Roundup Weed Killer

Dear Anne, I have a large maple tree in the middle of my lawn (30-40 foot high and 3-4 ft dia. trunk). I am completely redoing the lawn and need to first kill of the old lawn, which contains too many weeds. If I use roundup to kill the existing grass/weeds will the roundup hurt or damage the roots of the maple tree? Some of the maple tree roots are actually visible above ground. Once I have killed the existing grass I plan to add more top soil to cover the exposed roots. Thanks,
Larry P., Rocklin, CA

Anne's response:

I would not apply soil to cover the roots of the maple tree. They have grown out of the soil because its not soil they are happy to grow in. Covering the roots with soil will reduce the oxygen supply to the tree. The advantage of using Roundup is that it is a contact killer that is absorbed through the leaves of plants. If there are not cut places on exposed roots it is safe to spray the area with Roundup.