I've always been a fan of simply taking some wood and actually setting a flame to it, though you obviously have to be careful of how you handle that...wouldn't want to be setting off fire alarms or burning a building down, now, would we?
Um if you want to paint it on a model use Scorch brown as a base, then Slowly Dry brush lairs of Grey from light to dark then maybe give it a devlin mud wash
Get a brown base down and then layer it up to black at the edges. After that do a very fine drybrush of grey to get a charcoal effect right on the edges.
What I've done on ruined fantasy buildings (for Mordheim) is paint the wood as normal, then simply drybrush black fairly heavily on the "burnt" ends to represent a thick heavy soot/ash.
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