Strength Grading

What is strength grading?

BSW produce strength graded timber to specifications laid down in British Standards - BS4978 for visual grading and BS EN 519 for machine grading. Timber is graded to strength classes. The most commonly specified grade for softwood carcassing is C16.

What are the benefits of strength grading?

Strength grading is undertaken primarily to provide timber of sufficient strength for a particular function, e.g. a floor joist or a roof truss. At the same time strength grading helps to ensure that both the design and the timber sizes specified are economical.

How do BSW strength grade?

After kiln drying to an average moisture content of below 20%, the timber is passed through a highly automated grading line where a combination of measurements are taken which closely relate to strength. From this, the machine determines the Strength Class of the piece of timber. The machines are calibrated daily and the company operates a third party certification scheme under ISO 9000.

To satisfy British Standard building regulations, structural timber used in buildings must be kiln dried to an average of approximately 20% or less and be marked DRY or KD. The grade stamp must also contain information on the species - the strength class - the graders licence number - the BS number and the certification body logo or mark.

If the moisture content is higher, the timber must be marked WET. Wet timber must not be used for internal structural use.

Strength class Chart Table showing the relationship between the strength class, the species of European/British grown timber and the General Structural and Special Visual Grades.

SOURCE AND SPECIES	GRADE RULES	STRENGTH CLASSES
		C14	C16	C18	C22	C24	TR26	C27	C30
BRITISH
British Pine	BS4978 vis	GS			SS
	S EN519	•	•	•	•	•	•	•
British Spruce	BS4978 vis	GS		SS
	BS EN519	•	•	•	•	•
Douglas Fir	BS4978 vis	GS		SS
	BS EN519	•	•	•	•	•
Larch	BS4978 vis	GS				SS
	BS EN519	•	•	•	•	•	•	•

SOURCE AND SPECIES	GRADE RULES	STRENGTH CLASSES
		C14	C16	C18	C22	C24	TR26	C27	C30
EUROPEAN
Redwood	BS4978		GS			SS
Whitewood	BS EN519	•	•	•	•	•	•	•