For home and sauna: scientists explain why it's essential to harvest logs in winter.

The Timing of Wood Harvesting Matters

The quality of wood, as well as the durability of structures and products made from it, depends on the season in which the tree was felled.

“The optimal time for harvesting materials is considered to be late autumn and winter. During this period, the movement of sap in the trunk stops, which reduces the risk of wood being infected by fungi and pests. Wood harvested during this time possesses high-quality characteristics and has a longer lifespan,” shared Andrey Tat'yannikov, senior lecturer at the Department of Construction Production and Geotechnics at Perm Polytechnic.

The reason is that in cold weather, microorganisms, pests, and insects that can damage the wood die off or burrow into the ground. Additionally, trees cut in winter have lower moisture content; they are in a dormant state, allowing excess water to seep into the earth. This type of wood is less prone to shrinkage, decay, and warping.

The longevity of the timber is largely determined by its content of resinous substances and essential oils. Wood harvested in summer contains less pine resin—a liquid mixture of resin acids and terpenoid hydrocarbons—due to increased metabolic activity and intense evaporation during the hot season.

The Best Species for Building a Bathhouse and House

When choosing materials, it is essential to consider the conditions of use, resistance to moisture, and decay. A house and a bathhouse are fundamentally different structures.

“For a bathhouse, hardwoods such as linden and aspen are suitable. They have low thermal conductivity, are moisture-resistant, and do not release resin when heated. Linden has a light fragrance and does not heat up, while aspen is also resistant to decay and retains steam well. On the other hand, the popular pine is not entirely suitable for construction because it releases resin when the bath is heated,” explains Kamil Sharafutdinov, a graduate student at the Department of Structural Engineering and Computational Mechanics at PNIPU.

Wooden houses offer excellent thermal insulation, reducing heating and air conditioning costs. The natural beauty of wood creates a cozy atmosphere, and the quick construction technology shortens building times. Furthermore, wooden houses are characterized by high strength when properly treated and maintained.

For constructing a durable residential house, hardwoods are preferable. Larch and oak are known for their strength and resistance to decay. Aspen should be avoided as it has significant shrinkage as it dries.

“The most popular choice is pine—it is an affordable and inexpensive wood with good thermal insulation properties. It is easy to work with, facilitating construction, but it is prone to decay and requires additional antiseptic treatment. Cedar is considered the most durable and wear-resistant wood for building a house; however, it is rarely used due to its high cost,” explains Kamil Sharafutdinov.

Defects in Wood

Experts from Perm Polytechnic highlight key points to remember when selecting wood for construction. Among them are rot—damage to wood caused by fungi, bacteria, and other microorganisms. This leads to not only color changes but also alterations in the material's texture, potentially resulting in structural failures.

Another common issue is knots, where holes may form that weaken the strength of the beam. They can lead to additional defects, cracks, or warping.

Cracks are splits in the wood that run along or across the grain. They may be caused by changes in temperature and humidity, mechanical damage, or structural violations of the beam, weakening the structure and reducing its lifespan.

Curvature is the bending of the trunk or wood. This defect can be caused by uneven tree growth, improper storage, or treatment of the timber.

Disadvantages of Naturally Moist Timber

Natural moisture refers to the percentage of moisture present in the wood at the time of felling. It is not regulated and can range from 30 to 80 percent according to GOST standards. Timber with a moisture content of less than 22 percent is considered dry.

According to expert Andrey Tat'yannikov, wood with a moisture content of up to 14 percent can be used for outdoor work. For interior finishing and furniture making, the moisture content should be even lower—6-8 percent.

“Wood has a heterogeneous structure, so the process of moisture evaporation is also uneven. During drying, the mass of the log changes, and when the timber is already fixed in the structure, the drying process can lead to cracks, gaps, and deformation of individual elements of the building. Therefore, it is crucial to dry the wood before construction,” explains Kamil Sharafutdinov, a graduate student at the Department of Structural Engineering and Computational Mechanics at PNIPU.

Dangerous Deadwood

Deadwood refers to trees that have died but remain standing. The drying out of trees can be caused by various factors: reaching their maximum age, severe frosts or freezing, lowering groundwater levels leading to moisture deficiency. Conversely, if the soil becomes too wet, the roots suffer from a lack of oxygen and nutrients, leading to their demise. Another cause of tree drying is the spread of harmful insects and mold.

“It is important to note that deadwood for construction can only be harvested in the northern latitudes of the Arctic—such as in Karelia. Dried trees from other regions of Russia, like Vologda, Kirov, and Nizhny Novgorod, are not suitable for harvesting logs. This is because inside trees that have dried in the moderately continental climate zone, adult beetles, pupae, and larvae reside. The trails, rot, and waste from insect activity drastically reduce the quality of the log,” says Andrey Tat'yannikov.

The Strength of a Building Depends on the Quality of Harvested Wood

Improperly harvested logs can crack even in a constructed house or bathhouse. The wood must be well and evenly dried before use; otherwise, residual moisture will lead to deformations that compromise the structural integrity.

“It is also essential to protect the wood from moisture contact during storage or during the construction process. Its moisture resistance can be enhanced with water-repellent treatments. Fire resistance can be increased with fire retardant agents. It is also crucial to treat the wood with antiseptic compounds to prevent infection from fungi and mold,” responds Kamil Sharafutdinov.

Correcting Existing Defects

If the damage is deep and compromises the integrity of the structure, the damaged section can simply be cut out and replaced with good wood. Saws and cutters are used for this purpose. If the defect is not too large, it can be filled in—such as with epoxy resin, wooden plugs, or wood glue. This will help maintain the integrity of the beam and prevent further crack propagation.

If the damaged area is under high load or subject to dynamic destruction, it can be reinforced. Staples or stainless steel rods can be used for this. However, it should be noted that inserts may affect the wood's flexibility.

To protect the log from decay, insects, and other external influences, it is treated with special compounds. This can include varnish, paint, or other coatings. However, it should be considered that such a protective layer may alter the color and texture of the wood.

Making the Right Choice

“When selecting material, the first step is to carefully visually assess its condition. Tap the surface. The sound should be dull and even, without creaks. The moisture content of the boards or beams can be checked with a special tool. The display will show the moisture percentage, which should be compared with what is stated in the accompanying documents. Additionally, the supplier must have a quality certificate,” noted Andrey Tat'yannikov, senior lecturer at the Department of Construction Production and Geotechnics at PNIPU.