Popular legend says that sawn lumber grows stronger with age. It’s a fact that old wood is stronger than on the day it was first milled. Old wood is at least 50% stronger than fresh-cut lumber.
Does wood lose strength over time? As wood grows older, decreased moisture content makes it stronger. It shrinks the most at the felling stage. Wood then continues to shrink as it dries which could take several years. Unless the cell structure breaks down due to wood rot, wood becomes stronger with age.
A piece of lumber has three specific areas (planes): radial, longitudinal and tangential. The least amount of shrinkage takes place in the longitudinal area. The overall appearance of the project is not affected because the shrinkage is almost invisible to the human eye,
The radial and tangential areas tend to shrink more than the longitudinal area. Woodworkers cut wood in a way that will minimize further shrinkage. If you cut the wrong way it leads to inferior projects.
Thoroughly dried wood is readily available in the market. It will be more expensive than semi-dried wood. Your projects will be of the highest quality if you use completely dried wood
Properties of Wood
Wood is mainly used for structural purposes. The facts given below summarize some of the basic concepts connected to the mechanical characteristics of wood. An important parameter influencing almost all mechanical properties is the moisture content of wood. Here are the main properties of wood:
- The strength of wood increases when the moisture content decreases. Air-dried wood which has a moisture content of 12% to 13% has higher strength properties than that of wood which has a 20% moisture content. Generally, you dry wood to 15-20 percent moisture for structural applications.
- You categorize wood projects under aesthetic, functional, and load-bearing. A wooden frame is aesthetic, a wooden door is functional and a cabinet is load-bearing.
- Aesthetic wood projects last the longest. You must refurbish them at the surface level to return to their original state.
- Functional wood projects get damaged easily. They are susceptible to environmental damage as well as damage from impact.
- Load-bearing projects don’t last long. Sometimes wood projects with a load-bearing function don’t last their expected lifespan.
- Wood is an orthotropic material. This means its properties are independent in three directions – longitudinal, tangential, and radial.
Another unique property of wood is its viscoelasticity, which means it has both plastic and elastic properties.
Elastic materials stretch when you place a load on them. Remove the load and they get back to their original condition. In contrast, plastic materials stay in the same stretched condition even when you release the load after a long time. You find that wood products exhibit a behavior that is between elastic and plastic.
You can take a bookshelf to illustrate the viscoelasticity of wood. When you place several books on the shelf, it sags because it has a limited amount of sagging deformation. Even when you remove the books from the shelf, it never gets back to normal. This is due to the residual deformation left by the viscoelasticity of wood.
The compression of wood and wood-based materials plays an important role in construction projects. If the compression strength or bending strength of a beam is not known, deflection due to bearing a load may cause deformation. We grade most softwood construction lumber based on load resistance, determined through a stress test.
Factors affecting the quality of wood
Many factors affect the quality of wood other than the variability of strength among and within the species. There can be natural defects and irregularities, factors related to the environment, and the effects of biological agents. Here are some factors that can affect the quality of wood:
- Wood exposed to the elements deteriorates quickly. Uncovered wooden bridges have a 20-year lifespan, whereas covered bridges last for over 100 years.
- Short-grained or cross-grained is when the direction of the grain is not parallel to the long axis of the wooden component. This may occur because of the spiral grain in the tree or how the wood is sawn. It can weaken the wood.
- Knots in the wood also weaken the wood.
- Loss in the strength of the wood results not only from the abnormal tissue and the direction of the grain of the knot but from the cross-grain of wood distorted around the knot.
Insect and Fungal infestation
Wood-inhabiting insects such as termites, carpenter ants, and beetles, destroy wood. The physical loss of wood causes a loss of strength.
Another major cause of deterioration in wood is a fungal infestation. There are four major conditions for the hyphae of the fungi to develop in wood. They are favorable temperature (70-85 °F is ideal), oxygen (20% or more air volume in the wood), moisture, and food.
The wood-staining fungi utilize the residues of stored materials in parenchyma cells of sapwood. But they do not attack cell walls.
