Nordic timber
Nordic sawn timber is produced by well-trained people. The technology required for production and handling of sawn timber is kept up-to-date by continuous new investments.
General features of Nordic timber
Good accuracy of measurements, smooth and even sawn surfaces and a moisture content required by the specific end users are technical properties, which allow the use of main grades of sawn timber in various end uses without further machining and handling. The same properties are important also for further processing of sawn wood. They allow for small processing allowances and reduce waste.
Sawn softwood is easy to machine and has an aesthetic appearance with good inner structure. The high percentage of heartwood is an advantage in the industrial use of sawn timber.
The good stability of timber is achieved in the sawing process through log dividing according to Nordic practice, in which the internal stresses are removed by heart splitting. Most of the pith also disappears due to the saw kerf in heart splitting.
Due to favourable strength to density ratio Nordic sawn timber is also suitable for all kinds of load bearing constructions.
The good hygroscopic and insulation properties are of advantage when using wood as building material: a wooden house is environmentally sound and pleasant to live in.
Wood is a product of nature, which is renewable and belongs to the natural cycle. The by-products produced in all stages of the production process can be industrially utilized in many ways without ecological problems.
Nordic Timber Grading Rules
“Nordic Timber Grading Rules” book is a Nordic set of commercial grading rules for Pine (Pinus sylvestris) and Spruce (Picea abies). The grading evaluates the properties of all four sides of sawn timber.
The book is published and edited by the Finnish Sawmill Mens’ Association, Swedish Svenskt Trätekniskt Forum and Norwegian Treindustriens Tekniske Forening.
The latest set of rules was published in 2016 and it is commonly known as the “Grey Book” because of the colour of the cover. Previous grading rules were in the “Blue Book” (1994) and “Green Book” (1960).
Nordic grading rules applies to both the domestic and export markets. It divides the sawn timber into grades according to the wood features. The Grey Book gives the maximum permitted values of wood features for each grade.
It is very seldom that several wood features with the maximum permissible values appear at the same time in one piece of sawn timber. Therefore, a parcel with normal distribution of wood features will be considerably below the maximum permitted values.
Great effort has been taken to make the book easy to understand. The language has been simplified and clarified. Terms, definitions and measuring rules have been specified and conform in principle with other rules used in Europe. Text and tables have been formed in a more pedagogic and logical way. All this is to make it easier to use in grading, training and as a guideline for the trade of sawn timber.
The grade classification of different qualities gives a guideline to division into the grades. The division of grades is not binding. Sawmills can after agreement with the buyer compile individual grades with their own grade names adapted to customer and product needs.
Instructions and rules
The basis for Nordic timber grading rules is that the sawn timber to be graded has been sawn according to the Nordic sawing practice and that the grade is defined from a sawn timber piece trimmed to the final length, or if not trimmed, to the intended length.
The main grades are U/S (Unsorted), V (Fifths), VI (Sixths) and VII (Sevenths or Domestic Sixths), unlike in the 1994 version, where the same grades were denoted with letters A, B, C and D. The latter system was not
Species
Redwood (pine) and whitewood (spruce) are graded separately. Unless otherwise agreed, sawn timber pieces of wrong species, which are by mistake mixed with the other species, will be graded into grade VII.
Determination grade
The grade determined on the basis of the number, location, type and the maximum permitted values of the wood features according to the tables.
- Each side of the piece shall be graded separately.
- The maximum values of wood features which are permitted in each grade on the worst metre of length are given in the tables.
- The grade is decided on the basis of the outside face and both edges.
- The inside face may be one grade lower.
When evaluating features, which are defined in percentages of total area, the (*) marked points in the frame above are not considered. In those cases the whole piece of sawn timber has to be considered in the determination of grade.
The values given in the tables are the higher limits of the permitted wood features in each respective grade. They are related to standard sizes at 20% moisture content.
Standard sizes for sawn timber
Thicknesses: 16, 19, 22, 25, 32, 38, 44, 50, 63 and 75 mm.
Widths: 75, 100, 115, 125, 150, 175, 200 and 225 mm.
Lengths: 1 800 – 6 000 mm in 300 mm or modules. Other lengths and modules can be agreed separately.
Special sizes are graded according to values to the nearest bigger standard size. Sizes over 75x225mm are, however, graded according to rules for 75×225.
Permitted size deviations
Following deviations from the nominal sizes of sawn timber are permitted:
Thickness and width: up to 100mm – 1mm + 3mm, over 100mm – 2mm + 4mm.
Length 1,800–6,000mm: – 0mm + 50mm
The average thickness and width of the sawn timber lot may not, however, be less than the nominal size.
Sahatavaraerään kuuluvien kappaleiden todellisen paksuuden ja leveyden keskiarvot eivät saa kuitenkaan alittaa nimellismittaa.
In grade VII greater deviations are allowed.
Shrinkage of knots
Shrinkage of knots is not considered if it stays with the permitted size deviations, otherwise the classification is done as for loose knot or knot hole.
Discolouration
Discolouration can be caused by bark or weathering, or as a result of kilning and storage. Usually it affects only to surface of the piece. It can be measured and is deeper than the permitted negative size evaluation, will be evaluated and classified according to the values of log blue stain.
Handling damage
Handling damage, which can be measured and is deeper than the permitted negative size deviation, will be evaluated and classified according to the values of barkringed knot.
Moisture content
The maximum moisture content for all sawn timber sizes is 24%. At least 93,5% of the pieces in a lot shall have an individual moisture content between the upper and lower limits, as defined in EN 14298.
When determining the moisture content for a sawn timber piece or a lot, moisture content standard INSTA 141 or EN 14298 can be applied. Requirements, which deviate from standard rule have to be defined in the contract.
At the change of moisture content less than 20% the following has to be considered:
A. The actual thickness and width can become smaller than the nominal size, which applies at 20%. The relation of 1% shrinkage to 4% reduction of moisture content holds true as a guideline.
Example: Nominal size 50x100mm at 20% moisture content equals size 49x98mm at 12% moisture content.
B. The occurrence of fissures and warp may increase and exceed the table values if the timber is dried to moisture content below 20%.
Insect damage
Insect damage is not permitted. Such insect damages (holes) under 2mm that cannot be discerned at grading will be permitted, provided that the depth does not exceed the permitted negative size deviation. The maximum number of wood pieces with insect damage is 2% of the total number.
Width of annual rings
The width of annual rings varies according to the growing conditions of different forest areas. Requirements concerning the average width of annual rings are therefore not given. If special requirements are needed they have to be defined in the contract between the seller and buyer.
Other grade inclusion
The guideline is that at least 90% of the pieces in the lot shall not have wood features with values exceeding the maximum permitted values for a contracted grade.
Other grading applications
If there are specific reasons to deviate from these grading rules, e.g. because of different species etc, or to consider a wood feature outside these rules, it must be agreed in the contract.
