There is a real difference between water-shedding and waterproofing when it comes to roofs. In building construction, waterproof means resistant to hydrostatic forces, while water-shedding means a roofing system that redirects water to roof drains, gutters, or scuppers.
How Water Reaches the Right Place
On steeply sloped roof systems, there is no need for the roof system to be perfect. Water generally will run downhill, and for various shingles and shakes, slopes of 3 inches (or more) are adequate to get the water to the valleys and gutters.
For low-slope roofing – generally defined as a slope of 0.25 inches per foot, or 2%, the minimum slope required by code for new construction – workmanship is far more critical. The installed membrane also relies upon accessories such as pitch pans, flashings, and functional roof drains, as well as protection against abuse by other trades.
For reroofing, buildings require “positive drainage,” or no standing water 48 hours after rain falls. When retrofitting an existing roof, we have an opportunity to note whether the existing roof ponds water and to either add new drains or use tapered insulation to eliminate those ponds.