jmc

Imagine the square whose diagonal would be PQ. Clearly, that square would be formed of 9 of the shaded squares. The formula for the area of a square from its diagonal is $A=\frac{d^2}{2}$, therefore, the area of that imaginary square is 18. Thus, each smaller shaded square has area 2, making for a total of $\boxed{32\text{ square cm}}$ for the entire shaded area.