A wide range of products is manufactured from wood material, ranging in size from small timber sections and thin laminates through chips and shavings down to wood fibres. The physical properties of the materi- als produced reflect a combination of the sub-division of the wood, the addition of any bonding material and the manufacturing process. The physical prop- erties then determine the products’ appropriate uses within the building industry. Many of the products are manufactured from small timber sections or timber by-products from the conversion of solid timber which otherwise would be wasted. Compressed straw slabs and thatch are additionally included in this section.
The product range includes:
laminated timber;
cross-laminated timber;
structural insulated panels;
laminated veneer lumber;
plywood;
blockboard and laminboard;
particleboard;
fibreboard;
wood wool slabs;
compressed straw slabs;
thatch;
shingles;
‘Steko’ blocks;
flexible veneers.
Within the European Union, whenever wood- based panels are used in construction, compliance with the Construction Products Directive must be demonstrated. This may be achieved by adherence to European Harmonised Standard for wood-based panels EN 13986 (BS EN 13986: 2004 in the UK).
This requires that products used in construction com- ply with its specifications and also to the additional performance-based criteria within the various EN standards listed for each specific material. Most Euro- pean countries now use the mark on boards and panels to show compliance with this harmonised stan- dard.
LAMINATED TIMBER Manufacture
Large solid timber sections are limited by the avail- ability of appropriate lumber; in addition, their
Table 4.12 Strength classes to BS EN 1194: 1999 for homogeneous and combined glulam
Glulam strength classes GL24 GL28 GL32 GL36
Homogeneous glulam:
Strength class of laminates C24 C30 C40 Combined glulam:
Strength class of outer laminates C24 C30 C40 Strength class of inner laminates C18 C24 C30
calculated strength must be based on the weakest part of the variable material. Laminated-timber sec- tions overcome both of these difficulties and offer additional opportunities to the designer. Laminated timber is manufactured by curing within a jig, lay- ers of accurately cut smaller timber sections which are continuously glued together with a resin adhesive.
Laminates may be vertically or horizontally orientated.
The use of strength-graded timber and the staggering of individual scarf or finger joints ensures uniformity of strength within the product; although, under BS 5268-2: 2002 and BS EN 387: 2001, large finger joints through the whole section of aglulammember are per- missible. The manufacturing process ensures greater dimensional stability and fewer visual defects than in comparable solid timber sections. Laminated timber may be homogeneous, with all laminates of the same strength class of timber, or combined, in which lower strength class laminates are used for the centre of the units.Table 4.12gives the European strength classes to BS EN 1194: 1999 for the two alternatives. Lami- nated timber manufactured from spruce or pine and phenolic or urea/melamine formaldehyde resins would normally achieve a Euroclass fire performance rating, under the conditions specified in BS EN 13986: 2004 of Class D, subject to testing.
Forms
Sections can be manufactured up to any transportable size, typically 30 m, although spans over 50 m are possible. Standard-size straight sections (315×65 and 90 mm; 405×90 and 115 mm; 495×115 mm) are stock items, but common sizes range from 180×65 to 1035×215 mm. Sections may be manufactured to order, to any uniform or non-uniform linear or curved form. The majority of laminated timber structures are manufactured from softwoods such as European redwood or whitewood, although the rib members within the roof structure of the Thames Flood Barrier
T I M B E R A N D T I M B E R P R O D U C T S 1 3 7
Fig. 4.26 Thames Barrage, London.Photograph: Arthur Lyons (Fig. 4.26) were manufactured from the West African hardwood, iroko.Figure 4.27illustrates typical lam- inated timber arches, columns and portal frames as generators of structural forms. The aesthetic prop- erties of laminated timber can be enhanced by the application of suitable interior or exterior timber fin- ishes. Steel fixing devices and joints may be visually expressed (Fig. 4.27) or almost unseen (Fig. 4.28) by the use of concealed bolted steel plates. Laminated tim- ber performs predictably under fire conditions with a charring rate of 40 mm per hour as defined within BS 5268:4-1: 1978. Preservative treatments are necessary when the material is to be used under conditions in which the moisture content is likely to exceed 20%.
The three service classes of glulam structures relate the environmental conditions.
Service classes for glulam:
Service Class 1 Internal conditions with heating and protection from damp.
(typical moisture content <12%) Service Class 2 Protected, but unheated condi-
tions.
(typical moisture content <20%) Service Class 3 Exposed to the weather.
(typical moisture content >20%)
CROSS-LAMINATED TIMBER
Cross-laminated timber (CL) is similar to conventional plywood, except that the laminates are thicker, and the panel thicknesses are generally between 50 and 300 mm, although 500 mm can be produced. The max- imum panel size is governed by transportation limits which are normally 13.5×3 m, but the production limit is 20×4.8 m. The panels manufactured from spruce, larch or pine are classified by surface quality which is planed or sanded.
