Timber - Strength & Grading Of C16 & C24 Structural Timber
Strength and Grading – C16 and C24 Grading of Timber
The strength of timber is a function of species, density, size and form of member, moisture content and duration of applied loading, together with strength-reducing characteristics such as slope of grain, knots, fissures (splits parallel with the grain as a result of drying/shrinkage) and wane (a rounded edge of a timber section).
Strength grading methods have been devised to classify timber using either visual strength grading or machine strength grading methods.
Visual or machine strength grading overcomes the inherent variability of structural timber by assessing the strength and stiffness of individual pieces and assigning them to appropriate strength grade and/or strength class which has known and dependable mechanical properties.
A strength class is simply a group of species/strength grade combinations which have similar properties. The solid timber strength classes range from C14 to C45 for 12 softwoods and from D30 to D70 for six hardwoods. The numbers represent the characteristic bending stress for each strength class.
Timber used for structural applications must comply with the correct standards and strength requirements as stipulated by building codes and regulations.
Structural or ‘carcassing’ timber is graded by strength according to British Standard BS EN 140811-1: 2005 i.e. C16 or stronger C24 grade.
The C16 rated timber is home grown but C24 grade is only available from imported Russian and Scandinavian timber.
C16 graded timber is kiln dried to reduce moisture and is primarily used for internal building work in floor and roof joists. C24 graded premium structural timber is a kiln dried stronger, more demanding grade and has fewer/smaller knots than C16. C24 is best used where higher loads or wider spans are required. C24 will carry a higher point load than C16 grade.
The Building Regulations require structural timber to be strength graded and marked dry or kiln dried. A grade mark should appear on all pieces of structural timber. If there is no grade mark the piece must not be used for structural purposes.
Quality Grading Stamp – Softwoods
Since timber is a natural product its quality depends greatly on how and where it was grown on the species, on the age at which it was cut, and many other factors.
Sawn softwood which is imported in large quantities into this country is graded according to rules which vary slightly according to the exporting country. Normally however, grading is based on the number of allowable defects.
Six grades are recognised in the United Kingdom. Grade 1 Clear and Grade 1 are generally suitable for joinery and high class structural work, whilst Grade 2 is for general structural and carcassing. Grades
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3 and 4 are for general use where neither consistent strength nor appearance are essential.
The Shipping Mark is the stamp that timber suppliers put on the end of each plank and it indicates the shipping company and the grade.
The ‘crown’ in the image above indicates that the timber is unsorted quality, the ‘star’ generally means fifths, and the ‘plus’ sixths. Unsorted and fifths are qualities used where the visual aspect is important, sixths are used for hidden and constructional work.
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