Moisture content, displayed as a percentage, is the amount of water by weight within a length of timber in direct comparison to the mass of the raw, dry timber present. For example, if the total weight of a freshly felled log is 1000kg, and the weight of the raw dry timber contained within the tree is 500kg, then the moisture content of the tree is 100%, as the weight of the water is equal to the dry weight of the timber. As such, it is possible for timber to possess a moisture content of more than 100% if the weight of the water within the tree is more than the weight of the dry wood, though this is not particularly common. The water is stored within the cells of the wood fibres and provides sustenance to the tree whilst it grows. Despite having been felled, the cells are still able to exchange water and resin within the structure of the timber. Due to the nature of these porous cells, water can be freely expelled or drawn in, in relation to the surrounding environment. To reduce the moisture content within a length of timber to a suitable level, it must be dried or seasoned. 

Seasoning is a process by which timber is dried deliberately and precisely to minimise warping and twisting, over a fairly long period of time; at least 12 months for use in UK construction. The desired moisture content of a piece of seasoned timber is determined by its intended use. Typically, the moisture content of structural timber as it comes from the merchant will be around 15-18%. Joinery and Furniture grade timber will possess a lower moisture content, around 8-12%, being more stable and less prone to movement due to internal stresses caused by acclimatisation. The typical moisture content for timber, both in furniture and structural timber, in a house has that acclimatised and stabilised in humidity and temperature is around 6-10%. This means that whilst often timber from the merchants may possess a higher moisture content, once the building is properly weathered in and is allowed to dry out adequately, the moisture content of the timber will fall to a lower level.  

The process of seasoning timber correctly involves stacking the timber evenly, spaced with small bits of wood called stickers, to allow air flow between the timber or logs and let the excess water evaporate. In this industry, to speed up the time required to season a log, kilns are often used, which through a means of steadily heating the timber, can dry out the wood faster.

Uneven seasoning can cause the timber to warp, crack, or split. This can occur due to incorrect storage during natural drying, or by drying the timber at too high of a temperature in the kiln. If the timber is not properly stacked with stickers to allow for air flow, mould and fungi might start to grow, attracting insects as well. 

As is the case for larger piece of timber, trapped moisture may also cause issues in the final piece. The moisture meter takes surface readings of the moisture content based on the conductivity of the amount of water present. For thick pieces of timber, especially slow drying hardwoods, small holes may be drilled into the timber to check for the internal moisture level. In some instances, due to the irregular structure of the timber, pockets of trapped moisture can be missed. This timber could be sold under that the assumption that it has been properly seasoned, leading to issues further down the line. These issues can include warping of the final piece as it dries or affecting the finishing process of furniture and joinery components.  

The modern timber manufacturing industry meets regulated standards for the exact moisture level of timber to be sold. A traditional rule of thumb for drying times before the implementation of measuring devices follows a year per inch thickness of a board. For example, a rough sawn board that is 8 inches wide by 2 inches thick will take approximately 2 years to dry, provided it is stored with adequate air flow. As the timber dries, shrinkage of the wood is typically, though the amount varies by moisture content and species.  

Preservation of timber

As we’ve already discussed, timber is a living, organic material, and as such is susceptible to many forms of decay and attack that are found within the natural world. Untreated, raw wood products are attractive food sources to many species of insects and fungi. To protect the materials that we use in construction from harm caused by these types of infestation, they can be treated with chemicals or otherwise engineered to be more durable and resistant. The most common type of timber treatment, as used in construction, is known as a process called Tanalisation. In this process untreated timber, typically spruce or other structural softwoods, having already been sawn and seasoned, is placed in a large pressure sealed tank. This tank is then filled with a chemical called Tanalith E and placed under vacuum. Tanalith E, or copper azole is a preservative solution that contains a number of chemicals that lend themself to deterring or eradicating harmful insects and organisms, as well as somewhat increasing resistance to weather. Within the vacuum, the wood preservative is drawn deep into the cells of the timber, saturating the cellular structure of the timber with preservative. The timber is left to soak for a while in the pressure treated tank filled with preservative, and after this is allowed to dry naturally, fully completing the preservation process. Depending on several factors such as the exact process or thickness of timber, the preservative content within the wood can vary. Some building codes call for the cut ends of timber to be treated again with preservative, as the initial round of treatment may not have fully saturated the timber.  

Other methods of preservation can sometimes be implemented, such as acetylation, application of oils or other chemicals to the timber, heat treatment, or other pressure treatment methods. However, to all intents and purposes, the most used, and more importantly the most specified type of treated timber to be used in modern UK construction is tanalised “pressure treated” timber.  

