Timber

The elements that determine the soundness and quality of a wooden construction are its design, its installation and, obviously, the performance of the materials it is made up of. In this respect, wood is a material that boasts numerous assets. Wood is highly efficient in withstanding major stresses.
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Wood properties

Durability and use class (EN 350-2/EN 335/EN 460)

Due to its organic nature, wood is likely to be damaged by biological agents such as insects and fungi. The natural durability of wood reflects its intrinsic resistance to biological attacks. These risks depend on the exposure of wood to moisture. Before designing wooden decking or weatherboarding, it is essential to choose a suitable wood species. This choice is determined by various parameters such as durability, aesthetics, mechanical performance, machinability, environmental impact , availability, price. In order to choose the wood, special attention must be paid to the identification of climate conditions and external stresses specific to the environment where the decking or weatherboarding will be installed, as well as the class of use of the chosen species. In fact, it is imperative that the wood feature durability, natural or conferred, sufficient to withstand relevant moisture conditions. Wooden decking is typically assigned to class of use 3 or 4. Weatherboarding is assigned to class of use 3. The durability of wood is obviously one of the major properties to be taken into account when choosing the species. The durability is essential for designers, architects and contractors as it allows to choose the species to be avoided and/or the preservation treatment required depending on the type of structure, its purpose and its location. Wood used for construction of decking must show durability type 1, 2 or 3.
Durability(heartwood) Description Equivalence(for information only)
1 highly durable > 25 years
2 durable 15 to 25 years
3 moderately durable 10 to 15 years
4 slightly durable 5 to 10 years
5 not durable < 5 years
Natural durability is a notion that cannot be dissociated from conditions of use of the wood. Standard EN 335-1 describes 5 use classes:  
Use class Service conditions Examples of use
1 Wood always dry.Wood moisture always < 20% Interior carpentry sheltered from moisture: parquets, interior staircases.
2 Dry wood, not in contact with the ground, whose moisture may occasionally exceed 20% (surface wetted temporarily or accidentally) Ventilated framework and structure.
3 Wood not in contact with ground, subjected to alternately moisture and drought (e.g., inclement weather or condensation).Frequent moisture < 20%. All outdoor vertical construction or carpentry components exposed to rainfall: weatherboarding, windows, etc. Unventilated framework. Sheltered components showing condensation.
4 Wood in permanent contact with the ground and fresh water.Permanent moisture < 20%. Piles, pillars, solid or glue-laminated timber in contact with the ground, wood immersed in fresh water, etc.
5 Wood in permanent contact with sea water. Port constructions, landing stages, breakwaters, etc.

Resistance to termites

Resistance to termites considered is the natural resistance of the heartwood in accordance with NF EN 350-2 or else resistance provided by an adapted preservation treatment in accordance with requirements of standard NF B 50-105-3 for the use class claimed. D: Durable M: Moderately durable S: Sensitive

Stability

During their service life, wooden deck boards undergo, depending on climate fluctuations, shrinking and swelling cycles. During said cycles, timber is subject to warping, splitting, gerses , etc. It is important that the designer be familiar with the properties of each species in order to end up with a quality structure regardless of the species chosen. To do so, the following scale of values was selected in the summary table of species: PS: Unstable MS: Moderately stable S: Stable

Hardness (NF EN 1534)

Four level classification: A: 10 to 20 N/mm2 B: 20 to 30 N/mm2 C: 30 to 40 N/mm2 D: > 40 N/mm2

Aspect quality

Wood is a heterogeneous material by nature that boasts a certain number of singularities originally or that show up as it ages (knots ,gerses ,etc.). The presence of said singularities on a deck board does not mean that it is or will become unsuited to the planned use. The impact on its technical specifications is often non-existent.

Geometric properties required for deck boards

Maximum width/thickness ratio for the board cross-section

In order to prevent excessive warping during the service life of the board, the width/thickness is limited to a maximum threshold. This threshold depends on the species’ intrinsic natural stability. Hence, the more the species is said to be stable during the shrinking and swelling phenomena, the higher the allowed upper limit. Authorised values are listed in the attached summary table (“Species” menu).

Minimum thickness of boards

For reasons of stability and safety, it is recommended not to choose board thicknesses lower than the values listed in the attached summary table (“Species” menu).

Upper ridge of the boards

The boards’ edges must be cassées . In the event of a rounded edge, the bend radius will exceed or equal 2 mm.

