When you ask your oak framer about what oak they’re using, they may come back and say we all use the same type of oak (Quercus robur). This basically is a true statement.
However, the question you should be asking is what grade has the structural engineer specified for your building, to your specifications, on your site? This should always take into consideration elevation, orientation, and wind loadings, and come with reassurance that the grades purchased are that specification. They should also be able to confirm that they have qualified timber graders involved in the manufacturing process, to guarantee that the correct standards have been met.
Another term that you might see is “English-sawn oak”. These are trees that have been sourced in Europe, brought to the UK, and then cut in the UK.
When purchasing an oak frame building, you should always remember there are varying oak grades:
- QPA and QP1 are appearance classes EN957-1. These then relate to Strength Classes EN 338, which a structural engineer uses to specify the strength class the oak needs to be.
- QPA relates to D30, which is what is used as a standard strength grade.
- QP1 relates to D24; some engineers may use this, but it means you would need a larger section of timber to pass the same calculations.
When ordering from Europe, you will find that they classify their grades as written below:
QP1
- Sawn timber with particularly sharp arrises, a permitting wane less than 10% of the face width across no more than 30% of the length. This width tolerance is increased to 15% for sections above 250 x 250mm.
- Sound sapwood is permitted on two arrises if the total width is less than 15% of the face width.
- Fully or partially intergrown sound knots are permitted if the diameter is less than half the face width.
- Two dead knots are permitted per linear metre if the diameter is less than one-quarter face width.
- Boxed heart is permitted, as well as traces of heartwood on both faces. A slope of grain less than 12% is permitted and should not exceed 20% locally.
- Brown streak, black holes, and brown pith are all permitted on a number of pieces.
- Unsound knots, frost crack, ring shake, star shake, curly grain, bark pocket, unsound sapwood, rot, and white holes are all excluded.
QPA
- Sawn timber with sharp arrises; in case of pieces longer than 3m, a wane less than 10% of the face width is permitted across no more than 25% of the length.
- Sound sapwood is permitted on two arrises if the total width is less than 15% of the face width.
- Fully or partially intergrown sound knots are permitted if the diameter is less than one-third of the face width.
- Dead knots are permitted where equivalent to two dead knots with a diameter less than 15mm per linear metre.
- Boxed heart is permitted, as well as slight traces of heartwood, on two faces. A slope of grain less than 7% is permitted and should not exceed 12% locally.
- Unsound knots, end shake, frost crack, ring shake, star shake, curly grain, bark pocket, unsound sapwood, brown pith, brown streak, rot, and holes are all excluded.
What We Purchase
Fresh Green Oak Beams’ Standards
The following requirements must be met, as well as any additional ones specified on our purchase order and/or timber list:
1. Sawn timber with sharp edges.
2. Tolerance of small quantities of sound sapwood on two edges maximum, covering less than 30% of the width of the faces and edges.
3. Sound knots are accepted if the diameter is below 50% of the total width for timber cross-section size > 20000mm2 or 25% of the total width for timber cross-section size < 20000mm2 (this is reduced to 20% for all timber cross-section sizes if D40 grade is specified on the purchase order and/or timber list). Knots are accumulative if longitudinal separation is less than twice the width or 300mm (whichever is the lesser), or when the grain has not fully recovered.
4. Grain angle does not exceed 1 in 4 for timber cross-section size > 20000mm2 or 1 in 10 for timber cross-section size < 20000mm2 (this is amended to 1 in 12 for all timber cross-section sizes if D40 grade is specified on the purchase order and/or timber list).
5. Heart splits and frost is accepted on ends if they are beyond the ordered length.
6. Dimensions width & depth tolerance: 0mm + 5mm. Length – 0 mm + mm.
7. Distortion and bow should not be more than 10mm over 2m in length, spring 8mm over the length of 2m, and a twist no greater than 1mm per 25mm width in 2m.
8. Unnatural staining from machinery is tolerated up to 10% of each face.
9. Timber with the following defects will not be accepted:
a. Woodworm
b. Bug holes
c. Heart shaking
d. Soft knots
e. Inner bark, or black vein
f. Wane
g. Brown Stain
h. Brown heart
i. Ringshake
Images of some of these defects are shown on the attached sheet for illustration purposes.
Packing Requirements
1. Packs will need to be identified by customer name and order number. The size needs to be identified at one end of each beam.
2. An identifying colour will be allocated to each supplier, which needs to be placed at the end of each beam. A clear and concise packing list/delivery note.
3. The maximum weight of an oak bundle is 3000kg.
4. Timbers must be stacked separating each layer with sticks every 750mm to prevent bowing.
5. Stacks must be in descending order.
6. Banding straps must be used every 1.5m.
7. If metal straps are used, a protector must be used to prevent timber staining.
8. When ordering “stock”, the same-sized beams need to pack together. Customer order requirements (random lengths) need to be kept together.
Oak Graders
It is also worth noting that there are certain characteristics of an oak beam that will make it unsatisfactory from a structural point of view. If used, you are risking the failure of that beam. This is why it is essential to have a qualified oak timber grader grading the beams that are going into your building. This matters because there will be more potential for structural defects in these timbers.
If we brought a standard grade with our grading we would reject about 35% to 40% of them. Our oak timber graders are also trained to grade to our standards and they are tested and re-certificated every year by BM TRADA.
So, the chain of custody is the structural engineer using the Eurocodes to design the frame. They will take into account the positioning of the building in the country, elevation, and orientation, while also allowing for wind loadings in the calculation.
In Europe, there are around 55 to 60 oak mills that are capable of producing oak beams to a commercially-required level, with the aid of highly experienced timber agents. We have only been able to identify 5 mills that can consistently produce the quality of oak we demand for our buildings. From the next group of mills, of which we estimate there to be about 30, we could buy the standard grades for around 10% less than what we buy our grade for. From this grade we buy, we also have to allow for a 40% rejection rate when it goes through our 4 stages of quality grading.
These mills will have a consistent enough quality to deliver those grades. The rest would not be able to maintain the quality of the standard grades to make them viable options for beams. So, when a manufacturer says we all use the same oak, they are only referring to the botanical group. They are not referring to the quality and grading of the beams that are actually going to be used in your frame.
