Answer: Cutting the roots could certainly kill the trees, especially if it is more than a small proportion of the root system. And if the trees survive, they will regrow the roots. Adding soil on top of the roots will kill sensitive trees such as oaks, and maple roots will grow right back through it. Tree roots can make it difficult to grow grass, grass also does not grow very well in shade if the trees are shading your yard. You might consider using a groundcover instead, possibly English ivy or Vinca minor for example. I'm sorry I can't be more encouraging.
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