There’s More to a Roof Replacement Than Replacing the Roofing
Replacing an existing roof system is a required maintenance item that all building managers will encounter at some, or multiple, points over the life of a building. It sounds pretty straightforward – hire a contractor to remove the existing roof system and install a new roof system in kind. However, from code requirements to building appurtenances, roof replacement can be much more involved than it initially seems.
Thermal Resistance Considerations
With recent code changes requiring buildings to focus on energy efficiency and limiting environmental impacts, roof system insulation thicknesses have increased considerably. Depending on the climate zone, new roof systems typically require continuous insulation thermal resistances of R-20, R-25, or R-30 above the roof deck to comply with the International Energy Conservation Code. These R-values correspond to three-and-a-half to five inches (3.5” to 5”) or more of flatstock insulation. To direct stormwater toward roof drains, tapered insulation is added on top of the flatstock, increasing existing roof systems with two inches (2”) to as much as fifteen to twenty inches (15” to 20”) of combined insulation at roof edges depending on the building configuration and tapered insulation layout. This increased roof system thickness often impacts rooftop equipment, roof penetrations, penthouses, and rising walls.
Roof Penetrations
Raising rooftop equipment may require a mechanical engineer during the design phase to define scope and will require temporary disconnection of the unit, extension of electrical lines and ductwork, curb extension, curb replacement, or installation of new rooftop dunnage to accommodate the increase in height, and reconnection of the unit. This temporary shutdown may need to be coordinated with interior occupants to limit the impact of building operations, especially in critical facilities, such as hospitals. If the building has existing rooftop communications equipment, in addition to requiring coordination with the provider’s contractor for modifications/raising of equipment, a temporary shutdown may be required during roof system replacement to allow workers to access the area. The cost associated with this system “down time” may dictate a shorter construction period depending on the outcome of a cost-benefit analysis.
If the building has existing skylights, the owner will need to consider whether these units should be maintained, raised, or infilled based on the building’s interior layout and finishes. If skylights will remain part of the new roof system, fall-rated glazings or other protection, such as rails or cages, will need to be considered for OSHA compliance. If the existing roof system has smoke vents or smoke hatches at stairwells, it will need to be confirmed if these units can be decommissioned or if they are required to be maintained. If new smoke hatches are to be part of the new roof system, connection to the building’s electrical and fire alarm systems, fusible links, and the unit aesthetic (whether to incorporate daylighting, a single or double leaf unit, unit slope, etc.) need to be considered.
Vent pipes may need to be raised to provide adequate clearance above the new roof system. For example, the Massachusetts Uniform State Plumbing Code (248 CRM) requires vent pipes to extend eighteen inches minimum to twenty-four inches maximum (18” to 24”) above the roof. Roof drains may need to be replaced with new drain bowls with integral extensions to accommodate the increased insulation thickness. Similarly, chimneys may need to be extended to maintain appropriate clearance above the finished roof surface for venting. Fire separation walls between building sections may need to be extended to maintain adequate protection.
Rising Walls
If there are penthouses or rising walls above the roof, these walls may require repairs to address moisture infiltration that could enter into the new roof system or the building interior and present as an apparent roof leak. The lower limits of the walls and flashings may also need to be reconfigured as the increased thickness of the roof system could result in existing base flashings now terminating within the new roof system. This can potentially allow moisture infiltration into the roof system. Roof access doors may need to be raised, which will require structural engineering to raise the head of the door opening. Doors may need to be reduced in size by maintaining the door head and raising the sill, which could restrict access. Existing windows may need to be replaced with smaller units, louvers may need to be raised or modified, and existing wall penetrations may need to be rerouted. Depending on the existing wall system(s) and the modifications required, this can considerably increase the project cost and schedule. Therefore, when making design modifications, such as raising flashings or wall penetrations to accommodate current roof replacement, it is important to consider how to minimize modifications needed during future roof replacements. Providing additional flashing height now could possibly alleviate the need to raise flashings again during the next roof replacement.
Gravity Load
Depending on the existing roof system and roof deck type, the increased insulation thickness may result in the new roof system weighing more than the existing roof system. If the new roof system increases the dead load on the building framing element by more than five percent (5%), the International Existing Building Code requires structural analysis to determine if load-carrying structural elements need to be altered or replaced to support the increased loading from the new roof system. This analysis and any required modifications can add additional time, cost, and interior disruptions to the project.
Stormwater Drainage
Stormwater drainage at the new roof system via roof drains, gutters and downspouts, or scuppers will need to provide adequate primary drainage capacity. The roof system may need to provide independent secondary (emergency) drainage to a readily visible location in the event the primary drainage system becomes obstructed. Other plumbing code requirements, such as acceptable materials for drainage system components and installation of strainers at drain bowls, will also need to be identified and addressed.
Selecting Your New Roof System
And of course selection of an appropriate new roof system is an important part of the roof replacement. It is crucial to consider wind uplift resistance requirements, whether a continuous air barrier is required, placement of a vapor retarder based on dew point analysis, and the required durability of the roof covering. The new roof system design may also need to consider solar reflectance requirements and solar-ready requirements depending on the climate zone and jurisdiction of the site. Therefore, it can be advantageous to employ a building enclosure consultant to provide design services for a roof replacement.
Kimberly Kilroy, PE, RRC, CDT
Project Manager, Gale Associates
Email address: [email protected]
Kimberly Kilroy, P.E., RRC, CDT is a Project Manager in the Building Enclosure Design and Consulting Group at Gale Associates, Inc. She manages and conducts roof, wall, and window evaluations, analysis, and design, including the development of technical specifications, drawings, and details for competitive bidding and construction. Ms. Kilroy performs construction administration services, including submittal review, construction meetings moderation, and intermittent on-site review through project completion.
