To factor the expression \( 9z + 18 \) using the greatest common factor (GCF), we first identify the GCF of the coefficients.
The GCF of \( 9 \) and \( 18 \) is \( 9 \).
Now, we factor out \( 9 \) from the expression:
\[ 9z + 18 = 9(z + 2) \]
So, the factored form of \( 9z + 18 \) is \( \boxed{9(z + 2)} \).