Question
We have a mature blue spruce approx. 12' from our house.  It is about 15" in diameter and 35-40' tall.  Our neighbour has recently began a excavation for an addition to their home.  They have excavated a large hole at least 8' deep only about 4' away from our tree. My first concern is, will this increase the likelihood of this tree falling on our house in strong winds?  Secondly will this seriously harm our tree?  Thirdly having a tree this close to both houses is its root system likely to compromise the foundations?  Thank-you so much for your help!

Answer
No there will be NO problem with the roots from the blue spruce. At maturity the tree will have a spread of about 15 feet and the roots will spread out  about the same BUT unless there is a break in the foundation or the tree is right on top of a septic line the roots will not cause any problem. They have a tap root going straight down and feeder roots out from the trunk in the first 2 feet of soil. These feeder roots should cause no problem to the foundation.

It will depend on if that was the only root severed. If the roots was cut digging a hole and this was the only one I would thing there would be little damage to the whole root system. In the very least you might loose one limb but I would not expect any damage to show up.

Now if a trench was dug through the root system if could cause damage. IF more then 30 % of the roots were cut then I would expect some dieback of the foliage. If you think of the area of the roots under the tree as a pie and you draw a line across the pie shape where the trench was cut and the cut piece is more than 30% then you may have damage to the crown of the tree.

The stability of the tree would not be effected since spruce have the tap root that will hold the tree up. I would not think this would be a problem. But I would expect to see some dieback  of foliage on the side damaged. I would try to off set this by fertilizing the tree with 10-10-10 fertilizer at the rate of 1 lb of fertilizer per inch of trunk diameter. Scatterer the fertilizer around the tree and water it in good--apply the fertilize just before a rain storm and you will not need to water it in. This will increase the health of the remaining root system.