exhaust fans in bathrooms and kitchens and run them for an additional 15–20 minutes after use.
Read more: Is your home’s wall moldy or slimy because you planted trees against the wall?
Controlling protective film
- Check the condition of the exterior wood finish every 6 months. Especially before and after the rainy season
- Repaint when you start to see a dull film, cracks, or when water no longer beads up Don’t wait until it peels off
- Always apply termite and mold protection before installing new wood Because once installed, you won’t be able to paint the back again
