If you are building any kind of structure out of wood, particularly if it is to be outdoors, one of your biggest enemies is rot. Yes, wood is a naturally occurring substance, and over time, it can degrade, especially when there is moisture involved.
To minimize the potential damage, it is important to know the types of rot there are and what causes them, as well as how to prevent it. That’s exactly what we will discuss in this article.
3 Types of Wood Rot
Let’s start by looking at the three main types of wood rot.
Brown rot is one that you may be familiar with, and it is often called dry rot, with the reason being that the wood will often look both brown and dry. This type of rot targets the cellulose in the wood, and as the cellulose is destroyed, the wood shrinks, turns brown, dries out, and starts breaking off into cube-like shapes. The process is known as cubical fracture.
This type of rot thrives in moderate temperatures, and once it takes hold, it is very hard to stop, as it spreads very quickly.
The next type of rot you might find is white rot, which you will be able to recognize by the wood taking on a white or yellowish color. Moreover, white rot will cause wood to feel very soft and spongy. Unlike brown rot which targets cellulose, white rot fungi target the lignin in the wood (the other main structural component of wood). When the lignin breaks down, it leaves the white cellulose behind, thus the name, white rot.
This type of rot thrives in the same temperature conditions as brown rot, between 65 and 90 degrees Fahrenheit.
The third type of wood rot is soft rot, which thrives in a much wider temperature range, between 0- and 110-degrees Fahrenheit. It is most commonly found in fallen trees and logs, but in some cases may affect structures.
The fungi that cause soft rot break the cellulose in the wood down, usually leaving behind what resembles a honeycomb beehive. This type of rot usually starts in the center, in deep cavities of old logs, and it is a type of rot that progresses at a slower pace than either of the others.
What Causes Wood Rot?
To be precise, there are two things that cause wood rot, but that said, they are really just one thing. What we mean is that it takes specific types of fungi combined with relatively high levels of moisture. Fungus is what actually rots the wood, but for any fungi to grow, the wood needs to be constantly damp, and there needs to be a certain temperature too.
So, when it comes down to it, wood rots due to a combination of high moisture and fungi.
4 Conditions for Wood Rot
What we want to make clear here is that although at the high level, the cause of wood rot is a combination of moisture and fungi, there are a few other conditions that need to be met for wood to rot. Let’s take a quick yet comprehensive look at what fungi needs to cause rot.
Fungi need to eat, and in many cases, the food is cellulose or lignin in wood. Wood is the food source that fungi need to reproduce. Wood also serves a second purpose for fungi, a safe place that is sheltered from extreme weather and temperatures.
Fungi require the wood to be constantly wet or damp to grow. Wood generally needs to have a moisture content of at least 20% to 30% for fungi to thrive.
Most fungi that cause wood rot require the temperature to be between 65 and 90 degrees Fahrenheit, with soft rot surviving in a temperature range of 0 to 110 degrees Fahrenheit.
To thrive, fungi require the air to contain at least 20% oxygen.
What Types of Wood Are Susceptible to Rotting?
Another thing that is important for you to know is that some types of wood are less susceptible to rot than others. If you expect to choose the most rot-resistant wood, you need to be aware of the points as outlined below.
In general, softwood is much less resistant to rotting than hardwood. Softwood is more porous and lets moisture and fungal spores inside much easier than hardwood. Some of the worst types of wood in terms of being susceptible to rotting include pine, maple, aspen, hemlock, alder, elm, birch, buckeye, poplar, and beech.
If you plan on building anything outdoors, always use pressure-treated wood. Without getting too scientific, pressure-treating allows wood to be more resistant to rotting, UV damage, and other forms of damage. However, keep in mind that pressure treating doesn’t make wood waterproof (read this guide to learn how to do that). Either way, untreated lumber is not something you want to use for building outdoor structures.
Sapwood is that layer of light-colored wood that is right inside of the bark of a tree, the very outside layer of wood. Although it has some uses, sapwood is not ideal for outdoor structures, as it has virtually no rot resistance, and this is true whether we are talking about softwood or hardwood.
Younger trees usually contain fewer extractives (waxes, fatty acids, resin acids, and terpenes) than older trees. Higher amounts of extractives allow for greater rot resistance.
Best Rot-Resistant Types of Wood
Just to put you on the right track, the most rot-resistant types of wood out there include Spanish cedar, mahogany, teak, cypress, redwood, cedar, white oak, black cherry, and black walnut.
7 Tips to Prevent Wood from Rotting
Now that we have the basics down, let’s go over some crucial tips to help prevent that wood of yours from rotting.
1. Choose the Right Wood
As we mentioned above, choosing the right type of wood for the job is essential. Don’t use some low-grade softwood that is susceptible to rotting. You need to start strong, and that means using the best possible wood.
2. Make Sure the Wood Is Pressure-Treated
Although pressure treating does not make wood waterproof, it does make it more resistant to rotting. For decks, patios, sheds, and other such things, using wood that has not been pressure treated is not recommended in the least.
3. Use a Fungicide
If you are really worried about fungi taking hold, use a special fungicide designed for wood. You can apply this before and after painting, which will kill fungal spores. This is something you can do on a regular basis for the best results.
4. The Right Coating Makes a Difference
If you don’t paint, stain, or varnish your wood, it will be far more susceptible to rotting. High-quality waterproof paint, stain, or varnish will help keep the wood dry, thus robbing fungal spores of the moisture they need to grow. Make sure to get the right product for outdoor applications.
5. Create a Slight Pitch
As you should know by now, moisture is your enemy. When it comes to stairs and patios, build them with an ever so slight pitch or angle. This way, instead of pooling up and remaining on the wood, water will run off.
6. Limit Ground Contact
The ground is wet, so the more of the wood touches the ground, the more often it will be wet and the longer it takes to dry. Limited ground contact is essential to prevent wood from rotting.
7. Remove Plants and Vines
Plants contain moisture that can seep out onto wood, plants cause wood that they cover to retain moisture for longer, and they block out sunlight too. Removing vines and other plants will allow wood to remain dryer than it otherwise would.
Can Wood Rot Be Repaired?
Ok, so the actual rotten part of the wood cannot be repaired. Rotten is rotten and that’s it. However, you may be able to scrape away the rotten parts, make sure that the rot has not spread far, and then fill in the rest using sawdust and wood glue, or any other wood repair method. Totally rotten wood cannot be repaired.
I also wrote about how to fix wood rot, so you might want to read that article too.
There you have it people, everything you need to know about the types of wood rot, what causes wood rot, and everything you can do to prevent your wood from rotting. It all starts with the right kind of wood!