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Eyeball-able summary added.

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edit approved Oct 5, 2021 at 11:28

Vorac

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18

TL; DR;

Measure the trunk diameter,

multiply by 18

don't touch anything within that distance from the tree.

E.g. a trunk of diameter 20cm(8") would expect a 3.6 meter(12 foot) radius of healthy undisturbed roots at bare minimum.

There is some guidance here

It is difficult to say how much of a tree's root system can be removed. I prefer to talk about how close to the tree you can cut roots. I teach a concept called "Critical Root Radius (CRR)" which recommends not cutting roots inside the CRR. This Critical Root Radius (radius, not diameter, in feet) is calculated by measuring the diameter (not radius or circumference) of the trunk (in inches, not feet) at four and one-half feet from the ground and multiplying by 1.5. A tree with a five-inch diameter trunk (at the four and one-half foot level) will have a CRR of 7.5 feet. The CRR provides a general guideline.

and here

Balance between the tree’s crown (top) and root system is important for maintaining healthy trees. When roots are lost for any reason, the imbalance creates stress. A tree usually has 4 to 7 major roots. Cutting just one of them within a few feet of the trunk can remove up to 25 percent of the root system.

There is some guidance here

It is difficult to say how much of a tree's root system can be removed. I prefer to talk about how close to the tree you can cut roots. I teach a concept called "Critical Root Radius (CRR)" which recommends not cutting roots inside the CRR. This Critical Root Radius (radius, not diameter, in feet) is calculated by measuring the diameter (not radius or circumference) of the trunk (in inches, not feet) at four and one-half feet from the ground and multiplying by 1.5. A tree with a five-inch diameter trunk (at the four and one-half foot level) will have a CRR of 7.5 feet. The CRR provides a general guideline.

and here

Balance between the tree’s crown (top) and root system is important for maintaining healthy trees. When roots are lost for any reason, the imbalance creates stress. A tree usually has 4 to 7 major roots. Cutting just one of them within a few feet of the trunk can remove up to 25 percent of the root system.

TL; DR;

Measure the trunk diameter,

multiply by 18

don't touch anything within that distance from the tree.

E.g. a trunk of diameter 20cm(8") would expect a 3.6 meter(12 foot) radius of healthy undisturbed roots at bare minimum.

There is some guidance here

It is difficult to say how much of a tree's root system can be removed. I prefer to talk about how close to the tree you can cut roots. I teach a concept called "Critical Root Radius (CRR)" which recommends not cutting roots inside the CRR. This Critical Root Radius (radius, not diameter, in feet) is calculated by measuring the diameter (not radius or circumference) of the trunk (in inches, not feet) at four and one-half feet from the ground and multiplying by 1.5. A tree with a five-inch diameter trunk (at the four and one-half foot level) will have a CRR of 7.5 feet. The CRR provides a general guideline.

and here

Balance between the tree’s crown (top) and root system is important for maintaining healthy trees. When roots are lost for any reason, the imbalance creates stress. A tree usually has 4 to 7 major roots. Cutting just one of them within a few feet of the trunk can remove up to 25 percent of the root system.

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created Aug 22, 2011 at 22:49

Jeff Atwood

There is some guidance here

It is difficult to say how much of a tree's root system can be removed. I prefer to talk about how close to the tree you can cut roots. I teach a concept called "Critical Root Radius (CRR)" which recommends not cutting roots inside the CRR. This Critical Root Radius (radius, not diameter, in feet) is calculated by measuring the diameter (not radius or circumference) of the trunk (in inches, not feet) at four and one-half feet from the ground and multiplying by 1.5. A tree with a five-inch diameter trunk (at the four and one-half foot level) will have a CRR of 7.5 feet. The CRR provides a general guideline.

and here

Balance between the tree’s crown (top) and root system is important for maintaining healthy trees. When roots are lost for any reason, the imbalance creates stress. A tree usually has 4 to 7 major roots. Cutting just one of them within a few feet of the trunk can remove up to 25 percent of the root system.