If you have both tarp 'ends' start at the floor and go over the pipes you can weigh them down with a length of wood - or chucks of 4x4. If the top still sags, you can run a wood board vertical to your cross beams and then staple left over tarp to cover up that support board.
However, sometimes, a tarp sag looks GREAT in a haunt. I try to avoid square corners and neat looking lines.
However, sometimes, a tarp sag looks GREAT in a haunt. I try to avoid square corners and neat looking lines.