Processing timber

When a tree is felled and in good condition (not rotten or possessing a major defect), it is ready to be processed into usable timber. Some of the softwood that we use in construction in the UK is grown domestically, but the majority is imported from other regions, such as Scandinavia and Russia. These regions are colder in climate, providing an environment in which most construction related softwoods thrive. Once felled, the trees are debranched and debarked, often being cut down to more manageable lengths at the felling site. They are then transported via truck or boat to a sawmill. The mills process the logs into commonly used timber sizes and shapes, and store them as part of the seasoning process. Often the timber will be treated at the same location, as many sawmills have the facilities required to treat timber. In some instances, these locations also act as merchants where processed timber can be purchased directly.  

Whilst there are a handful of specific methods of sawing logs into timber, there are 2 main descriptors that are used to identify the qualities of a length. By this, it is meant that a board processed from a tree can be cut radially or tangentially. This is in reference to the appearance of the annular rings of the tree within the end grain of the board. A radial cut produces a length of timber in which the annular rings of the tree present themselves at a 45-degree angle or more, in reference to the widest edge of the board.  

In contrast, a tangential cut produces a length of timber in which the annular rings of the board present themselves at less than a 45 degree angle, relative to the widest edge of the board.  

These different cuts bear no structural relevance in modern UK construction, though in certain on site applications it is important for a trained operative to be able to identify and use these characteristics in an effective manner. These cuts present a different aesthetic and finish to a length that is cut respectively, and this is an important factor to observe when the timber is used in a more refined way, such as joinery or cabinetry. In smaller pieces used in joinery, radially cut boards possess more consistent and straight grain, causing them to be much more stable than a tangentially cut board. This straight radial grain also presents a more aesthetic surface than that of a tangentially cut board.    

There are a handful of primary methods that are used in the machining and milling of modern structural lumber. There are many colloquial terms used to describe these methods, with the geography and language of a region altering the exact phrasing used. That being said, each method involves a different cutting procedure and result, and as such each produces a board with different characteristics, which in turn favour different applications. Here we will look at the most common forms of milling, and the terms associated with them.  

Plain Sawn

Plain sawn timber is the most common type of cut that can be found on site and at the merchants, and is a simple, traditional, and very cost-effective method of producing usable timber from logs. In this method, boards are cut tangentially from the log, with the log being rotated in increments of 90 degrees at regular intervals to produce a varied range of full width boards. This type of milling is the most common in the production of timber for use in construction, and creates a perfectly stable and efficient board, though much less desirable for use in joinery or cabinetry due to its relatively ugly appearance.  

Rift Sawn

Rift sawn timber is produced through the most expensive method of processing logs into usable timber, due to its comparatively higher percentage of waste. In this method, each board is milled perpendicularly to the annular rings of the log, thus producing consistent grain patterns in the lengths that are cut. The grain within these boards presents itself in a consistent straight character, minimising flecking in species such as oak, and creating a very stable and beautiful board. It’s important to note that these boards are radially cut, but the resulting timber is very expensive due to its technical milling process and high percentage yield of waste to usable timber. The timber is a very sought after material for high end furniture and cabinetry, as 1 log milled in this fashion will produce a full matching set of timber lengths in terms of grain orientation and appearance.

This method can often be utilised in association with quarter sawn timber, minimising waste. The largest most desirable boards of each quarter are milled in a rift saw fashion, with the remaining timber being milled in a quarter saw fashion.  

Quarter Sawn

Quarter sawn timber, like rift sawn timber, portrays an aesthetic appearance for use in fine joinery as well as possessing stable characteristics. This method sees a log being cut radially into 4 even quarters, with each quarter then being sawn radially to the right-angled centre of the log. This method, whilst still more labour intensive than plain sawn and live sawn methods, offers less waste than that of rift sawn timber. The consistent milling direction of each quarter produces slightly varied grain directions in the resulting timber.  

Live sawn timber

Live sawn timber, also known as through and through cut timber, is a method that involves making full width cuts through the log to produce wide boards. The logs are cut at an even thickness all the way through by a mill, producing the widest possible boards from a tree. The boards are trimmed of their wane after the process and shaped to the nearest standard size possible - producing little waste in the process. This method is also sometimes called plain sawn depending on the area, though the rotated plain sawn method is never called live sawn or through and through sawn. Approximately 60% of the boards produced from 1 log in this method are tangentially sawn, and the remainder end up radially sawn, as can be seen in the diagram. These boards, due to the way their grain is presented, can often be more susceptible to cupping than other types of cuts, however the capability for this method to produce wider boards lends itself to the construction industry and the need for wider standardised boards.