Question
QUESTION: I am considering building an outdoor structure next to a large pine (~25 in trunk diameter).  Building the structure would require me trenching across on one side of the tree (~2ft next to the trunk), to a depth of 24 in.  I have seen conflicting information as to what this would do to both the stability and health of the tree so any advice would be appreciated. Thanks.

ANSWER: The general rule is not to cut more than 20% of the roots of a tree before there is damage to the health of the tree. If you take the outline of the outer most limbs and draw a circle around the tree and then draw where you are going to trench and if the cut of the "pie" is more than 20% you will damage the root system of the tree. The roots are between 18-24 inches deep. The stability of the pine may not be harmed since the pine has a tap root that will hold the tree. But the cutting of the feeder roots will effect the water and food up take and will cause at the very least die back of some or all limbs on that side of the tree. The other thing with pine is in some sandy soil there is a fungus that will attack pine roots that are damaged and could cause the tree to die. Longleaf pine seem to do ok with this type root cutting but the other pines sometimes have trouble.
I would try thinking about maybe pilings and not trenching--the holes for the posts will not be a problem--the trenching is the problem. You did not say what type of structure so I am just guessing maybe a deck. Or even it is a room a crawl space would work rather than a slab and footing.


---------- FOLLOW-UP ----------

QUESTION: Thank you for your prompt reply.  My structure is indeed an outdoor kitchen on a slab foundation.  At the same time the area I am planning to cover was already mostly under concrete, except right next to the tree where I had enclosed gazebo (built as a deck).  Given that I don't think the tree was getting much water on that side anyway, do you still foresee issues?

Answer
If you extend the present concrete slab toward the pine I think you will get some die back of the limbs closest to the slab. If you do this I would fertilize the rest of the trees roots with 10-10-10 fertilizer at the rate of 1 lb per inch of diameter of trunk. This will help the remaining root system cope with the loss on that side.