Weight won't be an issue. most roof bars are rated for 40-75kg each, so multiply by 2 and your looking at 80 to 150kg, and even then you can safely go higher..
Pad the roof racks (old towels, sheets etc).
Make sure the padding is secure.
Flip the table upside down, and lift onto the racks.
Use tie downs to secure the table to the rack, ensuring it won't move left/right, up/down or forwards/backwards.
Start driving, hang you hand out the window to feel the table, confirm there is no movement.
Keep driving.