Reroofing your facility generally only occurs every two to three decades. However, a new piece of rooftop equipment or penetration may be added more frequently. Maybe skylights are installed to increase the light levels within a building. A more efficient HVAC unit could be replacing an older version. Perhaps new vent stacks are needed for interior improvements. Each of these examples is an alteration to an existing roof, and preparation and planning is critical for a successful installation.
The preplanning actually starts when a new roof is installed. Similar to keeping records of your vehicle fleet’s maintenance or elevator upkeep, it is prudent to keep a roofing file. At a minimum, the design documents, material samples, manufacturer information, work contracts, warranties or guarantees, and maintenance reports should be kept in this file. Ideally, an as-built set of drawings should also be included. A roofing file is the basis for understanding the makeup of the roof system when revisions are made to the equipment and penetrations on an existing roof. It is important to know the membrane type, insulation amounts, and whether there is a vapor retarder/air barrier utilized in the roofing system.
If the roof is covered by a warranty or guarantee, read it to ensure you are following the requirements, and then contact the manufacturer (or the third-party entity holding the warranty or guarantee) before initiating any work. The requirements of the warranty or guarantee must be followed so that the warranty is not voided. The type of system and warranty or guarantee length often determine which version of a construction detail is permitted. Not all construction details are equivalent; warranties and guarantees of longer duration require more conservative details. Manufacturers can provide acceptable flashing details for new rooftop equipment and penetrations.
When having work done, the roofing contractor should be authorized to perform the work by the manufacturer of the roof system. This is important for all roof types. Is your roof covered by a warranty or guarantee? If so, it is imperative to use an authorized contractor. It may be best to use the installing contractor because the installing contractor may also provide a warranty.
Matching the existing materials and roof system design is also crucial. This is much easier to ensure if you maintain a roofing file. Use the same membrane from the same manufacturer. This is especially important for TPO and PVC membranes to ensure seams are installed correctly. Use the same type and thickness of insulation and coverboard, as well as the same roofing component attachment methods.
For penetrations through a roof system (such as a vent stack) with an air barrier/vapor retarder at the deck level, the air barrier/vapor retarder must be connected (e.g. flashed or foamed in place) to the new penetration to ensure continuity within the system. Incorrectly detailed penetrations can cause or exacerbate condensation issues that could change system performance or cause premature degradation of components and possibly structural members. Condensation is typically excluded from coverage under manufacturer warranties or guarantees.
For new curbs for equipment, install the prefabricated curbs on top of wood nailer to eliminate thermal bridges. Code requires insulated curbs so that there is a minimum level of insulation. If there is an air barrier/vapor retarder, that layer needs to be incorporated into the curb/flashing detail. Also, new curbs might change the drainage path. If curbs are wider than 24 inches or so, install a small up-slope cricket to divert water around the new curb.
Make sure any necessary structural upgrades are done when adding a large or heavy piece of rooftop equipment. Creating an opening through a steel, wood or concrete deck likely changes the load path, and without the addition of new substructure – such as angle iron between steel joists, new 2-bys or beams between main wood framing members – a new opening in a roof deck could become a weak point, possibly resulting in a localized deck failure. It is best to consult a structural engineer or architect to determine if minor structural upgrades are required.
Appropriate planning and preparation before installing new rooftop equipment will help ensure that the installation is successful in the long term and that the warranty or guarantee covering the roof remains intact.
This article provides general guidance for information purposes only. You should always consult with a design professional when adding rooftop equipment on an existing building.
James R. Kirby, AIA, is the Building and Roofing Science Architect for GAF serving the East Coast.