ORIENTED STRAND BOARD (OSB)

www.osb-info.org

What is OSB ?
OSB is an engineered wood structural panel that is rapidly gaining popularity for various applications. It is a sheet material in which rather long strands of wood are bonded together with a synthetic resin adhesive. Sometimes in all three layers, but usually only in the outer layers of these panels, the strands are orientated in a particular direction. OSB varies in colour from a light straw colour to a medium brown depending on the wood species, resin system and pressing conditions.
OSB’s strength mainly comes from the uninterrupted wood fibre, interweaving of the long strands and degree of orientation of strands in the surface layers. Waterproof and boil proof resin binders are combined with the strands to improve internal strength, rigidity and moisture resistance.
OSB’s uniformity makes it ideal for a variety of uses. With no core voids, knotholes or delamination problems, you can use OSB with confidence and work without waste.

How is it made?
Wood is, without doubt, the best building material available. However, it doesn’t grow as uniform as one might like. The OSB industry has found a way to take smaller diameter, more economical trees, remove the knots, grain defects and irregularities and turn them into engineered wood shapes.
The OSB production process is detailed below:

 Key elements in the production are the Strander in which strands are produced. These strands retain the natural strength properties of the wood and are critical to the quality of the finished product. The dry strands are metered into the Blender where they are tumbled into a rotating drum with a fine mist of wax and resin. The resin/ wax bonds the strands in the pressing process and enhances the moisture resilience of the board. The coated strands are then conveyed overhead to the Forming Line where a “mat” of flakes is laid on a continuous belt conveyor by a Cut-Off Saw. This mat is loaded in a Press where, under high temperature and pressure, a sheet of OSB is formed. In the finishing area the product is trimmedand cut to produce the finished OSB board.

Advantages of OSB

High mechanical strength – comparable with that of equivalent grades of plywood and other structural panel products.
High rigidity
• Resistance to distortion, splitting and de-lamination
• Excellent strength-to-weight ratio
• Impressive durability – Dimensionally stable. Retains as-new performance levels throughout its long life when used in specified service conditions.
• Precise, predictable performance - Load bearing grades conform with exactly defined design and loading specifications in dry or humid atmospheres.
• Easy to work - Readily sawn, drilled, planed, routed and sanded. May be nailed, stapled or screwed close to the edge without splitting. Simple to glue, paint and stain.
• Flaw-free – No knotholes, core voids or points of weakness.
• Comprehensive choice – supplied in a variety of grades, a wide range of surface sizes, ready sanded or unsanded, plain-edged or tongued-and-grooved.
• Low ecological impact – No mature trees are sacrificed in the manufacture of OSB. Only small-diameter timber from ecologically sustainable forests is used.
• The product itself is fully recyclable.

What is it used for?

According to European standards, there are 4 types of OSB, according to use :

OSB/1 : boards for general purpose use and interior use in dry conditions
OSB/2 : boards for load-bearing in dry conditions
OSB/3 : boards for load-bearing in wet/moist conditions
OSB/4 : high-specification boards for load-bearing in wet/moist conditions

When it comes to new construction projects, OSB’s versatility makes it easy to work with a variety of applications, including :
· Wall sheeting – excellent strength and racking performance under all types of exterior cladding.
· Roof sheeting – uniformly sound and extra rigid to handle snow and wind loads, sacking for pitched tiles on slated roofs, structural decking on joists for flat roofs.
· Subfloors – strong, rigid and impact-resistant for underlayment, carpet or tiles.
· Single-layer floors – use OSB directly under carpet, lightweight concrete or hardwood.
· Underlayment – uniformly thin yet strong and finely sanded, OSB provides a smooth and uniform base for vinyl or tiles.
· I-joists – a high quality support system that minimizes deflection, provides for long spans and minimizes floor squeaking. Because it’s engineered, OSB can be custom manufactured to meet specific requirements in thickness, density, panel size, surface texture, strength and rigidity. OSB is also highly workable, making it easy to saw, drill, nail, plane, file, glue, paint and sand. This makes it an excellent choice for the replacement of ageing or unwanted building components, as well as new additions to existing structures. OSB is also used as concrete shuttering or framework.

