Inspection Insights: A Closer Look at Built-Up Roofs
SEE ROOFING PRODUCTS
Built-up roofing systems have been mainstays on institutional and commercial facilities for years. Maintenance managers have specified these systems seeking durability and flexibility.
But to ensure these systems deliver these and other benefits to their organizations, managers must ensure they are maintained properly. A properly managed and coordinated quality-assurance inspection program during installation can help managers and workers identify and address common performance problems with built-up roofing systems.
Inspectors first must understand the roofing system’s drawings and specifications before installation. A pre-construction meeting that includes the architect and consultant, the inspector, the roofing contractor’s foreman, the general contractor, the sheet-metal contractor and other interested parties enables all involved to discuss the project and its details, and to answer last-minute questions.
This discussion also might include what to expect from the inspector, such as how the daily and formal inspection reports — as well as any test sample results — should be distributed and to which individuals.
Experienced inspectors must understand that they are responsible for making sure everyone involved is fully informed and that the organization receives a proper return on its investment. For example, a contractor’s failure to comply with the approved plans and specifications should not be acceptable, and the issue must be reported to the foreman or the maintenance manager. A comprehensive inspection must cover the primary components of the installed roofing system.
Roof Deck Substrates
A good roof installation starts with specified original manufacturer’s unopened products delivered on-site, dry, tarped and stored on pallets so they do not become wet. The inspector must visually observe that the specified on-site materials are installed at the beginning of each site visit.
Materials that do not meet the specifications and are not approved as a substitute by the architect or engineer should be marked and removed from the site.
Before the contractor begins applying roofing materials, the inspector and roofing foreman should check the deck substrate for any deteriorated decking scheduled for replacement — if specified for re-use — and loose debris or small objects not removed after tear-off or installation of new decking material. The inspector also should check concrete decks for dryness by using a small amount of hot bitumen applied to the deck surface. The inspector then should check for frothing or bubbling of the bitumen, which would indicate that moisture is present. If frothing or bubbling does not occur, the inspector should attempt to peel the bitumen off of the deck. If the bitumen peels off easily with no resistance, the concrete deck is too wet and must be allowed to dry.
Installations often include a minimum of two layers of insulation boards, with offsetting and staggered joints to minimize the transfer of warm air through the roof system. The inspector should check all insulation joints and recommend that the roofing contractor fill in all gaps greater than 1/4 inch with insulation.
Failure to perform this action can result in warm inside air that migrates through the system, meeting cold outside air at the membrane level and resulting in the constricting and contracting of the built-up roof membrane, eventually causing the membrane to split open.
Inspectors should review insulation boards installed over steel decks per manufacturer product-data sheets for thickness requirements at spans over deck flutes. Also, if mechanical fastening is specified, the inspector must review the fastening requirements outlined in the specification with regard to the number of fasteners required at outside building corners, perimeters and the interior field of the roof area.
Inspectors also should carefully review specified fasteners and stress plates before installation.
Checking the bitumen temperatures at the tanker, kettle and the membrane application is critical to ensure the proper installation of a built-up roof membrane. The temperature gauges on tankers and kettles should rarely be trusted as in many cases they are either broken, inaccurate, or coated with bitumen.
Bitumen temperatures at the tanker and kettle must not exceed the flash point of the bitumen that is listed on the container. For obvious safety reasons, the inspector should check the temperature of bitumen by using a hand-held infrared thermal detector.
The inspector also must check the temperature at the point of application and compare the findings with the listed equiviscous temperature (EVT) stamped on the container. The EVT is the manufacturer’s recommended temperature at the application point that will allow for a proper bitumen viscosity range and provide for uniform inter-ply mopping weights.
Essentially, bitumen temperatures that are too hot might result in the application of bitumen to spread out and provide for lighter mopping weights, thus reducing the amount of inter-ply waterproofing.
Conversely, if the bitumen mopping is too cold, the application of inter-ply bitumen might be thicker and cause membrane slippage and ridging. The inspector must check these temperatures several times throughout each day to be sure application temperatures are being maintained.
Most built-up membrane manufacturer’s rolls have chalk lines printed on the felt. The inspector should inspect the factory-installed chalk lines on the felt membrane during installation, making sure the installer does not sway outside the appropriate lines during installation. Doing so might allow for a multi-ply membrane installation that does not meet the specification requirements for the required number of plies.
A good practice is to review the leading edge of the felt roll as it is rolled into the hot bitumen. Proper bitumen inter-ply mopping weights typically would be achieved with a pool of hot bitumen observed at the leading edge of the felt roll.
If the inspector does not observe a pool of bitumen, there is a good chance that not enough bitumen is being applied at the inter-ply mopping, which can result in a felt-on-felt condition and lead to blisters in the membrane inter-ply, eventually resulting in membrane failure.
Fish mouths at the end of the membrane roll result from not enough bitumen installed at the ends of the felt roll. The inspector should mark these locations for repair, cut the cupped felt without damaging the underplies of roofing material, and reset the felt ply in mastic, allowing it to lay flat. A one-ply cover felt should be installed over the repair area extending minimum 12 inches on each side of the repair.
Inspectors should always be aware of workers walking on recently installed membranes. This action can damage the membrane and displace the interply bitumen, causing blisters to form where boot prints leave a void. In most installations, built-up membranes are covered with river-washed gravel in a flood coat of bitumen, or a modified granulated cap sheet is installed.
Finally, inspectors should monitor the installation of base flashings so installers do not stretch the membrane into place. They should install base flashing sheets in lengths that are easily handled — about 10- to 12-foot runs, at most — and hand-pressed into the hot-bitumen application, making sure to achieve full adhesion to the substrate.
They also should secure base flashings along the top edge with nails through 1-inch-diameter discs or with a continuous term bar fastened to the substrate at 8-inch outside circumference with a sheet metal counterflashing cover.
Paying close attention to these essential issues is the first step to the successful performance of built-up roofing systems. Preventing problems at this point can ensure that these systems achieve their intended performance lives.
Tom R. Kaiser, RRO, is roofing division manager with StructureTec — www.structuretec.com — a building envelope consulting firm specializing in the restoration of building envelopes and roofs.