Wood and moisture

Wood is an hygroscopic material, i.e., it has the ability to absorb and release moisture compared to its environment. During its transformation and installation, its moisture content must be adjusted depending on the situation in which it will be used. Aspects linked to water play a critical role in the installation of wood and if not taken into account, may lead to numerous problems, including: Warping before or after installation, decrease in mechanical properties, drying flaws, etc.

The warping or bending phenomenon

The phenomenon of warping is inherent to wood. It represents one of the most frequent pathologies in wooden floor covering (decks, parquet floors). It must be allowed if it remains within acceptable limits. For further information, refer to, among others, report 2/2008 (book 9) of the CSTC and to applicable standards.

Parameters affecting the scope of the bending

Moisture content gradient within wood: the difference in moisture content between the top and bottom sides of the board constitute the decisive parameter in the warping phenomenon. In fact, it is this gradient that leads to a differential in the swelling or shrinking of the wood and that explains the warping that is sometimes concave (the top side has the lowest moisture content), sometimes convex (the top side has the highest moisture content). Wood sawing method: since wood warping is considerably greater (practically double) in the tangential direction of the fibres than in their radial sense, the sawing method (flat cut, quarter cut, rift cut) inevitably has an impact on the degree of warping. Wood species: Just like sawing, the wood species has an impact on warping, insofar as a wood that is less stable will display greater warping, for the same variation in moisture content. The boards’ width/thickness ratio : reflects the ration between the width and thickness of the boards. It is obvious that the higher the ratio, the greater the risk of warping. [/row]

The most commonly used species

HARDWOOD AND TROPICAL SPECIES

Species without sapwood for use without treatment Origin Density average kg/m3 (H=15%) Type 1 Suitability for use classes Type 2 Suitability for use classes Durability class (NBN-EN 350-2 Resistance to metropolitan termites Hardness Stability Maximum width/thickness ratio Minimum nominal thickness (mm) Comments
3a 3b 4 3a 3b 4
West Indian mahoganySwietenia macrophylla AL 550 2 S Change in colour in contact with iron and copper in humid environments
AformosiaPericopsis elata AF 700 1/2 S
Afzelia/DoussieAfzelia bipidensis AF 800 Yes Yes Yes Yes Yes Yes 1 D C S 6 19 Degrease surfaces before finishing. Risk of run-off
AmaranthPeltogyne spp. AL 850 Yes Yes No Yes Yes No 2/3 M C MS
Angelim pedra/SapupiraHymenolobium spp. AL 750 2 MS
AzobéLophira alata AF 1050 Yes Yes Yes Yes Yes Yes 1/2 D D PS 4 50 Limited use due to risk of warping
Balau/BangkiraiShorea laevis AS 950 Yes Yes Yes Yes Yes Yes 2/3 D C MS 6 19 Considerable variability of aspect and density.
Basralocus/AngeliqueDicorynia guianensis AL 750 Yes Yes No Yes Yes No 2 M C MS 6 21
BilingaNauclea diderrichii AF 750 Yes Yes Yes Yes Yes Yes 1 D C MS 5 27 Risk of warping.
BosséGuarea cedrata & Guarea laurentii (light bossé) Guarea thompsonii (dark bossé) AF 600 2 S Exudation of resin possible with G.c.
BubingaGuibourtia demeusii AF 850 2 MS Risk of warping.
ChestnutCastanea sativa EU 600 Yes Yes Yes Yes Yes No 2 M B MS 5 22
Sessile or pedunculate oak)Quercus petraea or robur EU 700 Yes Yes Yes Yes Yes No 2 M C MS 5 22 Darkens in contact with iron, in human environment.
CumaruDipteryx spp. AL 1070 Yes Yes Yes Yes Yes Yes 1 D D MS 6 19 KD (Kiln dried)
FramireTerminalia ivorensis AF 550 2/3 Darkens in contact with iron, in human environment.
Gonçalo alves/MuiracatiaraAstronium spp. AL Yes Yes Yes Yes Yes Yes D C MS 5 19 KD (Kiln dried)
IpéTabebuia spp. AL 1050 Yes Yes Yes Yes Yes Yes 1 D D S 7 19
IrokoMilicia spp. AF 650 Yes Yes Yes Yes Yes No 1/2 D C MS 5 21 Darkens in contact with iron, in human environment.
ItaubaMezilaurus itauba AL 850 Yes Yes Yes Yes Yes Yes 1 D C MS 5 19
JarrahEucalyptus marginata AS AU 800 1 MS
JatobaHymenaea courbaril AL 900 Yes Yes Yes Yes Yes No 2 M D MS 5 19 KD (Kiln dried)
KapurDryobalanops spp. AS 700 Yes Yes Yes Yes Yes Yes 1/2 M B MS 6 19
KarriEucalyptus diversicolor AU AS 880 2 MS
KeruingDipterocarpus spp. AS 800 3 MS
KosipoEntandrophragma candollei AF 650 2/3 S
Louro gamelaOcotea rubra AL 660 2 MS
MassarandubaManilkara spp. AL 1050 Yes Yes Yes Yes Yes Yes 1 D D PS 5 21 Risk of splitting and warping.
MakoreTieghemella heckelii AF 660 1 S
Meranti, Dark RedShorea spp. AS 680 2/3 S
MerbauIntsia spp. AS 800 Yes Yes Yes Yes Yes Yes 1/2 M D S 7 19 Degrease surfaces before finishing. Risk of significant run-off
MoabiBaillonella toxisperma AF 850 Yes Yes Yes Yes Yes Yes 1 D C MS 6 19
MovinguiDistemonanthus benthamianus AF 700 3 S
NiangonHeritiera utilis and H. densiflora AS 700 3 S Degrease surfaces before finishing.
OkanCylicodiscus gabunensis AF 910 1 MS
PadoukPterocarpus soyauxii AF 750 Yes Yes Yes Yes Yes Yes 1 D C S 6 19
Panga-PangaMillettia stuhlmannii AF 850 2 S
Black locustRobinia pseudoacacia EU 750 Yes Yes Yes Yes Yes Yes/No 1/2 D C PS 4 22 Very rare in great lengths and widths.
SapelliEntandrophragma cylindricum AF 650 3 S
SipoEntandrophragma utile AF 650 2/3 S
TaliErythrophleum spp. AF 900 Yes Yes Yes Yes Yes Yes 1 D D MS 4 27 KD (Kiln dried)
TatajubaBagassa guianensis AL 800 Yes Yes Yes Yes Yes Yes 1/2 D C PS 5 21 KD (Kiln dried)
TeakTectona grandis AS 650 Yes Yes Yes Yes Yes Yes 1 M B S 6 19 The durability of plantation teak (AF, AL) varies from 2 to 3. Degrease the surfaces before finishing.
TiamaEntandrophragma candollei AF 550 3 S Risk of warping.
TolaGossweilerodendron balsamiferum AF 500 2/3 S Degrease surfaces before finishing.
WengeMillettia laurentii AF 850 2 S

