Green Wood

[Foxy]

I learned something interesting today. A long time ago, there was a town where there was a big fire, and most of the buildings burned down. But there were some buildings which stayed standing because they were built with green wood. I think they were new, and maybe the wood was wet or damp? Anyway, maybe that is connected to why V.F.D. used to build their buildings with green wood, so they couldn't be burned down right after they were built. 

[Dante]

I don't think it's ever mentioned in the series itself, but similar information about green wood has disseminated through the fandom and is generally taken as being the reason why V.F.D. uses green wood (or "emerald lumber", as Sir puts it (U.A. p. 170)).