, its effects and causes for wood rot and decay.
I’ve published a lot of articles on damp and the effects of moisture in masonry materials however looking back, I’ve written very little about moisture in wood. So, here’s a new article that focuses on moisture in wood, how it’s measured, reasons for wood expansion and the causes for wood rot and decay.
Enjoy and thank you for reading Russ.
Wood is a fibrous substance we harvest from trees and trees are simply large perennial plants that consist of roots, trunks, branches and leaves. There are generally considered to be just two types of tree; hardwoods and softwoods. Hardwood trees are usually slow growing with broadleaves and are deciduous thus, shed their leaves. Softwood trees have a much quicker growth rate, they usually have thin needle like leaves and are mostly evergreen (they don’t shed). Because softwoods have a much quicker growth rate when compared to hardwoods, they tend to be less durable. The type of tree determines their appearance, properties, rate of growth and durability.
Roots are a tree’s anchor to the ground but are also used as a transportation network for water and other minerals that support growth. The trunk and branches grow by elongation, a biological term used for lengthening but also through secondary thickening where the width of the trunk increases. This is achieved by growing a ‘cambium’, a layer of living cells which lie just beneath the bark.
The living cells are ‘sapwood’ which are used to transport minerals and water. Each year of a trees life, the cambium cells move outwards thickening the trunk, evidence of this can be seen when a tree is felled revealing the concentric rings of growth.
All trees start as sapwood stems however, as the tree ages the trunk thickens and the need for structural support and stability becomes necessary, cells nearer the centre of the tree die off. These cells are no longer conducive to growth and instead are used to support the tree structurally, we refer to these cells as the ‘heartwood’. Heartwood cells are often recognisable by their darker appearance towards the centre of the trunk, this is caused by resins and other compounds which help increase durability and resistance to decay. Sapwood is generally lighter in colour and much more susceptible to insect attack and fungal decay, this is because the sapwood is where all the nutrients are stored and transported.
Lumber and timber, are both words which are commonly used interchangeably to describe the material produced from trees however, they all refer to the material through different stages of manufacture.
Wood, is basically used to describe the fibrous material that makes up the tree material and ‘lumber’ is a used to describe un-processed wood, basically felled trees ready for processing. We refer to wood that’s been processed for use as ‘timber’, whether this be rough sawn joists or beams for construction or plain sawn smooth timber used for decorative / architectural furniture and joinery. The process of harvesting timber from a tree isn’t quick. It takes around 40 years of growth to harvest softwood trees and around 150 years to reach maturity for hardwoods.
Trees are basically CO² carbon dioxide and water O² which through a process called photosynthesis, chlorophyll in the leaves use sunlight to create oxygen and glucose, allowing the tree to generate its own energy for growth.
CO² + H²O + photons = CH²0 + O²
(This can be read as carbon dioxide plus water plus photons (energy from sunlight) gives a carbohydrate (CH2O) and oxygen)
Tree’s are built on a foundation of just 3 polymers, cellulose, hemi-celluloses and lignin and it’s these three polymers which are responsible for the versatile properties of wood, strength, flexibility and stiffness. Cellulose is responsible for strength and hemi-celluloses are responsible for flexibility, together these make up about 70% of the tree. Lignin is responsible for the characteristics of stiffness (30%).
Cellulose = Strength
Hemi-celluloses = Flexibility
Lignin = Stiffness
To wood destroying fungi and insects these polymers are basically sugars that organisms and insects feed on. Wood isn’t just a building product, to life on earth wood is also food!
Felled tree trunks are debarked and cut using a large circular saws and band saws. There are several ways the truck can be dissected and sawn into planks and the chosen method often dictates the characteristics, strength and appearance of the timber. Once sawn the manufacturing process of wood into timber also requires drying. This is necessary to remove the excess ‘free water’ and ‘bound water’ from the wood to gain strength and reduce dimensional movement of the product once in use.
Drying of timber is usually undertaken in a special climate-controlled kiln which regulates relative humidity, temperature and air flow. The moisture content of the wood is reduced according to the desired use for the timber. Construction timber is usually dried to a moisture content of 9 – 14% with timber used for the manufacture of furniture and household joinery dried between 6 – 9%.