RESINOUS SPECIES

Species without sapwood for use without treatment Origin Density average kg/m3 (H=15%) Type 1 Suitability for use classes Type 2 Suitability for use classes Durability class (NBN-EN 350-2 Resistance to metropolitan termites Hardness Stability Maximum width/thickness ratio Minimum nominal thickness (mm) Comments
3a 3b 4 3a 3b 4
Douglas/Oregon pinePseudotsuga menziesii EUAN 550 Yes Yes No Yes No No 3 S B MS 6 20 Degrease wood rich in resins before finishing.
Douglas/Oregon pinePseudotsuga menziesii (treated for use in class 3b) EUAN 550 Yes Yes No Yes No No 3 D A MS 6 20 Degrease wood rich in resins before finishing.
SprucePicea abies EU 450 4 S Degrease surfaces before finishing. Risk of run-off
HemlockTsuga spp. (treated for use in class 4) Yes Yes Yes Yes Yes Yes S A MS 6 20
European larchLarix spp. EU 600 Yes Yes No Yes No No 3/4 S B MS 6 20 Degrease wood rich in resins before finishing. Risk of warping.
Maritime pine Pinus pinaster (treated for use in class 4) EU Yes Yes Yes Yes No Yes D B MS 6 20
Scots pinePinus sylvestris (treated for use in class 4) EU 500 Yes Yes Yes Yes Yes Yes D A MS 6 20
Western red cedarThuya plicata AN 370 Yes Yes No Yes Yes No 2 S <A S 6 27 Poor superficial hardness (denting).
Yellow pinePinus spp. AN 540 3 S Degrease wood rich in resins before finishing.

Sources : NF DTU 51.4, NF B54-040, Tropix, CSTC, Belgian Woodforum.