Surface quality classification of cross-laminated timber:
Class C Standard grade (non-visible) Class AB Interior grade (residential visible) Class BC Interior grade (industrial visible) The structural panels, which may be used to form walls, roofs and floors, are easily clad with brickwork, tiling or rendering as appropriate.
The thermal properties of CL panels depend on the timber used. Density is usually within the range 470–590 kg/m3, and an average of 500 kg/m3 has a thermal conductivityvalue 0.13 W/m K. To achieve a targetU-value of 0.35 W/m2K a 100 mm CL panel, with internally a 25 mm service gap and 12 mm
Fig. 4.27 Glued laminated-timber beams.Photograph: Arthur Lyons plasterboard, and externally standard brickwork cladding and a 50 mm cavity, would require 75 mm of mineral wool insulation or the thermal equivalent.
A well-ventilated cavity and breathable insulation is required.
STRUCTURAL INSULATED PANELS
Structural insulated panels (SIPs) are prefabricated lightweight building components, used for load-
bearing internal and external walls and roofs. Unlike cladding sandwich panels, structural insulated pan- els can support considerable vertical and horizontal loads without internal studding. They are manufac- tured from two high-density face layers separated by a lightweight insulating core. The three layers are strongly bonded together to ensure that the compos- ite acts as a single structural unit. The outer layers of oriented strand board (OSB), cement-bonded par- ticleboard or gypsum-based products are typically
T I M B E R A N D T I M B E R P R O D U C T S 1 3 9
Fig. 4.28 Laminated timber beam metal joint detail—Wells Cathe- dral Restaurant.Photograph: Arthur Lyons
8–15 mm thick. The core is composed of a rigid cellular foam, such as polyurethane (PUR), polyiso- cyanurate (PIR), phenolic foam (PF), expanded (EPS) or extruded (XPS) polystyrene giving an overall unit thickness between 70 and 250 mm. The structural performance is predominantly influenced by the thick- ness and physical properties of the outer layers, and the thermal performance is largely determined by the width and insulating characteristics of the core material. Large panels (e.g. 5.9 m×2.4 m×162 mm, U-value 0.15 W/m2K) may be used for two-storey con- struction to Code for Sustainable Homes Level 3.
Structural insulated panels (typically 1.2 m wide by 2.4 m high) offer a thermally efficient and air-tight form of construction, which is rapidly erected on site.
Jointing between panels is usually some form of tongue and groove system. Sound reduction for separating walls is typically 58 dB depending on construction details. External cladding may be brickwork, wooden
Table 4.13 Thermal performance data for structural insulated panels (SIPs)
Core material Face material Panel thickness (mm)
Thermal performance (W/m2K) Polyurethane
foam
cement bonded particleboard
86 0.28
Polyurethane foam
oriented strand board
100 0.23
Polyisocyanurate foam
oriented strand board
140 0.22
Notes:
The thermal performance data (U-values) are typical for the listed SIPs when constructed with a brick outer leaf and 50 mm clear vented cavity.
panelling or rendering with plasterboard as the stan- dard internal finish.Table 4.13gives typical thermal performance data for structural insulated panels. The use of SIPs is one of the Modern Methods of Con- struction (MMC) promoted by the UK Government to reduce energy waste in new building. Currently, SIPs account for approximately 10% of new building methods construction.
LAMINATED VENEER LUMBER
Laminated veneer lumber (LVL) (Fig. 4.29), also known asmicrolam, is more economical than lami- nated timber as there is little waste in the production process. It is manufactured to three grades by laminat- ing timber strands with polyurethane resin under heat and pressure. In one process, logs are cut into flat tim- ber strands 300 mm long; these are then treated with resin, aligned and hot-pressed into billets of reconsti- tuted wood. In the other processes, 3 mm thick timber strands or sheets of veneer are coated with waterproof adhesive and bundled together with the grain paral- lel. The strands or veneers are pressed together and microwave cured to produce structural timber billets or sheets up to 26 m long. The versatile material is suit- able for use in columns, beams, purlins and trusses and can be machined as solid timber (Fig. 4.30). I-section joists, with LVL flanges and OSB webs, are suitable for flat and pitched roofs and floor construction. Metal- web timber joists combine LVL flanges with metal strutting webs. Untreated LVL has a Class 3 surface spread of flame classification (BS 476-7: 1997). Three grades of laminated veneer lumber are classified by BS
Fig. 4.29 Laminated veneer lumber (LVL)
EN 14279: 2004 according to their serviceability in dry and exposed conditions.
Grades of laminated veneer lumber:
Purpose/loading Environmental conditions Type
Load-bearing Dry (service class 1) LVL/1
Load-bearing Humid (service class 2) LVL/2
Load-bearing – exterior conditions (subject to testing or appropriate finish)
Exterior (service class 3) LVL/3
Notes:
Service classes are according to BS EN 1995-1-1: 2004.