Not all timber is kiln dried, some is air dried. This process involves simply planking and stacking the timber in a dry environment for a long period which allows moisture to escape naturally. Stacking timber for air drying is called ‘En Boule’ and usually results in less risk of distortion as water leaves the wood at a much slower rate. The En boule process however takes considerably longer, years in-fact, as such this method is generally restricted to high value timber.
As timber driers from the outside (the side exposed to atmosphere) a moisture gradient is established, basically the fibres outside are drier than the fibres inside which remains wet.
Moisture gradients can cause problems if the drying process isn’t controlled and moisture is removed unevenly. The drying outer fibres want to shrink whilst the wetter core fibres remain constraint, this often results in cracking / and shakes forming on the surface which can reduce performance, appearance and value of the timber. As bound water is removed by the drying process the mechanical properties of wood also improve, the timber becomes harder and stronger caused by tensioning of the fibres.
The drying of wood whether this be kiln drying or natural is a complicated and precise process that needs to be undertaken carefully. I found this out myself when air drying planks of beautifully sawn walnut and elm. I had these planks stored for three years in a dry, well-ventilated environment before I considered using them. I built a table and benches from the walnut and shelves from the elm. Even after years of air drying, once finished, I still noticed movement and even had a few wood munching critters leave my table two seasons later. Thankfully they’re all gone now, but it took some time. I manufactured the elm into shelves and once I’d made all the brackets and shelf planks, within just a year the timber had moved resulting in these beautifully distorted natural looking shelves. Of course, I could have waited a few years longer to ensure the wood was totally dry and stable, but I wasn’t concerned as I happen to like the appearance which makes the furniture so much more aesthetically pleasing. Well, atleast to me it does.
Timber is a hygroscopic material (meaning moisture absorbing) which never loses its ability to absorb or release moisture too and from the environment, this occurs even after the process of manufacture. This is why timber doors swell in the winter and shrink in the summer along with external environmental changes. It is this hygroscopic function that allows the timber to reach equilibrium with the surrounding environment, but also a function that increases its susceptibility to rot and insect attack.
So, why is the moisture content of timber so important?
Dry timber is relatively stable and provided the moisture content is kept below 20% there’s no risk of rot. Whilst woodworms and other wood munching critters can infest dry timber, the risk of infestation is reduced on the basis these critters proffer damp timber, particularly where there’s an element of decay. So, maintaining your timber in a dry condition is vital to reduce the risk of decay and infestation.
Once timber has been dried, processed and used in construction, monitoring the moisture content of timber is not something we tend to do regularly. It’s only when a problem develops, or a house is being sold do we usually direct our attention towards measuring the moisture content of wood.
Building surveyors measure the moisture content of wood using an electrical moisture meters. These are resistance or capacitance meters which are capable of quantitively measuring the amount of moisture in the material.
Because the properties of wood, irrelevant of species are similar it is possible to measure the electrical resistance in accordance too its moisture content accurately, we refer to these electrical readings in wood as ‘Wood Moisture Content’ (WMC). In reality however, these meters are not measuring moisture, just electrical resistance with a reading converted to percentage moisture content based on an algorithm.
There are of course other methods of establishing the moisture content of wood which are much more accurate than electrical resistance, these include techniques such as gravimetric over drying. This method is however intrusive if samples need to be taken from the home for moisture measurement, there’s also a requirement of highly sensitive and specialist laboratory equipment needed for the process’s. Timber producing factories will use this method due to its accuracy although this method of moisture measurement after timbers production is generally only undertaken by specialist diagnostic companies like ourselves as electric moisture meters offer a much quicker and far less intrusive alternative.
https://www.dryfix.net/gravimetric-analysis-yorkshire.php
Electrical moisture meters which as explained above only measure resistance not moisture, are good but only up to a point.
Water in timber can be present in two states. Firstly ‘bound water’ which is water present within the cell walls, then ‘free water’ which exists outside the cell walls within the cell lumen.
Electrical moisture meters are only accurate in measuring moisture in wood up to the ‘Fibre Saturation Point’ (FSP). Fibre saturation point, is the point where the cell walls are saturated without additional water yet being present in the cell cavities. When excess water enters to the cell cavities the conductivity of the material increases experientially thus, moisture meters become very inaccurate. In most species of wood FSP is generally around 29- 30% (MC) Moisture Content, that’s the percentage ratio of the wet wood to the theoretical weight of the dry wood.
