Modified Bitumen Roofing and Telltale Signs – Part IV of VII

The outline for the entirety of this seven-part series on modified bitumen flat roofing follows, with today’s portion in bold and today, we pick back up right where we left off at this the last article from earlier this week:

  1. What is Modified Bitumen Roofing?
  2. Hard Requirements 
  3. Best Practices
    • Cant strips
    • Strip-in flashing
    • Consistent Bleed-out
    • Granules in bleed-out
    • Primer at metal terminations
    • Liberal seal at application
    • Coatings AFTER installation
  4. Telltale Sign Of Experience
  5. Core Sampling 
  6. Rounded Corners and Why They are Better
  7. Bleed-out as a Sign for Proper Application 

We are talking about best practices and looking at a few different types of examples of signs that roofs are built correctly, and not necessarily just to the minimum standards but also in some cases above and beyond those minimum requirements.

In our last article we talked about cant strips and how they protect a membrane from undue stress at 90° corners at parapet walls, for example. We also talked about strip-in flashing and securement of the membrane under terminations such as drip edges and gravel stops. We also talked about consistent bleed-out and how important that bleed out is as a visual check on the quality of the torch application.

bleed out dimension
The picture of above shows the check of the bleed out dimension at the transition between lapped joints of a APP modified bitumen roof.

Granules in the bleed-out

Granules are not necessarily required by the building code book as it is written in black and white. In fact, granules are not used in all types of modified bitumen membranes.  For example, modified bitumen smooth membranes don’t have any granules at all. But granulated roofs are covered with this granule as an alternative to coatings such as is required with modified bitumen smooth membranes.

Where granulated roofs join at the seams there is an overlap, and that overlap generally has bleed out, as explained in our last article. That bleed out then like modified bitumen smooth membrane, is not covered with granules. Those granules protect the membrane from exposure to ultraviolet rays in the everyday sunlight. Granules can be installed directly into the wet area of bleed-out in the granulated modified bitumen roof. We don’t expect to see this type of application in small repairs, but on new roofs of modified bitumen membranes, this is a good sign of quality because it shows the installer has taken the extra step to make sure even the small details are done better, above and beyond.

target patches
At the target patches at the pipe penetrations, granules have been installed in the bleed out at the edge of the granulated modified bitumen membrane overlay.

Primer at metal terminations

Metal terminations, such as drip edges and gravel stops require a specific type of coating or chemical application before the modified bitumen is attached to the termination. This application is an asphaltic type of liquid material that is applied to the metal similar to a paint. If the emulsion at the underside of the roof membrane is welded or torched to the substrate metal termination, the modified bitumen membrane has a high probability of later detaching or delaminating from the metal substrate. The emulsion at the underside of the roof membrane is meant to be a solid at most temperatures and even when torch welded will later go back to being a solid when the temperature of the membrane decreases to average ambient temperatures. That change can cause the membrane to delaminate from dissimilar materials such as metals. Galvanized metals generally have an oil at the surface which is very difficult to remove and which can unintentionally act as a rebinding or decoupling agent. Asphaltic spray primer though can change the surface conditions by bonding to the metal substrate and also being a binder which is acceptable to the modified bitumen membrane.

Liberal seal at application

When a new modified bitumen or TPO roof is installed, they’re also generally several fasteners that need to be installed into the accessory materials. Coping metals, for example are an example of an accessory metal that must be attached with fasteners. We consider it a best practice to avoid installation ofthese fasteners in horizontal planes of the metal whenever possible, but these fasteners also need to be covered as a redundant measure to assure that water does not enter in through the opening between the fastener head and the substrate.  Elastomeric sealants are generally one of the best materials to use because of their inherent flexibility and adhesion or cohesion properties.  When we check roofs installed by other contractors as a conditions survey, we look for liberal applications of gun applied sealants at fastener heads and at joints in coping, for example.

aluminum metal coping
The picture below shows an aluminum metal coping at a modified bitumen smooth membrane flat roof. The membrane is not coated, but the coping has a liberal application of gun sealant applied at the seam.

Coatings AFTER installation

It’s often not in the contract for installation of modified bitumen membrane, to also apply a coating after the installation. In fact most quality contractors recommend the modified bitumen smooth membranes are left to season in the elements for three to six months. After such time it’s okay to apply the roof coating. It’s important that the substrate is prepared either by a trisodium phosphate alternative chemical wash or other type of cleaning. The problem we find most often though when we are working with new clients whom have worked with other contractors prior, is that they tell us that the prior contractor never told them that the roof needed to be coated at all and it’s important for clients to know they need to plan for coating after installation. There are a variety of different types of coatings that are available for modified bitumen smooth membranes, some of the best are the high reflexity type coatings because they can reduce the overall energy footprint of the building by allowing the roof to be a bit cooler in the summer sun because of the reflectivity.

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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