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. PageBreak
Does the Roof Drain Actually Drain?
Collection of debris at the roof drain can completely block the flow of water. During biannual inspections, the inspector should bring some trash bags up on the roof and use them to remove debris and vegetation from the strainers.
After inclement weather, observe areas of ponding water and try to figure what the cause might be. Possibilities include blockage of the downspouts and deflection of the roof deck. Now that we are adding a lot of insulation to meet more stringent energy codes, there will be less heat reaching the drain bowl, which may result in ice blocking the drains.
To stay on top of these potential issues, remember this handy mnemonic for roof maintenance: Always Be Conscious of Drainage. If ceiling tiles start dropping to the space below, it may be too late to avoid collapse. The blocked drains lead to excessive deflection, which in turn collects more water. Climate change may result in extremely intense rainfall, and if flooding is taking place in low areas, the same may be happening on the roof.
Why Dual Drains?
You may be aware that each bay of a multi-bay structure is supposed to have two means of drainage. The primary drain is located at the point of maximum expected deflection, and should be recessed to avoid ponding. The secondary means of drainage is raised above the plane of roof membrane, typically 2-4 inches above roof level.
Doing the math, water weighs about 5.2 pounds per square foot per inch of ponding. Without that secondary means of drainage, 4 inches of water would add 20 psf of environmental load, and many structures are only designed to withstand 20-30 pounds.
The secondary system may have wall scuppers, which in turn are placed 2-4 inches above roof level. The joke here is that we may have reroofed with more thermal insulation thickness and submerged those scuppers. Water flowing to the scuppers may flood the parking lot and landscaping instead of following the primary drains to storm sewers.PageBreak
Roof Geometry 101
Richard (Dick) L. Fricklas was technical director emeritus of the Roofing Industry Educational Institute prior to his retirement. He is co-author of The Manual of Low Slope Roofing Systems, and continues to participate in seminars for the University of Wisconsin and RCI Inc., the Institute of Roofing, Waterproofing, and Building Envelope Professionals. His honors include the Outstanding Educator Award from RCI, William C. Cullen Award and Walter C. Voss Award from ASTM, the J. A. Piper Award from NRCA, and the James Q. McCawley Award from the MRCA. Dick holds honorary memberships in both ASTM and RCI Inc.
Use of crickets and saddles redirects the flow of water around equipment curbs and towards the drains. These can be constructed of plywood or OSB on the deck level, or by using coverboards and tapered thermal insulation. For multi-bay buildings, inverted pyramids are strongly recommended, but these are more practical at the time of original roof design.
An alternative is to use lightweight insulating concrete sloped to drain, but this too is more practical in new construction.
However, all is not lost for existing roofs. Contractors have found that drain inserts simplify the resetting of existing roof drains to accommodate changes in roof elevation. The new drain sleeve is pushed down into the existing downspout and an expanded sealant ring assures that water cannot back up into the roof system. Keep in mind that the insert may reduce the rate of water flow and could result in a code violation.
What does the future bring? The recent tropical storms in the Philippines and Colorado suggest it is just a matter of time before code and insurance officials will have to upgrade wind velocity, hail, and rain charts to reflect our changing climate. Meanwhile, let’s make sure we have done everything possible to maximize our ability to direct water’s flow.
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