I'm trying to figure out how close to a tree I can excavate for a footing to an addition to my house, without doing unacceptable damage to the roots. The tree is some sort of Asian elm, about 50 feet tall, with two conjoined (inosculated?) trunks, each about 20" in diameter. The City of Portland, Oregon, where I live, provides the guidelines depicted in the first drawing below, using a tree with a 12" diameter as an example. The formula is to take the diameter of the tree in inches and use that number, in feet, as the radius of a "root protection zone." In the second drawing below, I have applied this formula to the tree on my property, and shown the encroachment of the proposed footing into the root zone. Two questions: 1. For the purposes of figuring the diameter of the tree, I reasoned that the area of two 20" diameter trunks is roughly equal to one 30" trunk. Is that a valid way of looking at it? 2. The proposed footing, with "notched" corner, seems fairly safe, according the guidelines. What about a simple corner as depicted by the dashed lines -- how risky would that be? Thank you for your consideration. Ray
1. Using one 30" trunk for your calculations should give you a bit of extra margin, since the cross sectional area of two 20" trunks is the same as the area of one 28.3" trunk. I assume that the encroachment shown in your drawing includes forms and some work space around them. 2. The simple corner extends significantly closer to the tree trunks than the notched corner; so the risk will depend on the configuration of the roots. If there happens to be a main root that needs to be removed due to the construction, then there could be some risk of weakening the tree's resistance to windfall.
I would say that one is tricky because some tree roots seek out concrete because water tends to pool there. The other problem that might happen in correlation with that is if you have to cut any roots more will grow and most defiantly seek out water. The tree you have is almost as tall as it can get so it is full grown. also changing the landscaping that drastically could change the water table that the tree is feeding off of. I believe that the Asian elm has horribly invasive roots. Those calculations are for a tree that is growing perfect roots I have not seen many trees that do : )
