根太 is read as ねだ (neda) and means joist.
ねだ
neda
ねだいた
floorboard
こんぼん
foundation
ねまわし
digging around the roots of a tree (before transplanting)
ね
(fishing) reef
こんじょう
character
ねっこ
root (of a plant)
こんき
patience
ねざす
to come from
こんじ
complete cure
ねづよい
firmly rooted
こんぽんてき
fundamental
こんてい
root