There's just about no worse callback than a leaky basement — unless it's a leaky basement with finished living space in it. That's why Indianapolis builder John Lawrence regularly includes sprayapplied foundation waterproofing in his projects. The basements in the 15 or so homes he builds each year are more than just below-grade storage cellars: They're true extensions of the living space, complete with carpet, textured walls, windows, and fireplaces. And the key to offering this high-margin upgrade is the waterproofing system. "Waterproofing is central to doing anything with a below-grade space," says Joe Carr of Custom Concrete Company in Westfield, Ind., the foundation contractor who builds waterproofed basements for Lawrence and others in that market. "You can't build below-grade living space