Wood
Lumber
noun
Wood that is lumber is lumber wood.
もくざい
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
ひまだったら、木材を運ぶのを手伝ってくれない？
If you have nothing to do, can you help me carry some wood?
「木材」は、まだほとんど加工されていない原料としての木のことを指し、「材木」は、その木材を加工してすぐ使える状態に仕上げたものを指します。
"Mokuzai" refers to raw wood material that is not yet processed, and "zaimoku" refers to wood that is a finished product and can be used immediately.
二日なら時間を作れるよ。いつもの木材置き場で話さない？
I can make time on the second. Why don’t we meet up at the lumberyard and talk?