The wood-staining fungi discolor the wood, but they do not reduce its strength. The wood-destroying fungi use enzymes to break down and consume the cell wall substance. This in turn produces various forms of decay or rot.
During the initial stages of fungal invasion, termed incipient decay, the effect on the strength of wood is insignificant. If the growth of the fungus is not contained, total loss of strength may result. The fungus, Chlorociboria, deposits a green stain in the wood and causes loss of strength.
You should control moisture to prevent the decay of wood. Decay-causing fungi cannot develop if you maintain a moisture content below 20%.
Factors affecting the strength of wood
As we mentioned above, the strength of wood increases over time. But wood can also become weaker under certain conditions as described below:
When wood dries below the saturation point of the fiber, the strength of the wood increases with the loss of bound water. The greatest increases are in compression along the grain – the strength of wood is almost doubled when you dry the wood to 12% moisture content.
The strength of wood increases threefold when oven-dried. The modulus of rupture of wood increases less, and the modulus of elasticity of wood increases least upon drying.
Temperature affects the strength of wood. It increases as the temperature decreases, and it decreases as the temperature increases. Over a range of naturally occurring temperature changes, the changes in the strength of wood are temporary. The effects of heat in reducing strength is least in dry air and greatest in moist air or steam.
The strength of wood is also related to the duration of loading. The strength of wood decreases depending on the long-term load periods.
How to make wood stronger and long-lasting
Generally, any type of wood lasts for 15 years or a little more. Wood can last for a very long time (hundreds of years) by reinforcing it using a non-biodegradable material. You preserve it in such a way as to protect it from environmental damage and load-bearing wear and tear.
Wood can even last 1,000 years if treated with the appropriate resin. Norwegian spruce was traditionally reinforced with resin from its own pines.
Painting is a way to make wood last longer. It helps to restore the look of the wood project, seal out moisture, prevent the wood from rotting, and reduce the effect of weathering.
Pressure-treated wood can last up to 80 years without any load, and 40 years with loads applied to it. It resists rot, so it bears the load.
Preservatives can greatly extend the life of wood by preventing rot. Different preservatives work in different ways. Creosote is a popular wood preservative. It is a strong-smelling, oily brown liquid usually made from coal tar. It is a fungicide, miticide, insecticide and sporicide.
This is lumber that is grown naturally in virgin forests. Almost all the continents have some areas where trees are not cut for commercial use yet. The trees grow very slowly here because of limited space and light. Due to the slow growth, the growth rings on the trees are tightly packed, which is very beneficial. Some benefits of old-growth wood are:
Even though pressure-treated wood is good, old-growth wood or lumber is the original rot-resistant wood. The slow growth process creates a greater proportion of latewood (summer or fall growth). Latewood is what adds this rot resistance to the old-growth wood.
Older trees also develop heartwood at their center. Not only is heartwood beautiful, but it is also very durable. It resists rotting in ways that other wood can’t.
Yes, wood moves! When the wood is dry it contracts, and when it is wet it expands. This can cause joints to open up, paint and finish to fail, and a lot of other problems. But, old-growth wood due to the tight growth rings does not move nearly as much- from siding and framing to doors and windows. Nothing changes shape.
Old-growth wood is much stronger due to its high density. This wood can carry heavier loads across longer spans.
Termites don’t like hardwoods. Old-growth wood is hard and dry. Termites prefer soft, moist wood which is easy to chew.
Old-growth wood is certainly a good choice for woodworking because it is sturdier, firmer, and resistant to rotting. You find these trees in virgin forests.
Environmentalists are seeking to preserve their pristine nature. Since old-growth wood won’t be readily available in the market, you can use new-growth wood.
You have to take note of the water content and also make it stronger and long-lasting by painting, pressure-treating, and using preservatives. Hopefully, this article has added to your knowledge of making wood stronger as it ages.
By understanding how wood gets strong or can become weaker, you can get the best out of the wood you use for your woodworking projects.