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Context provided by current standards

KVH® structural timber and Duobalken® and Triobalken® laminated beams have provided proven service for many years in meeting the increasing requirements associated with modern timber construction. The requirement for dry and dimensionally stable construction timber to be used for carpentry and constructional timber works is now specified in all of the main building regulations.

DIN 4074-1: Strength grading of wood - Part 1: Sawn coniferous timber

The grading criteria set out in DIN 4074-1 for classifying the load-bearing capacity of construction timber are based on timber dimensions related to an official value of moisture content of um = 20 %, in other words dry timber. If the timber is not graded in a dry state (TS labeling), suitable allowance has to be made, e.g. by sawing to oversizes. Even then, the grading criteria of cracks caused by shrinking and of curvature still remain unaccoun-ted for. Given that the only means of checking with certainty that all the grading criteria are met is if the wood is dry, the timber has to be carefully re-graded by the person handling the wood immediately prior to installation and rejected where necessary, because that person is duty-bound only to install timber which has the "required equilibrium moisture content", which means dry timber. If the timber were to be installed in a semi-dry or moist state, it would have to be examined in what as a rule would be a dry equilibrium state to see if any strength-reducing cracks had developed, which would represent an unacceptable situation. The time and effort that this requires can be avoided if KVH® and Duobalken® and Triobalken® are used, because the timber used in these products is only ever graded when dry and meets more stringent quality criteria than those specified in DIN 4074-1.

Bauregelliste A (Construction Products List A): Labeling with the "Ü" mark

Ever since 2004, German Bauregelliste A (Construction Products List A) specifies that grading standard DIN 4074-1 has to be used as the reference standard for the inspection mark. This means that only solid timber which has been graded when completely dry and bears an "Ü" inspection mark containing specific reference to DIN 4074-1 is unequivocally a regulated construction product. KVH® structural timber also has an additional symbol in the "Ü" mark to indicate the certifying body, because the product is finger-jointed. Solid timber without finger-joints will in future be permitted under European regulations to bear a CE mark of conformity as an alternative.

Duobalken® and Triobalken® beams also have to be labeled with the "Ü" mark, but instead of reference to DIN 4074-1 these labels have to contain the approval number and the symbol of the certifying body.

At present there is no European technical standard, which means that there is no associated CE mark of conformity available either.

ATV DIN 18334: VOB/C

Carpentry and constructional timber works

It is a requirement of the German general technical specifications for building works for carpentry and timber construction works (DIN 18334) (Allgemeine Technische Vertragsbedingungen für Bauleistungen [ATV] für Zimmer- und Holzbauarbeiten) that construction timber made of sawn softwood must always be dry before it is installed. The 2000 version of this regulation introduced more stringent requirements for timber house construction with regard to moisture content (um ≤ 18 %), dimensional stability and type of cutting.

According to the commentary on the standard [2], internal timber in multi-storey timber buildings has to have a lower moisture content because of the settlement characteristics of such buildings. This requirement is particularly topical now that the German Model Building Regulations (Musterbauordnung [MBO] 2002) have been incorporated in the building regulations of the individual federal states and allow the principle of permitting the erection of timber construction buildings of up to five storeys in height.

DIN 1052 – Design of timber structures

The dimensioning standard for timber construction is another standard which requires the installation of dry timber for Service Classes 1 and 2, on the basis that this is required to reduce cracks caused by shrinking and changes in dimensions. Furthermore, it is only possible to match softwood products between the grading classes specified in DIN 4074-1 and the strength classes specified in DIN 1052 if the timber has been graded when dry (TS). It also has to be taken into consideration that the characteristic strengths used for the strength classes only partially account for cracking.

Caution is required when installing timber without suitably adjusted moisture content

Where timber with a much higher moisture content than the required equilibrium moisture content is installed, DIN 1052 requires that the timber must be able to dry out afterwards and that the structural timber components themselves, along with all adjacent building components, must be resistant to deformation through the shrinkage which can be associated with drying. The construction work required here, and the greater risk of building damage, can be avoided if KVH® structural timber and Duobalken® and Triobalken® are used.

Example:In relation to the load-bearing capacity and exact fit of connections, DIN 1052 requires the following in respect of the installation of timber with excessive moisture content:

A wooden ceiling beam with a moisture content for internal timber of um = 20% can dry out to 6% in the climate of a heated residential building. For a beam of 24 cm in height a 14% change in moisture content would result in a mean shrinkage ratio of 8 mm. The concomitant settlement has to be accommodated without any damage occurring. It would be better to use dry timber from the outset, e.g. Duobalken or Triobalken beams with a maximum um = 15%.

  • The reduction of the characteristic values to 2/3 for the load-bearing capacity for nail connections exposed to pull out stress,
  • Deformation through shrinkage in relation to butt joints and contact connections to be taken into consideration in the verification of serviceability
  • The retightening of bolted joints

Here again, KVH®, Duobalken® and Triobalken® offer decisive advantages over normal sawn structural timber.

Wood preservation

Finally it should also be noted that the use of dry timber is an important precondition for the classification of component attachments in Use Class 0 in accordance with DIN 68800-2, in order to be able to dispense with preservative treatment.

In addition, a field study [3] demonstrated that building components made of kiln-dried timber used for interiors and for protected outside areas are "resistant to insects".