Where projects are undertaken with long term durability and beauty in mind - it is well worth considering Western Red Cedar which is long lasting, has excellent dimensional stability and endurance and contains natural oils that protect the timber form insect attack and decay.  It smells good too, Western red Cedar is probably well known its distinctive aroma.  Over time the wood remains subtly aromatic and the characteristic fragrance adds another dimension to its universal appeal. 

Browsing through the excellent Green Building Forum I was prompted to post this photograph by a discussion on the subject of whether green oak was a suitable material for external cladding. The interior of Green Dragon Barn is dominated by a massive green oak frame that supports the internal floor structure as well as the roof, but the same material has had many less obvious uses, including the fascia and barge boards (not to mention the window frame) pictured here.

These boards do no more than provide the edging detail for the roof, so a bit of movement or even cracking is not really a problem. A large area of cladding would probably be another matter. The big advantage is a maintenance-free and visually attractive solution that just gets better with age. This easily outweighs the marginal additional cost of the oak, especially when the cost of painting (and re-painting) of cheaper softwood is taken into account. Many barn conversions have soffit, fascia and barge boards in softwood stained or painted to a sort of chocolate brown that doesn't cope well with the sun, wind and rain.

Incidentally, the fixings in the picture are galvanised nails. Using galvanised reduces greatly the problem of blackening when oak comes into contact with ferrous metal.