Question
my contractor wants to cut into one of the roots from a large pine tree in building my deck.  will this kill my tree? it has several other roots visible but this one runs under my house and he says he will have to cut into it to lay the foundation.  let me know what you think. thanks for your help.... i don't want the tree to fall on my house later on.

Answer
If you cut only one of the roots it will not effect the overall health of the pine. The pine  does  have a tap root as well as a root system that spreads out from the trunk about 1 1/2 the distance of the foliage spread. A general rule is if you cut more than 30% of the roots the tree will suffer die back of the limbs on the side of the root cutting. The remaining root system and the tap root will hold the tree and it will not fall on your house from cutting this one root.
In this case I would think there will be no effect to the tree by cutting one root. It will not die from this.