Today, we looked at a modified bitumen roof membrane with several problems that go all the way back to the initial installation. As well, though some of those initial problems have at this point experienced farther deterioration, defect, and deterioration. In roofing, it’s common that small problems can fester and get worse in an exponential sort of way if left installed improperly or without modification, rebuild, or repair.
The problems on this route range from improper attachment from successive modified bitumen membrane layers to substrate layers on the upper ends where the membranes join at seams, deterioration related to a lack of a coating on a modified bitumen smooth membrane which requires a moderate to high reflectivity coating to protect it from exposure to damaging or deleterious ultraviolet rays, and a ponding substrate.
In almost all cases, where a roof substrate deck has issues of ponding or reverse grade at any point within a low slope roof system, those issues should be repaired or fixed before the roof membrane is installed. During the initial installation, the substrate must be checked with a level. Often though, as shown here, the original roof deck apparently shows signs of ponding because each of those ponding areas coincides with a dirt spot, even in dry conditions. As rainwater flows across a roof deck, it washes dirt and dust into the flowing rainwater, and then that flowing rainwater puddles up or ponds in a low point. After the water dries, the dirt and dust are deposited into that reverse grade or ponding area. So this is an example that shows you can find most ponding areas on a low slope roof even without using a level to check the grade for continuous slope.
Here, a significant amount of dirt and debris have accumulated. Plus, as you can see, a significant amount of other miscellaneous debris and/or building materials have been left discarded on the rooftop. This is often a sign of low-quality roof construction. But sometimes it’s not even the roof contractor that leaves these materials; it’s often other contractors such as HVAC contractors working on the rooftop after the roofing membrane has been installed. When these low-quality contractors come to do their work, they just discard their materials because they don’t have anyone checking after them to manage them properly.
Another one of the big issues or problems that we discussed above, related to the rooftop, is that this is a modified bitumen smooth material that has never been coated. Modified bitumen smooth membrane materials are required to have a moderate to high reflectivity paint coating applied to reflect and/or protect the substrate membrane from the deleterious effects of exposure to ultraviolet rays in sunlight. Types of carbon-based building materials which include modified bitumen roof membranes, wood or any types of exposed framing materials made of natural materials, and even some types of synthetic rubbers.
A very close view of this membrane shows the deterioration, with pronounced cracking and alligatoring in a three-dimensional shape. As the sunlight has broken down the materials of this rooftop, it’s left the top layers of the single-ply membrane all but defunct.
This breakdown occurs at a molecular level through a process called photodegradation. Here’s a detailed explanation of how this happens:
Modified bitumen membranes are primarily composed of asphalt (bitumen) that has been modified with polymers, typically styrene-butadiene-styrene (SBS) or atactic polypropylene (APP). These polymers enhance the membrane’s flexibility, durability, and weather resistance. However, without proper protection, they remain vulnerable to UV radiation.
UV rays, particularly UVA (315-400 nm) and UVB (280-315 nm), carry sufficient energy to break chemical bonds within the polymer chains of the modified bitumen. When these high-energy photons are absorbed by the membrane, they excite the electrons in the molecular bonds, causing them to jump to higher energy states.
This excitation can lead to several destructive processes:
- Chain Scission: The excited electrons can cause the polymer chains to break, reducing the molecular weight of the polymers. This process weakens the material’s overall structure and flexibility.
- Free Radical Formation: When bonds break, they often form highly reactive free radicals. These unstable molecules can initiate chain reactions, further degrading the surrounding material.
- Oxidation: UV radiation can accelerate oxidation reactions, especially in the presence of atmospheric oxygen. This process leads to the formation of carbonyl groups (C=O), which make the material more brittle and prone to cracking.
- Cross-linking: In some cases, UV exposure can cause polymer chains to form new bonds with each other, leading to a stiffer, less flexible material.
The bitumen component itself is also affected. UV radiation can cause volatile components to evaporate and oxidize the remaining compounds. This leads to hardening and embrittlement of the asphalt, reducing its waterproofing capabilities.
Over time, these molecular changes manifest as visible and physical deterioration of the membrane. The surface may become chalky, develop cracks, or lose its elasticity. The membrane’s ability to expand and contract with temperature changes is reduced, leading to further damage.
The rate of degradation depends on factors such as the intensity of UV exposure, temperature, and the specific composition of the membrane. In areas with high sun exposure, unprotected modified bitumen membranes can show significant degradation within just a few years.
This is why reflective coatings or granular surfaces are crucial for protecting modified bitumen membranes. These protective layers reflect or absorb UV radiation before it can penetrate and damage the underlying bitumen and polymer structure, significantly extending the life of the roofing material
In this article today, we’re really focusing mostly on the effect of installation problems with modified bitumen roofs, but as you can see in the picture below showing a built-up roof or BUR roof membrane, the same issues apply to other roof types. In this case, although the BUR roof does not have a thick synthetic layer in each of its plies, it still deteriorates, exposing the substrate textile backing after the asphaltic materials have deteriorated and washed away over time.
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.
