In a separate blog article, we talked about fasteners installed in a horizontal plane of a roofing membrane: “It’s never a good idea to bolt (or fastener) directly through a horizontal surface of a low slope roof and in most cases it’s actually against the building code (if not properly covered or overlayed by an approved method). However, DCRA does not inspect roof installations and it is therefore very important to hire a good roof contractor, a specialist in flat roofing, someone who not just has some experience but considers the technical aspects of building science and principles of flat roofing to be part of their life’s calling, when you’re repairing, replacing, or working on your roof system, even in cases where you’re installing a solar system. We recommend that our clients consult with us when they’re installing a photovoltaic solar panel system, even if our company is not conducting the panel installation. As a roofing specialist, We have particular training and experience on roofing systems that other contractors such as solar panel installers, HVAC system installers, and other building contractors may not possess themselves.”
At a horizontal or low slope area of a roof system, water does not flow quickly, in some cases water can pond (sit still in pudddles) and build-up, to some extent even when roof is built properly and especially in cases of snow loading or ice damming. Snow and or frozen precipitation can build-up and then melt and easily enter into horizontal areas of roofing. Where there are improperly installed, improperly sealed, or improperly flashed areas of fasteners through the horizontal membrane.or low slope membrane, the opening created at this location is an interruption in the otherwise continuous membranes.
At ponding areas or any other areas of a low slope roof system where water can build-up, Those areas of the roof are not only exposed to regular flowing precipitation but they’re also exposed to head pressure. Water head pressure or pressure head is the static pressure head of the built up water.
Overall, this is really a hydrostatic force calculation. This type of calculation and effect, as it affects a roof in a horizontal or low grade type configuration, is orders of magnitude different from the effect of a subgrade hydrostatic pressure calculation at a foundation or retaining wall minus the static load of the soils, but the principles are very similar.
The picture below shows an example of a roof coping installed with a hex head screw, used as a fastener throughout the length of installation. The coping metal, as it spans the roof parapet, has a horizontal surface and also has a formed edge on both sides which terminates at the lower area with a drip edge or angle of the metal away from the wall of the roof parapet. That screw has a neoprene gasket, used to seal the fastener to the coping, but that gasket alone is not enough to stop water from getting through the coping.
The next picture below shows an up close view of the faster head. You can see that the fastener is installed on a slight angle to the substrate metal coping. That slight angle means that the near Pringle escape is not an equal compression at all sides around the perimeter of the screw head. In fact, as can be seen on the right hand side of the photograph, the neoprene gasket is not even touching the aluminum coping on one side.
When installing a faster like this, not only is important to carefully drive the fastener down at a perpendicular angle, but the substrate material, below the aluminum coping, in this case likely historic low temperature fired common bricks (likely not a wood blocking material as is commonly found in modern commercial roof construction) must also be continuous and installed in a way that has no other materials which might catch the threads of the fastener and cause the fastener to drive off course, at an angle. The installer cannot predict or see that because the substrate is completely covered with coping metal, in this case.
One of the other shortcomings of this type of fastener is that the neoprene will not resist ultraviolet rays for a time period or time of longevity commensurate with the coping metal. Coping metal of this type will likely last over 30 years before it has significant oxidation or deterioration. By comparison, depending on several factors related to configuration and exposure to sunlight, the neoprene gasket will deteriorate at a much faster rate. Ultraviolet rays can accelerate the deterioration of materials like this in a matter of just a few seasons.
The photo collage below shows 3 images of fasteners installed directly through a roof coping. Since this particular roof coping is at a near horizontal angle, water from precipitation will not flow at a fast rate and will have a chance to run over and around the area of penetration. Gun sealants have been used at this location, but they will also deteriorate and cannot be guaranteed or even likely to have complete coverage at the area of penetration and/or opening through the roof membrane material.
For several reasons, it is best practice to avoid installation of fasteners in a horizontal plane of a roof membrane or provide additional supplementary coverage of those fasteners. It makes simple and obvious sense, for several important reasons, but we often see this type of poor installation done by other contractors.
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.
We encourage all of our clients, and all readers of this article and to our blog in general, to prioritize the value of quality construction and building maintenance, and develop a relationship with our company. You can learn a lot more on our blog. Feel free to check it out. If you have questions about the roof and related systems of your building in Washington DC, fill out the webform below and drop us a line. We will be in touch if we can help.