Modified Bitumen Roofing and Telltale Signs – Part V

The outline for the entirety of this multi-part series on modified bitumen flat roofing follows.  In two different articles this week we will talk about telltale signs of experience and the items for today are highlighted in the outline below:

  1. What is Modified Bitumen Roofing?
  2. Hard Requirements 
  3. Best Practices
  4. Telltale Sign Of Experience
    1. Uniform substrate
    2. No 90-degree outside corner
    3. Double overlay at pipe penetrations
    4. Crickets at curbs and bounded areas
    5. Careful detailing of flashings
    6. Gravel stops instead of drip edges
  5. Core Sampling 
  6. Bleed-out as a Sign for Proper Application 

Modified bitumen roll materials are an extremely common type of low slope or flat single-ply roofing material.  (Just because they are common doesn’t mean they are the best though, TPO, for example, is clearly better in most ways!)  These type of roofing materials are used across flat roof row homes and commercial buildings in Capitol Hill and Washington DC. Often, we’ll have the chance to see a roof that has been installed well, by others. Normally the majority of roofs installed by others are installed very poorly. Building owners generally hire the cheapest contractors and it’s not easy to tell the difference between good quality contractors and low quality contractors from the onset. Not many of them have blogs like this where they explain the facets and details of good quality construction. If they did, it’d be easier to know that they care. But at the end of the day, we all struggle to compete with the lowest quality contractors because they use their low prices to compensate for the lack of knowledge and they save money by cutting corners.

In those good quality roofs though, there are signs that make it visually apparent, signs that it’s different than the poor quality ones, even though they’re often built with similar or same material types. This applies not just to modified bitumen roots, but also to EPDM, TPO, and other types of low slope or flat roof systems. Today though we’re going to take a look at some of the telltale signs of quality in modified bitumen roofs.

We call them telltale signs because they tell a bigger story. A lot of times the visual evidence will look almost subtle but if you know what you’re looking for you can see there’s more to it beyond the surface and if you understand well how these building systems are assembled.

Uniform substrate

At a well-installed roof, even if it’s an old building, you can generally see there’s a very smooth substrate. Single-ply membranes, whether it be modified bitumen, TPO, EPDM, or other polymer, thermostat, or synthetic types of roof membranes are generally somewhat thin, even in cases where they’re strong and durable. In most of those roof system types, you can observe the condition of the substrate in terms of significant lippage or joints that are out of plane. When the surface of the roof looks very smooth, it’s at least a sign that the substrate layer of recovery board was well installed and it’s a area of work where cheap contractors will skip the proper preparatory work to save a buck.

uniform substrate

No 90-degree outside corner

Modified bitumen roofing materials should not be cut with a 90° outside corner. That corner should be cut carefully by hand with a knife, to make sure that the outside corner is not 90°. The point at a 90° edge of a roofing material isn’t supported on either side of the very tip of the angle.  Instead, it’s exposed to the elements on both sides of the tip of a sharp cut angle. The sharp cut angle should be just cut off of the outside corner of the roofing material and that way, when cut with a radius, there is no little point that sticks out. The little point does not have the ability to protect itself from exposure to the elements at each side of itself. Therefore the little point will wear out faster and begin to delaminate. Once it begins to delaminate it works a little bit like a sail in the wind and as the force of wind starts to pull it up more and more and as time goes on it will lead to failure and or greater delamination.

90 degree outside corner

This picture shows a different angle of the same type of condition but here it’s even worse because it’s also raised up above the remainder of the field membrane.

field membrane

Double overlay at pipe penetrations

Even though this roof has not been properly coated, you can see that the membrane at the pipe penetration was properly installed because it has both a flashing that wraps around the base of the pipe and then a Target patch that lays around the pipe but directly onto the near horizontal roof surface. Often cheap roofers will skip this step and just install one or the other which is an omission of half of the required flashing.

pipe penetration

Upkeep, Care and Maintenance of Washington DC Roofs

This case study is a perfect example of why a professional roofer should be used for all types of roof installation, even things that seem as simple as a balcony.  Consulting with a roofing professional, like Dupont Roofing, familiar with the specific climate conditions and the building science of roofing in Washington DC is advisable to ensure the roof assembly is appropriately designed and constructed to manage the flow of water, ice damming, capillary action and condensation effectively.

The upkeep, maintenance, and general care of flat and mansard roof systems should be driven by an understanding and passion for historical methodologies, waterproofing principles, engineering and building science. Washington DC, a city built with both vintage charm and contemporary modernities, residential and commercial buildings of substantial value. 

The roofs of these buildings are their defensive shield from the harsh elements of nature. To our clients, as well as all readers of this article and our blog, we emphasize the importance of quality construction and active building maintenance.  Our website includes informative resources you can use to understand and learn about best practices on preservation of your building.  If you are in need of further guidance on the roof and its associated systems for your Washington DC property, we are here to assist, where we can. Simply contact us or complete the webform below and drop us a line and we’ll respond if we can help.

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