OSB board, structural element with an important role in the strength of wooden houses

Articles about timber frame houses are always accompanied by photos of the walls being assembled in the factory or being installed on the foundation. These are the phases where, in addition to the structure made of wooden posts and beams, you can also see the OSB boards enclosing the walls. And this is where the questions arise. Is it compulsory to install OSB? What is its role? Isn't it too fragile? Will the house withstand the wind if it has that thin OSB inside? Doesn't it trap water? Won't mold attack it? And on and on! There are even some who say that such panels should only be used to build sheds or animal shelters. It is totally wrong to consider OSB as a low-strength material made from scrap wood.

OSB belongs to the category of industrialized wood, it is very strong and has properties that make it very suitable for use in construction. The combination of its strength, its ability to stiffen other structures and its relatively light weight make it an extremely versatile material. It is designed for the construction industry, being used as load-bearing slabs in the construction of timber frame houses, as a stiffening material for prefabricated wooden panels, for walls, floors, floorboards, roof elements. It is mounted on at least part of the wooden wall structure, its positioning and fixing being part of the structural design and strength calculations. Unlike other building materials, OSB is lighter, ensuring the same stability and strength of the house at a lower foundation load.

What is OSB

This is not the first time we've talked about OSB. And even though it has been the subject of a previous article, which you can find hereI believe that some aspects need to be reiterated because they make it easier to understand why it is preferred in construction. OSB means Oriented Strand Board, i.e. a board of oriented battens. Shingles are large slivers of wood, 1-3 mm thick veneer, 2.5 cm wide and 15 cm long. The increased strength of the panels is due to the way these chips are placed before being glued and pressed. In the outer layers the direction of laying is lengthwise (the length of the chips is parallel to the length of the panel), while in the middle layer the direction is widthwise, perpendicular to the outer layers. Such an oriented arrangement of the layers is also used in plywood or CLT, resulting in panel strength and stability.

Initially, the chips are sprayed with adhesive (a moisture resistant synthetic resin), laid in 3 perpendicular layers and then pressed at high temperature and pressure. Finally, the panels are formatted (cut) to standardized dimensions, cooled and packaged. The result is elastic and uniform panels containing 95% wood. The remaining 5% is glue and additives that give resistance to moisture, insects and mold. The cutting of the boards from the original panel is done taking into account the chipping, so that the length of the board matches the length of the panel. This is very important because the boards have a bending strength and modulus of elasticity (bending stiffness) about 2.5 times greater along their length, considered as the axis of strength, than along their width. This is also taken into account when cutting on site.

Types of OSB. Not every type of OSB can be used in the structure

OSB boards come in different types and thicknesses, depending on their properties and area of use. How do we know which type of OSB to choose? From standard EN 300, which divides OSB into four classes. The classification refers to the applications and environment for which the type of board is suitable and the loads it can withstand. The division into the 4 distinct groups is based on mechanical performance and relative moisture resistance.

• OSB 1 - boards used indoors in a dry environment - this includes general purpose, furniture and interior trim boards, i.e. those without load-bearing capacity
• OSB 2 - load-bearing boards used in dry environments
• OSB 3 - load-bearing boards for use in damp environments
• OSB 4 - high-strength load-bearing boards for use in damp environments (does not mean that the board can stand in water).

Why OSB is used in construction

The edges of the original board, as they leave the factory, are protected with wax against moisture absorption, but if they are cut, the protection is no longer valid. Class 3 boards can be used both indoors and outdoors and are used by builders in a structural role to make walls, floors and roofing elements.

The thickness of the boards is different and can vary from 6 to 40 mm, with 14 different thicknesses in this range. Whether they are used for stiffening panels for walls, ceilings, floors or roofs, the thickness of the OSB used will be chosen according to the area of use and the loads it has to withstand. The strength of the board depends on the thickness and will be thicker the more weight it has to support. The choice of board for a wall or floor is based on strength calculations and structural design, and must be at least 15 mm thick to be used as a stiffening element. Fixing is also based on strength calculations, which determine the number of fixing points between the board and the timber frame and the distance between them. This calculated stiffening of the wall structure, called bracing, will make the house resistant to wind and earthquakes.

OSB panels have very good dimensional stability, high strength and light weight. They have a good strength-to-weight ratio and long life and are resistant to moisture and pests. Their properties also contribute to optimized thermal and sound insulation, fire protection and a good strength-to-weight ratio. They are quick and easy to install using any wood fastening system (nails, screws or staples). Timber frame houses with OSB stiffened walls are lightweight and strong, with very good earthquake resistance.

OSB cannot be directly plastered, nor can it be left loose on the outside as its water resistance is low. On the inside, the OSB panels are faced with plasterboard and then finished. On the outside, thermal insulation boards are applied over the OSB, which can be plastered and finished. On the frame, if OSB is used on both sides of the wooden elements, an additional vapor barrier must be installed on the inside, and all OSB joints in the inner layer are taped, thus preventing air and heat loss.

Houses on prefabricated wooden structures

In the USA, Canada and European countries OSB panels have long been used to stiffen the walls of timber frame houses. Romanian companies have been building in this way in European countries for many years and they are highly appreciated. In the last few years they have timidly started to be built in our country. Fortunately, since last year there seems to be an exponential demand for wooden houses. Advantages such as speed of construction, clean building sites, high energy efficiency, low weight of the structure resulting in lower foundation costs, high earthquake resistance are just some of the advantages of such constructions.

We have written a lot over the years about timber-framed houses built by Romanian companies at home and abroad. To convince ourselves of their advantages, we talked to builders and owners, we asked about their behavior in difficult areas. House built by Dimmer in Corsica, where winds can exceed 150 km/h, whose owners recommended the builder to four other islanders or the one built by Litarh in Arbonne-la-Foret, 70 km from Paris, mentioned by the magazine "Construire en bois" as one of the most successful houses in the Paris region, are just two examples of such constructions.

The easiest way to avoid misperceptions about a material or construction method is to document it. There are many sources of information on timber frame houses where you can find out why they are considered sustainable construction. You are not destroying forests by building with wood, you are using a renewable source, replacing some of the building materials that have a negative impact on the planet's health.