We know that it happens – is it allowed? More importantly – does it correctly grade timber?

The latest version of EN 14081-1:2016 contains a general clause (5.1.1) with the following text:

“Structural timber that has previously been graded shall not be re-graded to the same or different grades unless the method of determining characteristic values has made allowances for changes to the timber population caused by the previous grading.”

You can read more about that here and here, but perhaps it is enough to say that the basis of grading is the testing of a representative sample of timber to see what the properties of the graded timber actually are.  Implicit in this is the assumption that the before grading population is always the same (near enough) as the representative sample.

Let’s examine a very simple example – imagine we have graded 200 pieces of timber, and the characteristic (5th percentile) strength is only just what is required for the target strength class. If we  actually knew how strong those pieces really were (by testing them) then we could rank them in increasing order of strength – and the one that is 10th in the list (5% of 200) would have a strength that is higher (just) than the characteristic strength defined for the target strength class.  Everything is fine.

Now imagine we took 100 of those pieces of timber, and we took them in such a way that favoured stronger pieces.  Now, if we rank the our 100 remaining pieces in order of increasing strength we would find that the one that is 5th in the list (5% of 100) has a strength that is less than the characteristic strength defined for the target strength class.  Everything is not fine.  This happened because we took away proportionally more of the better 95% than the worse 5%.

In real life, we don’t know the actual strengths of the pieces which is why we cannot specify a strength class with a minimum value.  Instead we use the 5th percentile because we can design on the basis of 5% of pieces being weaker than this, even if we don’t know which pieces they are. (This is not unique to timber, we never have perfect knowledge of any material)

Now imagine that we took those pieces of timber out before we did the grading – we’d end up with the exact same problem.  The grading wouldn’t work because we’ve done something that changed the population.

This is what happens when you re-grade timber that was already graded.  The original grading changed the population – so if we apply re-grading without accounting for that change the re-grading probably won’t work.  Accounting for that change is very hard, and very specific – and as far as I know it’s never done.

Re-grading rejected timber is an extreme case of this – because you really only have the poorest pieces.

So – you should not do it, but it is allowed?

Well, yes…it is allowed.  Just not advisable.

This previous post explained why EN 14081-1:2016 is not yet regarded as the harmonized standard for structural timber, because it is not yet cited in the OJEU.  This means we can legitimately look at what is written in the (now withdrawn) previous version. EN 14081-1:2005+A1:2011

The previous version had the same text, but it was in clause 5.3.6, which is under clause 5.3 – specifically about machine strength grading.  Technically, it was therefore not forbidden to simply apply visual grading rules to machine reject (or graded) timber.  This was an oversight in the writing of the standard, which is why the revised standard moved this text to a general section.  Just because you are not forbidden from doing something, doesn’t mean it’s ok!