C and D – what do they stand for?

The European Standard EN 338 contains a list of strength classes for structural timber.  There is a set primarily for softwoods which begin with the letter C (e.g. C16 and C24) and a set for hardwoods which begin with the letter D (e.g.D24 and D30).

It is sometimes said (e.g. 1, 2, 3) that the C stands for Coniferous and D stands for Deciduous.   This doesn’t really make sense.

Firstly – the 2016 revision of EN 338 allows hardwoods to be assigned to C grades…so they are no longer exclusively for softwood.  But there is a more fundamental issue…

Softwoods are conifers.  Conifers are gymnosperms – cone bearing seed plants.

Hardwoods (or broadleaves) are angiosperms – flowering plants.  More particularly, they are dicotyledons (they have two seed leaves) putting them in a different category from bamboo (which is a monocotyledon, one seed leaf)

Whereas…

Deciduous means that the tree loses its foliage for part of the year – as opposed to evergreen, where the foliage is kept all year.

Hardwood is not synonymous with deciduous.   Many tropical timbers are evergreen – so it would certainly not make sense to think that the D in D70 Greenheart stands for deciduous!

And while we are on the topic – conifer is not synonymous with evergreen either!  The most familiar deciduous conifers are larches – but there are a few others too (list below).

So what do C and D stand for?  Well – they don’t need to stand for anything.  But if they had to – Conifer and Dicot would make slightly more sense.

 

Deciduous conifers:

Larches, which include
Larix decidua (European larch) [LADC]
Larix kaempferi (Japanese larch) [LAKM]
Larix x eurolepis (Dunkeld larch) [LAER]
Larix occidentalis (Western larch) [LAOC]
Larix gmelinii (“Siberian” / Dahurian larch) [LAGM]

Pseudolarix amabilis (Golden Larch)

Taxodium distichum (Southern / Bald cypress) [TADS]
Taxodium ascendens (Pond cypress)

Glyptostrobus pensilis (Chinese Swamp Cypress)

Metasequoia glyptostroboides (Dawn Redwood)

[XXXX] is the EN 13556 species code

 

2 Comments

  1. Hi, I hope you can help. I’m due to put a patched roof on my extension with the longest span being 5.2m using 8″x2″. I’ve been advised to use C24 grade or TR26 grade timbers. What is the difference between the two if any please ?

    • Hi Robert
      C24 and TR26 are similar softwood grades, C24 is a common one for general construction across Europe and TR26 is for the UK trussed rafter market. The design properties of TR26 are a little better than for C24, but I doubt in a way that makes any practical difference to you. More relevant will be the availability of the sizes you need and the price.

      More on TR26

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