Laminated veneer lumber for structural applications should be marked ‘S’ for structural application and ‘G’ for general application to pr BS ISO 18776.
Monocoque structures
Interesting and innovative built forms can be created using LVL (and other timber products) to create flat or curved form monocoque structures. These work on the well-established principles from the motor industry, in which the hard body skin acts in concert with any stiffeners to form the structure. Using this technology, structurally efficient and elegant forms, which may be slender, tapered, flat or curved can be produced. LVL is quickly becoming a significant material to complement
the more established products such as plywood, OSB and glulam, particularly because of its availability in very large sections.
PLYWOOD Manufacture
Plywood is manufactured by laminating a series of thin timber layers, or plies, to the required thickness.
The timber log is softened by water or steam treat- ment and rotated against a full-length knife to peel off a veneer or ply of constant thickness (Fig. 4.31).
The ply is then cut to size, dried and coated with adhesive prior to laying up to the required number of layers. Not all the plies are of the same thickness;
often, thicker plies of lower-grade material are used in the core. However, the sheets must be balanced about the centre to prevent distortions caused by differential movement. Plies are normally built up with adjacent grain directions at right angles to each other to give uniform strength and reduce overall moisture move- ment, although with even plywoods, the central pair of plies has parallel grains. The laminate of plies and glue is cured in a hot press, sanded and trimmed to stan- dard dimensions for packaging. Decorative veneers of hardwood or plastic laminate may be applied to one or both faces. Most plywood imported into the UK is
T I M B E R A N D T I M B E R P R O D U C T S 1 4 1
Fig. 4.30 Laminated veneer lumber (LVL) construction—Finnforest Office, Boston, Lincolnshire. Architects: Arosuo and Vapaavuori Oy.
Photograph: Courtesy of Finnforest
made from softwood (largely pine and spruce), from North America and Scandinavia. Smaller quantities of plywood produced from temperate hardwoods are imported from Finland (birch) and Germany (beech), while tropical hardwood products are imported from Indonesia, Malaysia, South America and Africa. Bam- boo plywood is made from a core of tightly compressed fibres with bamboo veneers on either face. It is coated with a lacquer but should not be used externally with- out preservative pre-treatment as bamboo is inherently not durable.
The standard sheet size is 2440×1220 mm, with some manufacturers producing sheet sizes of up to 3050×1525 mm or slightly larger. Sheet thicknesses range from 4 to 25 mm for normal construction use, although thinner sheets down to 1.5 mm are available for specialist purposes.
Under the European fire classification of construc- tion materials (BS EN 13501-1: 2007), an untreated plywood panel would normally achieve a class D-s2, d0 rating, excluding its use as flooring when the rating
Fig. 4.31 Manufacture and standard types of plywood
is class DFL-s1 (depending on a minimum thickness of 9 mm, a minimum density of 400 kg/m3and fixing to a non-combustible substrate [class A1 or A2] without an air gap. The secondary classifications ‘s’ and ‘d’ relate to smoke production and flaming droplets.)
Grades
Plywood is classified according to its general appear- ance and physical properties (BS EN 313-1: 1996).
The key characteristics are the form of construction, durability, and nature of the surface. The durability of plywood is largely determined by the bonding class of the adhesive used. This ranges from Class 1, to the most durable Class 3 (BS EN 314-2: 1993), which can be used externally without delamination, provided that the timber itself is durable or suitably protected against deterioration.
Bonding classes for plywood:
Class 1 Dry conditions (suitable for interior use).
Class 2 Humid conditions (protected external
applications, e.g. behind cladding or under roof coverings).
Class 3 Exterior conditions (exposed to weather over sustained periods).
Phenol formaldehyde resins are most frequently used for the most durable plywoods. Marine ply- wood (BS 1088-1: 2003) is a combination of a moderately durable timber bonded with phenolic or melamine–formaldehyde resin. The standard class of marine plywood is suitable for regular wetting or per- manent exposure to salt or fresh water. The lower grades of plywood are bonded with melamine–urea formaldehyde or urea–formaldehyde resins. The rela- tionship between the natural durability of the timber against wood-destroying fungi and the Use Class to which the plywood can be assigned is described in DD CEN/TS 1099: 2007.
The quality of plywood is also affected by the number of plies for a particular thickness and the sur- face condition of the outer plies which range from near perfect, through showing repaired blemishes, to imperfect. Factory-applied treatments to improve tim- ber durability and fire resistance are normally available.
The standard BS EN 635: 1995 describes five classes of allowable defects (E, and I to IV) according to decreasing quality of surface appearance; class E is practically without surface defects. These are related to hardwood and softwood surfaced plywoods in BS EN 635: 1995 Parts 2 and 3, respectively.
