Structural Grading

THa / THb and TH1/TH2 graded oak

TH1 and TH2 are smaller beams, under 20000mm2 end grain area, or less than 101mm in one dimension. So 200×100 or 141×141 would be the largest pieces in this grade. TH1 is a higher grade than TH2. It is common for merchants to decline to supply TH1 unless you agree to have it in a joinery grade.

THa and THb are larger beams, over 20000mm2 cross sectional area. For example larger than than 200×100 or 142×142. Tha beams are cleaner than THb.

THa and TH1 will have a D40 strength grade, whereas THb and TH2 will be D30.

The moisture content should be below 20% unless they are wet graded. Structural oak is nearly always over 20% so you should expect to see wet graded listed most of the time. The charts below give the definitions of acceptable defects:

The Construction Products Directive (89/106/EEC) or CPD is to replace national standards to allow the free movement of material within the member states of the EU, CE marked material cannot be refused entry on technical grounds into any EU state.

A CE mark on structural oak means that it has been certified as meeting the standards set out in EN BS 5756 and EN14081. The CE mark should specify the following information:

Species code
Visual grade, THa/ THb or TH1/TH2
Strength grade associated with visual grade, D30 or D40
Wet or dry graded
Company and individual grader identification number
EN 14081 and BS 5756

Size Greater than 20,000mm2 or 100mm thick
Wane 1/3 maximum
Moisture content 20%
Fissures Not through

Not greater than 1m or 1/4 the length of the piece, whichever is lesser

Through

At ends only, not greater than width

Slope of grain 1:15 maximum or 1:6 within 500mm of ends
Shape
Bow Not greater than 10mm over 2m
Cup Unlimited
Spring Not greater than 8mm over 2mm
Twist Not greater than 1mm per 25mm width over 2m
Knots 1/5 maximum of thickness or width
Longitudinal separation of knots Knots accumulative if less than 2 x width or when grain does not recover
Bark pockets Within 500mm of the ends, not greater than the width.

Elsewhere not greater than 1.5 times the width or 1/5 of the length, whichever is lesser

Insect and fungal attack No active infestation or wood wasp holes
Size Greater than 20,000mm2 or 100mm thick
Wane 1/3 maximum
Moisture content 20%
Fissures
Not through

Not greater than 1m or 1/4 the length of the piece, whichever is lesser

Through

At end only, not greater than width

Slope of grain 1:4 maximum
Shape
Bow Not greater than 10mm over 2m
Cup Unlimited
Spring Not greater than 8mm over 2mm
Twist Not greater than 1mm per 25mm width over 2mm
Knots 3/4 maximum of thickness or width
Longitudinal separation of knots Accumulative if less than 2 x width or when grain does not recover
Bark pockets Within 500 mm of the ends, not greater than the width.

Elsewhere not greater than 1.5 times the width or 1/5 of the length, whichever is lesser

Insect and fungal attack No active infestation or wood wasp holes
Size Less than 20,000m2 or 100mm thick
Wane 1/3 maximum
Moisture content 20%
Fissures
Not through

Not greater than 1m or 1/4 the length of the piece, whichever is lesser

Through

At end only, not greater than width

Slope of grain 1:10 maximum or 1:5 within 500mm of ends
Shape
Bow Not greater than 10mm over 2mm
Cup Unlimited
Spring Not greater than 8mm over 2mm
Twist Not greater than 1mm per 25mm width over 2m
Knots 1/4 maximum of thickness or width
Longitudinal separation of knots Accumulative if less than 2 x width or when grain does not recover
Size Less than 20,000m2 or 100mm thick
Wane 1/3 maximum
Moisture content 20%
Fissures
Not through Not greater than 1m or 1/4 the length of the piece, whichever is lesser
Fissures
At end only, not greater than width
Slope of grain 1:4 maximum
Shape
Bow Not greater than 10mm over 2m
Cup Unlimited
Spring Not greater than 8mm over 2mm
Twist Not greater than 1mm per 25 mm width over 2m
Knots 3/4 maximum of thickness or width
Longitudinal separation of knots Knots accumulative if less than 2 x width or when grain does not recover

Download the pdf

PDF