Roffing System 12
After identifying the project’s requirements a roof system should be selected that optimally responds to an integration of the project’s requirements and the system selection criteria discussed in System Selection Criteria below. After the roof system is selected, drawings and specifications are prepared to communicate the roof designer’s design concept to a professional roofing contractor. This section also covers warranty considerations, key elements of drawings and specifications, and construction contract administration.
SYSTEM SELECTION CRITERIA
ROOF SYSTEM SELECTION
For most roofs, several different types of systems could serve quite well. But some roofs have unique characteristics that lend themselves to perhaps only a few systems. In order to select the most appropriate system for a project, ideally the designer should have a good understanding of the material and system options described in the Description section. On large (>15,000sf) and significant projects the designer should be a registered engineer, architect, or consultant that derives their principal income from roof design. Note that roofing system manufacturers specifically state that they do not design roof systems.
In the context of this section, system selection refers to selection from the primary system types discussed in the Description section (such as BUR, modified bitumen, single-ply, sprayed polyurethane foam, metal panels, asphalt shingles, slate, or tile), as well as the selection of membrane materials within system types (such as type of modified bitumen, type of single-ply membrane, type of surfacing on an SPF, type of metal panel profile, or type of shingle or tile), and where applicable, the attachment configuration (fully adhered, ballasted, mechanically attached, PMR, or loose-laid air-pressure equalized).
- Factory Mutual (insurance underwriters’) requirements.
- Structural engineer to do wind uplift analysis per ASCE 7.
- Required R-value/u-factor for energy code compliance.
- Cool roof: yes or no.
- Aesthetics: can the roof be seen from above?
- Interior and exterior temperature/humidity parameters.
- Owner’s risk tolerance (i.e. data center versus common commercial space).
- Owner’s ability to maintain the roof.
- Roof access to public?
- How much mechanical equipment on roof?
- Need for early enclosure with temporary roof to facilitate construction?
- Local trade practices and preferences.
- How much subsequent construction over low roofs while working on walls above?
- Ability to reach the roof in the future such as in high-rise situations.
- Requirements for fire resistance rated assemblies.
- Smoke developed/flame spread criteria.
- Hail resistance.
- Hurricane zones: high winds and elimination of small missiles (ballast).
- Owner’s expectation for warranty coverage: will the owner expect and pay for a warranty that extends all the way to the predicted wind speed.
- Life-cycle costing.
- Where is drainage and what type? Is the structural deck sloped?
- How is secondary (overflow) drainage to be accomplished?
- Design of parapets.
- Available height for base flashings.
- Presence of animal fats or other exhausts that can harm some membranes.
- Presence of other contaminants, such as jet fuel for roofs at airports that can damage some membranes.
- For re-roofs: there are many more criteria
With an understanding of the available system options, consideration of the following technical and non-technical criteria can lead to the selection of the most appropriate system and details for a project.
- System demise
- Contractor familiarity and availability
- Maintenance intensity
- Technical considerations
- Available technical support from manufacturer.
- Veracity of manufacturer’s training programs for roofing installers.
- Coverage and fairness included in manufacturer’s warranty.
- Types of roofs in vicinity
- Implications of sustainable roof design
It is critical that the selected system sufficiently satisfy all of the criteria. Specific system selection recommendations are given later below.
Roof system design life should be a starting point in the selection of roof systems. Most buildings utilize a minimum 20-year roof design life. Since many roofing systems have not been in production for 20 years, the designer should consider proven systems as a first step in obtaining roof design life.