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Making The Grade - Dry Graded Structural Timber

Timber is a wonderful material - strong, durable, beautiful and renewable. Highly popular as a construction product in both traditional and modern applications and frequently preferred on environmental grounds, information on the correct specification of structural timber has never been more important.

In the UK, most structural timber used in construction is softwood - Redwood or Whitewood. Building Regulations require that all structural timber used in buildings has to be:

  • Strength graded at a moisture content appropriate to the service environment in which it will be used and marked DRY or KD (signifying kiln dried), if it is to be used internally.
  • Stamped with the appropriate grade marks, on each piece of structural timber.

For internal construction in the UK, Regulations (including BS5268: Part 2) require that structural timber must satisfy the dry grading requirements, i.e. dried to an average moisture content of 20% or less, with no piece exceeding 24% at the time of grading. Timber satisfying this requirement will be marked 'DRY' or 'KD'.

Where structural timber is to be used in a wet service environment, or has a thickness greater than 100 mm, the dry grading requirements are not applicable, although all other aspects of strength grading are required. Strength graded timber destined for such end uses will be marked 'WET'.

Bruce Muirhead, President of the STTA, which is campaigning to raise awareness of the importance of the supply of appropriate strength graded timber for structural use, says unequivocally:

"Everyone in the chain of timber procurement - designers, specifiers, importers, merchants, buyers and site staff - has to be aware of strength grading requirements and must demand the appropriate strength graded timber for the end use. If there is no grade mark on a piece of structural timber, or the marks are incomplete and there is no adequate explanation for its absence, the timber must not be used for structural purposes. The message is simple - if in doubt, ask".

What is strength grading and how is it carried out?

Put at its simplest, strength grading, formerly known as stress grading, is a method of determining the strength of a piece of timber destined for structural use. Specifiers and purchasers may encounter both the terms 'strength graded' and 'dry graded' used interchangeably. This is because although strength grading has been conducted for many decades, it is only relatively recently that moisture content requirements have been introduced into the Standards, hence the term 'dry grading'.

Strength grading may be carried out by either visual or mechanical methods; visually by individually licensed operatives and mechanically by companies operating individually licensed strength grading machines. As every grade mark contains a unique identifying number of the grader or grading machine, total traceability of each piece of graded timber is ensured - a vital factor in ensuring timber quality and building customer confidence.

The visual strength grading process is undertaken by specially trained individuals licensed by independent third party certification companies including, BM TRADA Certification, BRE QA and BSI QA. Visual strength graders examine each piece of timber assess it for the presence of naturally occurring growth characteristics which affect strength, including knots and slope of grain. Grading rules specify acceptable limits for these features. Visual strength grading produces two strength grades - GS (General Structural) and SS (Special Structural). BS4978 governs visual strength grading of softwoods.

Visual strength grading is carried out on both softwoods and hardwoods destined for structural use. BS5756 provides a single structural grade for hardwoods, HS (Hardwood Structural).

Machine, or mechanical, strength grading also requires the assessment of every piece of timber. Strength grading machines measure the stiffness of timber, as there is a correlation between the stiffness of timber and its other strength properties. Strength grading machines are licensed by independent third party certification companies, as are the companies which own and operate them. BS EN 519 governs the mechanical strength grading of softwoods. The machines can grade to the traditional strength grades, direct to a Strength Class or to a specialised strength grade such as TR26, for trussed rafter material.

Strength grading to the same Standards to those employed in the UK is also carried out in several overseas countries which supply the UK market. These overseas strength grading operations are also subject to the same stringent third party certification requirements as companies operating in the UK.

An independently validated system

The independent third party certification required for strength grading operations is further validated by the United Kingdom Timber Grading Committee (UKTGC). This independent body oversees the certification companies - in effect, 'checking the checkers'. The UKTGC is the ultimate arbiter of any timber grading dispute in the UK. It is an organisation with real teeth, with powers to suspend the licences of individuals and companies who are found to have contravened the rules.

Why use strength graded structural timber?

A key question and one which STTA President, Bruce Muirhead answers crisply. "As structural timber is a safety critical element of construction, the use of strength graded timber is required by Building Regulations".

Using strength graded timber which satisfies the dry grading requirements also brings additional advantages to the construction process. As a material which is 'fit for purpose' in structural engineering terms, it quite simply gives a better quality job. It is stronger and its dry nature increases its dimensional stability. This reduces the risk of shrinkage or distortion. The post-construction 'snagging' process is also much reduced. Dry graded timber - lighter because of its reduced moisture content - is easier to handle and it also machines better, giving a smoother finish, thereby providing improved appearance.

How do you identify strength graded timber?

Commenting on the importance of identifying the appropriate timber for structural use, Bruce Muirhead is clear that everyone in the supply chain has a responsibility. He says, "I want to flag up how important it is for everyone to know what grade marks to look for and to look for them proactively".

Each piece of strength graded timber is clearly marked and provides vital information. The key elements of these marks are:

  • The moisture content condition at the time of grading, i.e. DRY, KD or WET.
  • The strength grade or strength class of the piece.
  • The timber species or species combination.
  • Identification of the grader and company responsible for grading.
  • The relevant British/European Standard number.
  • The mark of the independent certification body.

The use of the term 'DRY' or 'KD' confirms that at the time of grading the piece of timber satisfied the moisture content requirements of the Standard. 'KD' indicates that the timber has been kiln dried as opposed to air dried. 'DRY' indicates that either air drying or kiln drying has been undertaken.

The strength grade or strength class confirms the minimum strength of the piece of timber. The strength class system was introduced by BS5268: Part 2. The requirements of each of the strength classes, for both softwoods and hardwoods, are satisfied by combinations of timber species and strength grade. For example, European Redwood or Whitewood of GS grade (strength graded visually or mechanically) satisfies the requirements of strength class C16, the same species graded to SS grade satisfies the requirements of strength class C24. In construction, the most commonly encountered strength grades/classes for softwood are C16, C24 and TR26.

The illustration below shows a typical grade mark, in this case from BM TRADA

Q mark example

It is essential that every piece of timber to be used for structural purposes is strength graded and appropriately marked. If there are any doubts about the validity of grade marks on timber then either the timber supplier or certification company should be consulted.

In conclusion, to ensure that the construction industry and its clients benefit fully from the qualities which timber delivers - strength, durability, beauty and environmental friendliness - the STTA and its Member companies are committed to supplying dry graded structural timber appropriate for the designed end use.

For further information Contact Us or The Timber Trade Federation (TTF) on Tel: 0207 839 1891 or The United Kingdom Timber Grading Committee who can be contacted through the TTF on the same number.

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