Use class for decking joists

9 Jul 2020
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United Kingdom
Long-time lurker, first-time poster

I'm starting my first deck build and have been researching materials needed to construct the timber frame.

One thing I've picked up quite early on is to make sure I use the right grade of wood for the joists, in terms of structural quality and longevity.

Under BS8471 there are 4 use classes in terms of preservative treatment:

Class 1 Internal building timbers with no risk of wetting or condensation.
Class 2 Internal building timbers with a risk of wetting or condensation.
Class 3 External building, fencing and landscaping timbers used out of ground contact.
Class 4 External building, fencing and landss used in ground contact.

The professional bodies are very clear in their guidance:

Builders’ merchants sell ‘joist material’ and some of it will be labelled ‘preservative treated’, ‘green treated’, or perhaps ‘brown treated’, or even just as ‘treated’. What does this actually mean? Joist material or carcassing is generally intended for use internally within a building. This means it may have received a level of preservative treatment appropriate to Use Class 2, for interior use only, where the timber is unlikely to be in contact with any water. It won’t therefore last if you use it outside under decking where it will inevitably get wet.
The message is very clear: use UC4, and definitely, definitely don't use anything less than UC3. I've spent this morning trying to track down UC4 timber in London, which seems to be virtually nonexistent.

Every timber yard except for one had no idea what I was even talking about, most just said "it's treated" and could offer no further details. Others said that it was UC2 "whatever that means... is fine for decking, we use it all the time".

Wickes sell a very small range of UC4 products (wrong sizes for me), and I found one old-skool timber yard miles away that can provide UC3.

Having all the supposed experts think I'm asking silly questions has left me feeling like I'm an idiot: am I over-thinking this? What am I missing here?
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