The performance specifications for plywood to be used in dry, humid or exterior conditions against the criteria of bonding strength and durability with respect to biological decay are described in the standard BS EN 636: 2003.
Biological use class conditions (formerly hazard class) for the use of plywood:
Use Class 1 Dry conditions (average moisture con- tent < 10%).
Use Class 2 Humid conditions (average moisture con- tent < 18%).
Use Class 3 Exterior conditions (average moisture content > 18%).
These biological use classes correspond to the service classes in BS EN 1995-1-1: 2004.
The standard BS EN 636: 2003 also has a classifi- cation system based on the strength and stiffness of
plywood based on bending tests. Plywood is assigned to a four part code specifying bending strength and modulus in both the length and width directions. Ply- wood sheets should be identified according to their intended application with ‘S’ for structural and ‘G’ for general use.
Uses
Considerable quantities of plywood are used by the construction industry because of its strength, versa- tility and visual properties. The strength of plywood in shear is used in the manufacture of plywood box and I-section beams in which the plywood forms the web. Increased stiffness can be generated by forming the plywood into a sinusoidal web. Plywood box beams can be manufactured to create pitched and arched roof forms as illustrated inFig. 4.32. Stiffened and stressed skin panels, in which plywood and softwood timbers are continuously bonded to act as T or I-beams, will span greater distances as floor structures than the same depths of traditional softwood joists with nailed board- ing. Such structural units can also be used to form pitched roofs or to form folded plate roof structures or barrel vaulting (Fig. 4.32). Plywood of 8–10 mm thickness is frequently used as the sheeting material in timber frame construction and for complex roof forms such as domes. The lower-grade material is extensively used as formwork for in situ concrete.
CORE PLYWOOD
The standard core-plywood products are blockboard and laminboard. Both are manufactured with a core of usually softwood strips sandwiched between one or two plies (Fig. 4.33). In blockboard the core strips are between 7 and 30 mm wide, but in lamin- board, the more expensive product, they are below 7 mm in width and continuously glued throughout.
As with plywood, the grain directions are perpen- dicular from layer to layer. Most core plywoods are bonded with urea formaldehyde adhesives appropri- ate to interior applications only. The standard sheet size is 2440×1220 mm with a thickness range from 12 to 25 mm, although larger sheets up to 45 mm thick are available. Blockboard may be finished with a wide range of decorative wood, paper or plastic veneers for use in fitted furniture. Variants on the standard prod- ucts include plywood with phenolic foam, polystyrene or a particleboard core.
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Fig. 4.32 Structural uses of plywood
Fig. 4.33 Core plywoods
PARTICLEBOARDS
Particleboards are defined as panel materials produced under pressure and heat from particles of wood, flax, hemp or other similar lignocellulosic materials. The wood particles may be in the form of flakes, chips, shavings, sawdust, wafers or strands (BS EN 309: 2005).
Boards may be uniform through their thickness or of a multilayer structure. Wood particleboard and cement- bonded particleboard are made from wood chips with resin and cement binder, respectively. Oriented strand board is manufactured from large wood flakes and is classified in BS EN 300: 2006.
Wood particleboard (chipboard) Manufacture
Wood particleboard (chipboard) is manufactured from wood waste or forest thinnings which are con- verted into wood chips, dried and graded according to size. The chips are coated with adhesive to approx- imately 8% by weight and then formed into boards (Fig. 4.34). The woods chips are either formed ran- domly into boards giving a uniform cross-section or distributed with the coarse material in the centre and the finer chips at the surface to produce a smoother product. The boards are then compressed and cured between the plates of a platen press at 200◦C. Boards
Fig. 4.34 Manufacture and standard types of wood particleboard (chipboard) are finally trimmed, sanded and packed. In theMende process a continuous ribbon of 3–6 mm particleboard is produced by calendering the mix around heated rollers.
The standard sizes are 2440×1220 mm, 2750× 1220 mm, 3050×1220 mm and 3660×1220 mm, with the most common thicknesses ranging from 12 to 38 mm, although much larger sheet sizes and thick- nesses from 2.5 mm are available.
Extruded particleboard (BS EN 14755: 2005) is manufactured by extruding the mixture of wood chip and resin through a die into a continuous board;
however, in this method the wood chips are pre- dominantly orientated at right angles to the board face, thus generating a weaker material. Extruded
particleboard is specified within four grades accord- ing to its density and whether it is solid or has tube voids.
Types
The durability of particleboards is dependent on the resin adhesive. Much UK production uses urea formaldehyde resin, although the moisture-resistant grades are manufactured with melamine–urea formaldehyde or phenol formaldehyde resins. Wood chipboards are categorised into seven colour-coded types to BS EN 312: 2003 according to the anticipated loading and environmental conditions. The standard specifies requirements for mechanical and swelling properties and also formaldehyde emissions. The first