I have re-seamed heavy duty plastic tarps - but I do a double seam -like your jeans - and have never had a problem with them leaking. My brother and SIL are using one I repaired a few yrs ago on the back of their house -they have torn off the back 1/2 to make a bigger bath -laundry room and sewing/craft rooms and its rained every other day since they started and the rest of the house has stayed dry.And I did not use Duct tape on the new seam..but I used fine fishing string instead of thread.