Question
Hello:

I live in London, England.

I have a 40-50 ft tall conifer in the garden. it has been there over 40 years.

To stop it drying out the surrounding grassed area, I cut through three surface-lying roots last week, probably each four feet in length and c. 4 inches in diameter.

I realize now that this was probably a very stupid action indeed... Could you let me know what the likely consequences will be?

Many thanks in advance for any assistance
John Moser

Answer
It is not necessarily the number or size of the roots but the % of the root system that is cut. IF you draw a circle around the tree where the limbs over hang to and then draw a line where the roots will be cut off and if this area cut is more than 25-30% of the overall circle then you will damage the tree by cutting the roots.  

It does not sound like you cut more than the 25% of the roots. And conifers have a tap root and can with stand more lateral root cutting than hardwood trees. I think you are OK with what you did and the health of the tree will be fine.