Read everything important on ponding water on low slope roofs
Low slope roof manufacturers generally require that all low slope roof surfaces have a grade greater than 1/8 inch per linear foot and no areas of ponding water. That means that there will be no areas of zero slope and no areas of ponding water. People across the industry commonly use the term “flat roof”, However, it’s technically more accurate to refer to them as low slope roofs because even flat roofs are required to have slope and they generally do. In many Washington DC rowhomes, there is an overall grade from the front of the roof to the back of the roof of a fall of a few feet. That overall grade is generally built correctly, but there are areas where the grade in the roof is found to be out of tolerance. This condition most often occurs at the wider area of the roof layout above the rear ells, at curbs built for skylights, roof hatches, or other installations. The rear ell is a common rowhome detail where the rearmost portion of the building is built thinner than the front of the house. Historic rowhomes were built this way to add an exposed area of sidewalk common normally at the kitchen areas so that there would be more finished ration and opening for windows to the outside at the side of the house.
Ponding Water on Low Slope Roof
The picture below shows an example of a rear ell, at the roof, where there is ponding. As the roof is framed to allow an overall grade, that continuous plane is interrupted by the parapet at the rear wall of the back of the building. But this is not the remote wall at the back of the building, it is a rear wall in the midspan of the overall roof. As the direction of grade and direction of the flow of water changes at this location, the plane of Roof becomes flat and in this case even backgrades or causes pounding.
Another example of the same condition shows a different building, in this case built with more drastic grade in the In the corner of the area just before the rear ell. However, this roof layout also has a defect in grade: The abandoned and covered over old roof hatch has a curb that is now ponding water just below the area of the rear end. So in this case, one area was repaired but the next adjacent area was built without slope to passively shed water.
Roof With Correct Angle
The same area of the same roof from a slightly different angle shows the context a little better because you can see the distance that the curb of the old abandoned rooftop access hatch is spaced away from the chimney and the corner of the rear ell. The curb, at this location, with this spacing from the old chimney is creating a dam and not allowing water to escape. If the angle or inside niche corner between the curb and the chimney also been built up with a more drastic angle, water here would also flow and drain properly. The effort was put into building up properly at the corner of the area above the rear ell but then probably in years later by other contractors when the access hatch was built or abandoned, a similar approach was not taken to build-up the area in that corner niche.
Smart proactive replacement, construction, upkeep and maintenance of low slope roof systems requires an enthusiastic interest and understanding of waterproofing principles and building science. Here in Washington DC, historic and modern residential and commercial buildings are extremely expensive and the roof and related systems provide the shield that preserves the building.
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