Question
We have a mature purple ash tree (20-25 year we estimate based on previous owner's stories) in our backyard that is maybe 20 feet from our house and 20 feet from our garage. We want to build a patio between the tree and the garage, but a lateral root is significantly above ground in that area. If we want to cut that root, what is the risk and what is the best way to sever it? (For instance, would it be better to simply slice off the top of the root versus severing it? Or sever it as far as possible away from the trunk?)

Thanks for any advice you can offer!

Answer
It is better to cut the root than to shave the top off. Shaving will allow fungi to enter the root and cause root rot disease. A clean cut root will heal over to prevent fungi infection. It is not how close you cut the roots to the trunk but how much of the root area you cut. If you think of the root system as being the inverse of the foliage of a tree cutting roots will effect the foliage and cause die back of the branches. If you cut more than 1/3 of the root system then the tree may also die. Cutting a large roots you can expect some foliage dieback but the tree should be ok. But remember over the years the tree will grow back roots under the patio and could in later year cause the patio to crack. IF you cut the root I would apply fertilizer to the tree to try to off set the damage. Use 10-10-10 fertilizer at the rate of 1 lb per inch of trunk diameter scattered around the tree and watered in good.