Once it has been determined that a roof needs to be replaced, managers should follow these best practices to tailor the roof type to the needs of their facility.
Compared to a restoration, a roof replacement is associated with higher costs and effort. A full roof assembly replacement goes beyond the roof sheeting and includes considerations for insulation, fire classification, wind elevation, and the performance of the structural roof deck and membrane. The requirements of modern building codes are stricter than they were 20 or even 10 years ago. When planning a roof replacement project, managers should consider the following considerations:
Structural membrane analysis. Depending on the applicable code, a licensed engineer may need to analyze the structural membrane of the roof and roof-to-wall connections for compliance with the IBC wind load.
Insulation thickness. Current building codes for energy savings may include minimum insulation requirements for R-2 through R-30. If the existing roof deck is shallow and tapered insulation creates the required slope of drains, the insulation depth can be well over 6 inches in some areas. If existing structural elements such as low parapet walls or penthouse thresholds were designed for a roof system with 2-inch insulation, these components must be modified to accommodate the new finished roof level.
Drains. Managers should analyze existing runoffs to confirm that their location and size adequately account for rainfall for preliminary planning. Replacing drains and adding new drains if necessary requires access to the bottom of the roof terrace. Suspended or finished ceilings must be removed and reinstalled accordingly.
Choosing a new roof with a low slope
After a manager has weighed the options and selected a roof replacement approach, the next point of discussion is the type of roof system and membrane. While it is easiest to stick to what a manager knows, an analysis of current requirements and new code requirements is a good idea. When choosing a suitable roof membrane system, managers need to consider roof usage, environmental factors and budget, among other things.
Roof systems that serve as additional spaces for tenant activities require a sturdy assembly that protects the membrane and a surface for pedestrian traffic. This system is likely to be an inverted or protected roof membrane arrangement (IRMA or PRMA) with insulation and overburden installed on top of the membrane. Suitable membranes for these systems include multi-layer bitumen, hot-applied rubberized, and reinforced liquid resin.
Green roof assemblies create an environment in which water is channeled evenly into the spoil pile and where root systems create overlapping seams between membrane panels – not ideal for waterproofing. The heavy congestion causes a logistical and financial burden if leaks occur. Managers can mitigate these factors by using a seamless application such as that provided by liquid-applied resin systems and hot-applied gummed systems.
Roofs, which serve solely as a waterproof cap for a building, offer the opportunity to use the greatest variety of membranes. Multi-layer bitumen systems, hot-applied rubberized systems, and reinforced liquid resin systems provide superior performance, but lower cost options may offer nearly the same performance.
Traditional or exposed membrane assemblies enable budget-friendly single-layer systems. However, managers should be aware that manufacturers offer single-layer membranes in a variety of thicknesses.
Over time, pedestrian traffic, water flow, ultraviolet radiation, and exposure to the elements erode the membrane surface. The thicker the membrane, the longer it will last. However, this maxim is specific to the membrane material and does not take into account the roof installation and details, the performance of which is based on sound design and high quality workmanship.
Environmental factors can also determine or influence the use of a particular type of membrane. For example, chronic exposure to corrosive exhaust gas requires a membrane with suitable chemical properties that will not degrade prematurely under extreme environmental conditions. Corrosive deposits require the use of membranes with a composition that has been tested and proven to be resistant to such exposure, such as a polyvinyl chloride (PVC) or ketone ethylene ester membrane.
The prices for roofing membranes vary. In general, single layer membranes are relatively inexpensive systems with basic labor requirements for installation. Multi-layer systems offer additional redundancy, which is always in demand for sealing and is usually more expensive than single-layer systems. Liquid applied resin systems and hot applied rubberized systems are typically the highest priced assemblies. They require on-site material preparation and labor-intensive installation, but provide a seamless waterproofing membrane.
Recommended course of action
Managers can use best practices to set up, review, and maintain the quality of a new roof. During the design phase, factory visits, adhesion tests (for roof restoration) and models for quality control are critical. For recovery projects, infrared and capacity tests can confirm that existing assemblies have not been affected by water. If testing reveals limited water damage to the system, targeted repairs can be performed prior to restoration.
After installing the restoration or new roof, installers should provide the manufacturer’s maintenance guidelines. This literature contains information at the time of the routine inspection, what to look for in roof assessments, general recommendations for care and maintenance, and instructions for notifying the manufacturer of problems.
Typical roof inspection items are: signs of stress; including wrinkles and blisters; Evidence of mechanical abuse such as punctures and cuts; unusual wear and tear from excessive pedestrian traffic; and indications of damage caused by chemical attack or other adverse reactions to substances.
Maintenance recommendations may include keeping the roof free of debris that can cause damage, cleaning the membrane with low pressure water or soft bristle brooms, and maintaining clear drains and barriers.
The roof system of a building is crucial for its comfortable use and durability. Choosing a suitable roof system and carefully maintaining the roof ultimately reduce problems and increase the return on investment.
Daniel L. Bishop, AIA – [email protected] – is a project architect in the office of Hoffmann Architects in Virginia, an architecture and engineering firm that specializes in the renovation of exterior buildings. He designs cost-effective, resilient solutions for building housing assemblies.