OSB is successfully used for high-quality and high specification packaging.

OSB is further used:
· in fair- and shop-fittings, for both structural and decorative components
· in furniture manufacture, not only as decorative furniture but also to reduce thickness and weight of furniture components (chair seats and backs, furniture frames, desk tops under lamination);
· for vehicle and wagon interiors;
· for hoarding and barriers, shutters and fences;
· for dry storage pallets.

How to use it

Storage and handling
Careful storage and handling is important to maintain boards in their correct condition for use; thus, OSB must be protected from rain and accidental soaking. Boards should be stored flat in an enclosed, dry building. When handling boards, the edges and corners should be protected against damage.

Working with OSB
OSB can be cut by a hand or power saw and machined (routed, spindled, planed and bored) with normal woodworking machinery. Tungsten carbide cutting edges are recommended for use with powered tools. Where smooth surfaces are required pre-sanded boards should be specified.

Joints and fixings
Conventional woodworking fixings and techniques can be applied to OSB which provides good holding power for screw fixings into the board faces; generally, edge fixing is not recommended. Parallel core screws should be used because they have greater holding power than conventional wood screws. A high ratio of overall diameter to core diameter is desirable. Drill pilot holes for all screw fixings. Typically, the holes should be 85 to 90% of the screw core diameter. Fixings into the board face should not be within 12mm of edges and 25mm of the corners.

Nails and staples can be used for lightly loaded fixings or to hold glued joints while the adhesive sets.

A wide variety of adhesive jointing methods can be used, provided the following simple guidelines are observed:
· the joint parts should be accurately machined;
· sharp cutters should be used to avoid tearing or burnishing the surfaces to be bonded;
· high solids content adhesive with low flowing properties such as polyvinyl acetate or urea formaldehyde are recommended;
· mating pieces should be accurately located and held under pressure while the adhesive sets;
· the width of grooves machined in OSB should be limited to about one-third of the thickness of the board. The depth of groove is typically about one-half of the board thickness;
· adhesive-bonded joints should be allowed to condition for several days before sanding and finishing; this avoids the appearance of sunken joints and is essential with high-gloss finishes;
· a tongue and groove joint is very efficient, provided the fit of the joints is not too tight to cause a split along the edge;
· when attaching lippings, the tongue should be machined on the solid wood piece.

Density, mass and sheet size
Board density and thus board mass varies depending upon the product, being affected by the timber species and the process used in manufacture. Typical densities are 600-680kg/m3. Thus, for example, a 2400 x 1200 x 12mm board will weigh approximately 20 kilogrammes. Other sizes are available or can be produced to order. Boards are produced with either square or T&G edges.

Wet conditions
OSB with enhanced moisture resistance (OSB/3; OSB/4) is not “waterproof”. The term “moisture resistan” applies to the adhesive binder which (within limits defined by EN 300) will not break down in the presence of moisture. Physical wetting of all grades of OSB should be avoided. When wet, OSB will increase in thickness.

OSB will not normally be attacked by wood-boring insects common in temperate climates, but panels made from aspen and spruce are susceptible to fungal attack under prolonged wet conditions; panels made from pine have higher moderate resistance to attack.

Use of OSB in load-bearing conditions, such as flooring

Different grades of the product are available for different levels of loading and different environmental conditions. Guidance on the use of OSB in these load-bearing applications is given in the European standard ENV 12872 “Wood-based panels: guidance on the use of load-bearing boards in floors, walls and roofs”, obtainable from national standards institutes.. Generally, for the same loading conditions, a thinner board of OSB can be used than a load-bearing particleboard. If there is a risk of wetting, the right grade of OSB should be used.

It is important that square edged boards are supported along all edges. OSB is stronger parallel to the long axis of the board. As such, both square edged and T & G OSB should be laid with its long edges across the joists.

Joists across the shorter edges should be staggered. It is important that edges around the perimeter of the floor are continuously supported, either on joists or noggings.
To allow for expansion, a gap of not less than 10 mm should be left wherever OSB boards abut any rigid upstand such as a perimeter wall, column or fireplace surround.