Question
I plan to excavate some land and would like to know what a douglas fir root system generally looks like below the ground so that I can avoid it and preserve the health of the tree if possible.  So far I've been able to find out that the root system spreads horizontally to roughly match the tree crown.  However, I've not had any luck with how deep the roots go relative to it's height and at what angle, as measured from horizontal ground, the roots grow (not sure if this will help but the tree is located in the Portland, OR hills and the soil is permeable enough to allow roots to grow freely).  Since I will be digging on the northwest side of the root system and the winds come from the south east then I'm assuming I shouldn't have any issues with the stability of the tree but any advice I can get on this is welcome.

Answer
Generally the roots system is within the first 24-36 inches of soil and extends out about 1 1/2 times the width of the branches in all directions. Firs have a tap roots system also that will extend deep into the soil just below the trunk.

You can cut up to about 20% of the roots without causing serious damage to the tree. Normally, twenty percent of the tree's roots can be cut before any signs of stress will appear, however, keep in mind that tree roots do extend outside of the protected root zone. Think of the surface under a tree as a pie and slice through the "pie" to determine the amount of the roots that were cut. The damage will show up either this growing season or the next by the foliage starting to die back.
Healthy root systems below ground are vital for tree vigor and longevity. Roots are responsible for water and mineral nutrient uptake, energy storage, and anchorage. If for any reason tree roots are damaged, tree health will be jeopardized.
Because roots work quietly out of sight underground, most people have a poor understanding of this important subterranean network. In general, roots grow where the resources of life (water, oxygen, and mineral nutrients) are available. They usually will not grow where there is no oxygen or where the soil is compacted and hard to penetrate. This need for oxygen explains why a majority of tree roots are located in the top 12 to 18 inches of soil. Root systems are also extensive. They often extend outward from the tree trunk to occupy an irregularly shaped area 1 1/2  to 2 times larger than the crown (branch) spread. It is easy to see why any type of soil disturbance near trees can, and usually does cause damage. As trees mature in the landscape they attain a rather delicate balance with their surrounding environment. In fact, trees grow best in an environment of minimal change.  

You should not have any problems with stability since firs have the tap roots and you are not cutting much of the root system.