Question
Would there be an effect on my Douglas Fir trees if soil was added above the established root systems?

Answer
Smothering Roots by Adding Soil. Most people are surprised to learn that 90 percent of the fine roots that absorb water and minerals are in the upper 6 to 12 inches of soil. Roots require space, air, and water. Roots grow best where these requirements are met, which is usually near the soil surface. Piling soil over the root system or increasing the grade smothers the roots. It takes only a few inches of added soil to kill a sensitive mature tree.

You can safely cover tree roots as long a sit is not more than about 2 inches of fill. More than this will damage the roots. I would use sand since you are going to sod the area. And since you are going to sod. Sand is better than clay soil since it will not compact as hard and damage the roots.  I would just add sand to even with or just above the roots and lay the sod on top. Since the water table is not far beneath the surface over the years the roots may again become exposed from the growth. When this happens just add more sand.