Understanding the Ideal Slope Range for Low-Slope Roofing Projects
Last week we took a look at the roof shown in the picture below. A TPO roof was added to a rear part of the building and joined into an existing older roof but instead of connecting it properly, they made a transition using a piece of pre-bent aluminum flashing. This is completely Incorrect and will definitely lead to problems in the future. All signs of the installation indicate that the roofers who did this didn’t have any experience installing roofing materials. There were additional pictures in last week’s article, but you can see the same element in the picture below and you can identify the materials used. They applied a gun sealant along both sides of the edge of the metal and fastened directly through the horizontal plane of the roof without a proper cover on top of the fasteners. By all accounts, this is incorrect In many ways.
Today, here in this below, we’re going to show more pictures of these issues coming from different angles and talk more about why these materials are not appropriate to be used in these ways in low slope roofing. We’re also going to talk about the difference between low slope and installation methodologies for moderate to high slope roofing
Caulkings and gun sealants can be very effective secondary means of ceiling, especially at the tops of vertical flashings, for example. Here though, in the horizontal plane as the only means to stop water entry through the area of transition, this is incorrect and does not work in a low slope roof application, particularly in the field of the membrane. TPO stands for thermopolyolithin. It is a particularly strong modern membrane, one of the most modern of all types of low slope roofing membranes, and although it’s very strong, it does have particular requirements for installation. Although the building code has very few requirements for this type of installation, the building code enforces the manufacturer’s installation instructions and manufacturers here in the United States have a strong bonus to make their requirements for installation exacting. Not all requirements fit perfectly in the context of Capitol Hill in Washington DC row homes, because these roofs are not necessarily the primary market for the manufacturers, but nonetheless, the rules for installation are relatively consistent and do a pretty good job of creating a set of standards that should be followed in almost all cases throughout the types of installation in Washington DC.
Whenever you see a pre bent aluminum flashing, essentially smushed down to a near horizontal position, in the field of a low slope roof, it’s almost an instant indicator or sign that there are problems or unorthodox elements in the installation of the roof. There are no typical standard approved reasons why this should have been built in this way. If a prevent piece of flashing is installed in the horizontal plane of the membrane then it can only really be secured down with fasteners in the horizontal plane which by covering with nothing more than gun applied sealant, is not appropriate.
Judging by this first sign, as an indicator or a red flag from there anyone looking at this roof with the knowledge of proper installation methodologies should begin to look at the other particular elements of the installation, and one of the next glaring details is the field seams. Here the field seams have not been welded properly.
TPO flat root seems must be welded, very similar to modify bitumen seams or even EPDM seems, but in this case, Tupac must be sealed with a hot air gun, whereas modified bitumen lapped seams are normally sealed with a actual Flame torch. EPDM seems are sealed with chemicals that actually weld the material on a chemical level.
By comparison though, asphalt shingle roofs are overlapped, each course is successively laid above the course below, but they’re not actually sealed down to one another. Asphalt shingles actually have a very thin adhesive strip, generally cover with a thin type of cellophane which protects it from getting stuck to the shingle below in packages during shipping, but that adhesive actually creates a light connection to prevent lifting up or blowing apart in light winds, but it doesn’t actually create a weld or strong bond to the shingle below so the joints from one shingle to the next are actually open, but there’s a big difference between shingle roofing and low slope roofing.
The big difference between low slope roofing is that unlike medium to high slope roofs, gravity alone will not keep water from entering the joints between the successive courses of membrane on a low slip roof. A low slope roof is basically any surface that is less than about a 2:12 slope. That’s about a 16.67 percent slope. Shingles shouldn’t really be used on any roof that is less than about a 2.5 to 12 ratio.
In this coming week, we’re going to look at another series of photos at the field membrane of this roof and at the lower termination where the roof membrane meets the gravel stop and the rearmost termination where it changes from a low slope membrane to the transition where it feeds into the gutter.
In this coming week, we’ll also show pictures that look closer at the transition directly in the photo below. You can see, there is a shadow below the edge of the seam because it’s not an actual seam, it’s just a lap joint where the upper membrane lays loosely on top of the membrane below. And a low slope roof like this, less than and about 2.5 to 12 ratio, this is not acceptable. With this type of membrane, TPO, it’s not even acceptable in a slope greater than a 2.5 to 12 ratio because unlike asphalt shingles, TPO membrane is not fastened at short course spacings throughout the roof.
Problems like this could be easily avoided by using a better contractor. We recommend every building owner in DC who values the longevity of their roof (and their investments) and building use a contractor who values the simple and important principles of proper roof construction like Dupont Roofing DC. Learn more about our company and the proper techniques of working with roofing on historic buildings in Washington DC here on our blog at DupontRoofingDC.com, and you can call us at (202) 840-8698 and email us at dupontroofingdc@gmail.com.
