I'm designing a storm sewer system and one of my structures is very tight on vertical space (dist from an existing 18" RCP invert to the finished grade). I am limited on how much I can raise the finished grade because of the grade of a nearby road. Currently, my structure has 4" of clearance from the outside crown of the existing 18" RCP (2.5" thick walls) to the 8" precast structure lid, but I really need to find something smaller. The structure also has to accomodate an 18" RCP going into and out of the box. Anybody know any options for loosing a few inches?