5.1. SR 09-19-1994 - Laserfiche WebLink

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37 ...MINIMIZING PONDING USING ....LAT INSULATION DWIGHT F. JENNINGS and DWIGHT D. BENOY INSPEC. Inc. Minneapolis, Minn. In the 1950s, designing totally flat roofs was common practice. When roofs were designed to be totally flat, the end result usually was not. Unless camber was built into the structural system to allow for anticipated dead and live loads, deflection between bearing points resulted in low points on the roof. Typically, roof drains were located along beams or bear. ing walls, which turn out to be the high points in the roof system. There also was very little room for error in construc- tion. Everything, from the footings on up, had to be con. structed to very close tolerances in order to end up with equal elevations across the building. The industry gradually learned that ponding water acted to reduce the serviceable life of a built-up roof. Getting the roof dry enough to patch a leak was also a problem. And, much more water entered the building if leaks did develop. Most major manufacturers in the United States eventually . came to recommend V.-inch-per-foot slope for new roof design using built.up roof membranes or manufactured single-ply membranes. However, there are many, many buildings out there already that have no slope and have excessive water ponding. The most opponune time to cor- rect this condition is when reroofing becomes necessary. Ideally, eliminating all ponded water would be the best for the long-term performance of the roof system. However, there are times when eliminating ponded water is not prac- tical because of the complexity of the solution or limitations posed by the existing conditions. It is imponant to keep the design as simple as possible. A complex design may be the most effective, but costs will rise proponionately, and the contractor may have difficulty in actually executing the work. In these cases, the cost to eliminate the ponds exceeds the advantage of doing so. Many expens believe a roof pond that dries within 48 hours of a rainfall will have little effect on the long-term performance of a roof membrane. . REROOFING DESIGN APPROACHES Once the existing conditions and the cause of the ponding problem are determined, approaches to the reroofing design can be developed. Many times the addition of roof drains or scuppers is not feasible. Adding insulation of varying thick- ness, not necessarily tapered insulation, can be a viable solu- lion in many cases. flat and Tapered Insulation System In response to the industry movement toward slope in roof systems for complete drainage, manufacturers are market. inltapered insulation to give slope to relatively large sec. tions of roof area. Tapered insulation board is available in several different types, including polystyrene, perlite-aggre- pie, isocyanurate, cellular glass and fiber glass. The slope available in each type varies. The slope should be sufficient to achieve positive drainage, yet not so great that insulation thicknesses become difficult and costly to deal with al the details. Maybe it's not necessary to use tapered insulation to slope the entire roof area. There may be existing deck slope, no matter how slight, to take advantage of, even in an essen. tially flat roof. Let's look at a simple example of a 50.year-<Jld building with an essentially flat concrete deck. The deck has gradually deformed and sagged between suppons through the very slow process referred to as "creep". To compound the prob- Jem. new HVAC equipment has been suspended from the deck. The net result is a slight slope in the roof section from the suppons to the center. The roof is drained at one roof edge through scuppers over one of the bearing walls. The result is substantial ponding in the central ponion of the roof area (Figure 1). The reroofing design could include tapered insulation from the scuppers to the opposite roof edge to eliminate ponding. But half the existing roof is .a1ready sloping in the direction for drainage to the scuppers, so why not take advantage of it. Flat insulation placed over this half of the roof will have a net positive slope toward the scuppers. Tapered insulation with enough slope to counteract the negative slope of the deck in the other half of the roof will provide a net result of positive slope across the entire roof area at the membrane level. This approach minimizes the insulation thickness so one can better deal with height limitations at existing flashings, such as through-wall flashings, and at large mechanical units. Likewise, blocking requirements at the details may be less. The final product provides the desired drainage at less cost than a fully tapered system. Tiered flat Insulation System Another, typically less costly, method of reducing ponded water is to increase the thickness of insulation in steps, or tiers, from the drainage points to the perimeter of the area. These tiers are typically made in Ih-inch or 'II-inch incre- ments with tapered edge strip installed to provide a smooth transition from one tier to the next. The same basic concept as in the first example is appli- cable here. Take advantage of the existing deck slope on half the area. In this case, the insulation is stepped down in four increments rather than a continuous taper (Figure 2). The steps are located more closely in the area of greatest ponding, or negative slope, to minimize the existing highs and low'. The disadvantage of this approach is that it does not com. pletely eliminate the ponding in all cases. However, it can reduce the depth or ponding to a tolerable level by displac. .