The terminology bound / free water describes basically the mobility of the water in the cells. Bound water is chemically bound to the cellulose molecules inside the cell walls. Free water is simply that, water not bound to the structure which instead occupies voids in the cell lumen.
Understanding movement in timber and Fibre Saturation Point for moisture in wood
Understanding what fibre saturation point means to timber is important as the moisture content of wood and it’s saturation limit affects its, behaviour, appearance, performance and vulnerability. Movement in timber is a function of moisture and occurs only when moisture is increased or taken from the cell walls. Note it is only the fluctuation of ‘bound water’ within the cell walls specifically that causes movement in wood and this movement will occur until the timber reaches it’s FSP (Fibre Saturation Point). Above the FSP moisture threshold, movement stops the cell walls are saturated and can no longer take on additional water, any additional water then occupies the cell lumens and no movement occurs.
Timber has three orthotropic axes therefore movement can occur tangentially, radially and longitudinal however most movement occurs over the tangential and radial axis. As the moisture content of considered ‘dry timber’ increases (bound water loading), the dimensional stability decreases upto its fibre saturation point.
It’s not uncommon to see the effects of this movement in buildings particularly where insufficient expansion gaps have been left around flooring causing the floor surface to buckle. This happens when moisture levels in the timber increase and there is insufficient room for expansion.
Moisture expansion in wood
1 – Longitudinal 0.1% (parallel to the grain)
2 – Tangential (along the radial rings) 5 – 10%
3 – Radial (across the radial rings / across the grain) 3- 6%
Wood above 30% FSP is dimensionally stable although at this stage the wood becomes much more vulnerable to decay and infestation. Decay is essentially a process of decomposition where wood destroying fungi feed on specific wood polymers reducing the wood substance. Different wood decaying fungi will attack the timber in different ways.
Essentially there are two types of wood destroying fungi, those are the infamous ‘dry rot’ Serpula lacrymans (there is only one dry rot), and many different species of ‘wet rot’. Rots/fungi can also be subdivided further into categories ‘brown rots’ and ‘white rots’ which determine how the rot/infection decomposes the wood.
Dry rot is a ‘brown rot’ as too are many species of wet rot Coniphora puteana the cellar fungus and Anrodia vaillantii mine fungus are both ‘wet rots’ and ‘brown rots’. Brown rots attack the ‘cellulose and hemi-cellulose polymers causing very distinctive ‘dark appearance and cuboidal cracking of the timber. This is however much more pronounced with a dry rot infection, when compared to other brown rot infections.
White rots will too attack celluslose and hemi-celluloses however, white rots also attack the lignin polymers which causes the timber to appear lighter in colour resulting in a decomposition that longitudinally separates the fibres causing a string like appearance. Asterostroma and Donkioporia are both white rots. If there’s one thing you should remember from reading this blog post it is, ‘that a white rot can never be dry rot’!.
If you are unsure what type of infection you’ve come across there is a really useful flow chart for identifying rots in the BRE ‘recognising wood rot and insect damage in building’ book pages 10 – 15.
https://www.brebookshop.com/details.jsp?id=327313
All rots require a sustained high moisture level sufficient enough for the spore to germinate and sustain growth. Unlike moulds which are considered to be ‘opportunistic’ rots are more resourceful and require sustained conditions before gemination.
All rots start out as spores that germinate into hyphe. The hypha is a long branching structure similar to roots in trees that transport water and nutrients. Many hyphae then merge and fuse together in a process called ‘anastomosis’ which creates a mass of mycelium and develops on or within the wood and breaks down the wood for food. In some circumstance rots many also develop a sorophore, often referred to as a fruiting body or bracket, example below.
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Sporophores are most common in dry rot infections although wet rots also develop sporophores, they’re just seen less often. Sporophores are basically the flower of the organism, which develop to disperse new spores to the atmosphere to continue the infection. Spores from fungi are omni-present (meaning everywhere and anywhere) however, will only germinate when conditions to growth are conducive and sustained. You simply can’t hide from the spores so maintaining your timber in a dry stable condition is the best way to reduce the risk of a rot infection.
Finally, if you made it this far ‘Thank you’ and I hope you found this article useful and informative. If you have any questions or queries about these subjects please don’t hesitate to get in touch.
