Technical Perspective: Built-up Roofing
Is a wrinkled base sheet still a concern?Wrinkles in any roofing system are a concern. To overcome this problem, some manufacturers offer an SBS rubber modified-asphaltic base sheet that incorporates the features of a strong fiberglass mat with a coating containing a blend of synthetic rubber and a high quality asphalt. The modified asphalt blend lends elasticity and flexibility to the sheet. This unique combination is designed to assist the sheet to lay flat in all nailable specifications. In addition, the industry has learned to manufacture the sheet with a minimum amount of tension and the roofing contractor has learned to allow the sheet to relax sufficiently and to fasten the sheet to prevent wrinkling problems during installation.
Is “phased construction” ever acceptable?Yes. One acceptable method of “phasing” a BUR job is to apply the insulation and four plies but delay the application of the surfacing until the end of the project. This method keeps gravel from getting tracked across the roof and into the plies while the roofing operation is in process. It is important for the roof to be clean and dry before completing surfacing. However, it is not acceptable to apply two plies of a four-ply system — usually to “dry in” a roof — and later apply two additional plies. In this case, two plies with an additional two plies defies good roofing practice since dirt and moisture may be trapped between these layers and can affect the integrity of the membrane.
If a storm forces the crew to “dry in” the roof, the acceptable solution is to use a minimum of two plies of fiberglass felt with a very light squeegee coat of asphalt to make the roof water tight. The job is then completed by cleaning debris and moisture from the roof and finishing with four plies laid up 8 1/2 inches apart as designed.
What is the advantage of Type VI felt versus Type IV?Initially, Type VI fiberglass felts were board-like and stiff to handle. While they provided a stronger roof than Type IV fiberglass felts, they were a little more work to install. More recently, the Type VI felts have been redesigned so that they are easier to handle and not so board-like, without taking away from their ability to stabilize and strengthen the BUR membrane. Their additional strength and ability to stabilize more of the waterproofing asphalt per ply continues to be the real strength of today’s BUR system. The reason that BUR fiberglass ply felts work so well vs. organic felts is their increased strength and natural porosity. This porosity allows the mopping asphalt to flow freely between and through the plies while fluid, and to improve the bond strength of the system. As the asphalt cools to ambient or roof top temperature, it becomes a solid again, resisting deformation and providing a completely strengthened and watertight membrane. This provides for a more homogeneous membrane assembly than just alternating layers of felt and asphalt.
What are the pros and cons of hot vs. cold-applied BUR?With tighter restrictions and the desire to avoid odor of hot-applied asphalt, some owners have turned to “cold-applied” built-up roofing. This is occurring more often since many school districts now offer year-round schooling and the staging of a hot kettle can be a safety concern around children. Cold-applied asphalt adhesives fill a niche that hot asphalt products cannot. The finished membrane has the same basic waterproofing whether it was installed with hot or cold asphalt. There can be some concerns until the “cold-applied” finally sets up, which could take as much as 30 days, depending upon the conditions at the time of application. Additionally, there may be concern about the odor and solvent content of cold-applied asphalt adhesive.
One method to continue the use of hot asphalt is to stage the kettle in a restricted area away from children and to use a filter, after-burner or other odor control during installation. One fume recovery system eliminates odor, fume and particulates at the kettle. Filters aren’t needed with this method as the gases passes through a thermal converter where they mix with fresh air and are superheated, eliminating 99 percent of the odor and fume.
Another method uses low-fuming asphalt that eliminates most odor and fume at the kettle and lugger. With this method, the asphalt’s polymer packaging melts in the kettle and floats to the surface, creating a skim layer that traps the fume and odor inside. This method also helps to eliminate some of the odor and fume at the point of application as well. When the need for low fume asphalt is over, just simply add standard asphalt to the kettle. Since the low-fume asphalt is compatible with standard asphalt, there is no need to drain the kettle. Simply add standard asphalt, allowing the low-fume asphalt to mix until depleted.
Which membrane system best handles abuse?Historically, gravel-surfaced built-up roofing systems have had an excellent track record for damage resistance. The success of conventional BUR can be attributed to its multi-ply construction, the strength of the fiberglass ply mat, the stabilization of the waterproofing asphalt, and the damage protection provided by the gravel surfacing. In addition, protection boards are typically used in high traffic areas such as the walking area from the access hatch to and around the high maintenance equipment.
Regardless of the membrane used, the owner is best advised to limit traffic and prevent other trades from access to the completed roof. While built-up roofs have natural defenses, the best protection is still to limit access to the roof. Always keep a log of who has been on the roof, the reason for it and where they went.