Does new C16 grade wood need preservative treatment?

[Tony]

Builders have overhauled part of my roof and its ready for me to start lining it.

It never occurred to me to ask the builders whether it was prudent/necessary to treat the new spars and purlins with a wood preservative before sealing everything up. The wood is all stamped "C16". Do i need to?

thanks Tony

[John Armstrong]

C16 is the structural grading of the timber. It should also have been pressure treated but might not have been.