How much backfill is OK?

[KYGardener]

We have two old red oaks that have roots exposed from erosion over a long period. We're landscaping now and I want to put a small covering of dirt over those roots and have a retaining wall planned to keep future erosion in check. The canopy is about 13' on each side of the trunk and we're staying away from that with the wall. I want to know if covering up those areas and under the tree with soil will likely smother it? Planning on about 4-6" on the roots with 10" where the retaining wall is highest out 12 ft from the trunk. Hope this is clear enough for you to help us do the right thing? Thanks!

 

[Tom Dunlap]

Backfill with the best organic soil that you can find. Composted leaf/woodchip mulch is a good option too. Keep any of the backfill away from the trunk.

 

[treeseer]

We have two old red oaks...The canopy is about 13' on each side of the trunk ... Hope this is clear enough for you to help us do the right thing? Thanks!

Not very old if the branches are only 13' long. Pictures would be very useful. Aslo see Avoiding Damage at the link below.

Roots are often much longer than branches. Get you trees inspected and cared for by a pro; you wnat to be very proactive with red oaks, a species with many disease pressures.

 

[Kneejerk Bombas]

Pictures would help.
Generally though 4 inches of good soil should be tolerable to the trees, 6 inches is getting dangerous. If you can, add 2" each year. This will allow the roots to grow up into the new soil and give the tree time to acclimate to the new soil level.
Adding a retaining wall will be much like taking a trencher and cutting the roots off at the point where the wall goes in. If the trees are in good health, you stay 12' away, take extra caution not to compact the existing soil, and the wall is only on one side of the trees, the trees should be able to stand the root area loss.
Now if you add the wall and 6 inches of soil, all at the same time, you could easily kill the trees.