wait is 木材 just wood or what

so i was walking past this construction site near the station (i think it was near the 西口 exit anyway) and there was a sign saying 木材 everywhere. i thought would was just Ki 木 ?? why is there a second kanji here lol. like is it fancy wood or tree material? my phone keeps changing wood to would... ugh anyway do i need to use 木材 or is 木 fine? i just want to talk about lumber i think