Brettsperrholz (CLT): Composition and Raw Materials

• Brettsperrholz is a building timber product consisting of at least three orthogonally glued layers.
• These layers are made of lamellas consisting of high-quality boards or wood-based materials.
• Popular trade names are KLH, BSP, BBS, and CLT.

Curved Brettsperrholz

• Curved Brettsperrholz is a type of Brettsperrholz with a camber exceeding one percent of its span.

Raw Materials

• Saw timber, boards, posts, and often side lumber are used.
• The use of side lumber can lead to higher added value.
• Mainly spruce wood is used, but also larch, pine, Swiss pine, and other types of wood.
• The thickness of the saw timber used is usually between 19 and 34 millimeters with a wood moisture content of 12 ± 2 %.
• Values between 8% and 18% are also possible.
• The number of layers is usually 3 to 7 layers but can also vary between 9 and 13 layers (the number of layers is always odd).
• Brettsperrholz is produced in small or large panels.

Gluing

• The surface is usually prepared by conventional planing or using the Rotoles method (better gluing possible).
• Both cold and hot gluing are possible for bonding.
• Hot gluing is performed using high-frequency presses.
• Commonly used adhesives are one-component PU adhesives, phenolic resins, and amino resins (melamine-urea-formaldehyde).
• Depending on the adhesive, the pressing pressures are between 0.2 and 1.2 Newton per square millimeter.

Qualities

• Brettsperrholz is divided into different qualities: SI (visual quality), NSI (non-visual quality), ISI (industrial visual quality), and DL (ceiling layer lengthwise to the panel direction).
• A distinction is also made between sanded, planed, and surface-treated variants.

Properties

• The crosswise gluing of the individual layers results in dimensional stability.
• Drying the lamellas to the desired moisture content helps ensure freedom from cracking.
• Dimensional change in the panel plane: 0.01% per 1% change in wood moisture
• Dimensional change normal to the panel plane: 0.2% per 1% change in wood moisture

Advantages

• High degree of prefabrication
• No additional bracing required
• Rainproof shell construction possible in a shorter time
• Favorable ratio of dead weight to strength
• Use of local softwoods
• Creative and innovative building possible
• Good thermal insulation values with smaller wall thicknesses
• Sustainable raw material wood

Applications

• Wall elements
• Ceiling elements
• Roof elements
• Use in house construction, commercial construction, residential construction, industrial construction, agricultural buildings, bridges, and public buildings
• Brettsperrholz can be well combined with steel, concrete, glass, and other materials, leading to a wide range of applications.

Structure according to Euronorm 16351:2011

• At least three layers of construction timber are required.
• Layers may consist of different types of wood, provided their technical properties regarding swelling and shrinking do not differ significantly.
• Layers may account for up to 50% of the total height of the Brettsperrholz product.
• Wood-based materials may not be arranged in two adjacent layers (except for solid wood panels).
• Thickness of a single lamella: minimum 12 mm, maximum 45 mm
• Gluing of the narrow sides

Bonding of the Narrow Sides

• Gluing of the narrow sides of adjacent lamellas within the building timber layers.
• Universal Finger Joint Connection
• Only components with an identical cross-section and without regular changes in the fiber direction may be glued.

Minimum requirements for manufacturing

• Initial testing of the product
• Establishment and operation of in-house production control
• Initial inspection by a notified body
• Verification of the adhesive strength of board lamella joints and glue lines
• Testing of the adhesive strength of the universal finger joint connection
• Fire behavior tests
• Ongoing self-monitoring and periodic external monitoring (twice a year)
• CE marking of the finished products
• Skilled personnel with proven knowledge regarding gluing and planing
• Each European Brettsperrholz product must be marked with a unique identification number that complies with the CE marking information.