Hello and thank you for contacting Ask Extension. Shaving or cutting the roots of an established Norway Spruce is extremely risky for several reasons. Norway Spruce trees have a shallow root system so removing large, structural roots that are close to the trunk can severely compromise the tree’s health and reduce it’s ability to withstand strong winds and a heavy snow load. It can also allow diseases to get into the tree and cause additional stress.

If root shaving is necessary (and I think it is), follow these guidelines: do not remove more than 25% of the functioning root under the tree canopy and do this in the cool weather of late winter or early spring when there is less danger of dehydration. Do not shave roots after trees start growing in the spring as this will deprive the tree of water when it needs it the most. Consider extending your pruning project over several years to give the tree an opportunity to regrow supporting roots elsewhere as you gradually shave off parts of the root. Removing roots close to the main trunk may create a structural hazard. This area is called the Critical Root Zone and if cutting roots in this area it should only be done in consultation with a certified arborist. Here is a link to find a certified arborist near you. https://www.treesaregood.org/findanarborist (ISA Michigan | Directory)

Here is a link to the publication where I found this information.

https://s3.wp.wsu.edu/uploads/sites/2076/2024/07/C137-Tree-Surface-Roots.pdf