At some point in the life of most buildings, the roof is going to leak. Every facilities manager's goal should be to increase the length of time between roof completion and its ultimate failure and replacement.
There are steps that can be taken during design that will make a difference in how much maintenance a roof will need and how easy it will be to perform. Before designing a maintainable roof, determine the intention to do maintenance, the expectation of roof life and the relative damage possible by a roof leak.
Handling the leak concern first, the intended use of the space below the roof will help determine how maintenance should be prioritized. Storage space for items that can tolerate weather or moderate moisture may dictate a "leak-fix" maintenance schedule, where nobody worries about the roof until it leaks.
The other end of the leak-tolerance spectrum is to have high-tech equipment with zero-tolerance for water. With these systems, inspection becomes crucial to stopping problems from becoming leaks.
Expected roof life For a facilities manager who has short-term expectations, a long-lasting roof may not be acceptable from an initial cost basis. Those needing to live with the roof until the second replacement should be interested in improving the life of the first roof and reducing the cost of the replacement.
For specifiers dealing solely with districts that look at cost first, some of the following ideas will not add significantly to total cost and may still work.
If the intention is to reduce initial cost, the maintenance likely will be more expensive. If the district does not plan maintenance, then increasing slope and changing flashing details are important. Use of water-resistant roofing materials also will help reduce damage from a roof leak. Some systems demand maintenance, including roofing systems having impermeable vapor or air barriers; monolithic decks, such as concrete; and those that will be subjected to known abuse. Leaks will cause damage over large areas before the water finds its way into the building's interior.
Some important points to consider when designing a roof system: -Drainage/water-removal systems. -Rooftop traffic control. -Access. -Management of the roofing asset.
Water-removal systems There are three helpful things to know about getting water off the roof. First, water sitting on a roof can increase the amount of damage, as there is more water to get in the system when the roof leaks. Second, roofs require drainage so that the water is not there to accelerate deterioration of the roof membrane. Third, water removal helps reduce the dead load on the building. Water weighs 5 pounds per square foot for each inch of depth. If the weight causes a structural deflection, then additional water will flow into those areas and increase the load, which increases the deflection until structural collapse is the means by which the roof drains.
The ICBO Uniform Building Code and the BOCA Standard Building Code both require that all roofs have a minimum 1/4:12 slope to drain. In addition, good roofing practice suggests that drains be spaced no more than 75 feet apart. As roof maintenance includes cleaning drains, overflow scuppers are unlikely to be used, but should be employed at a height not more than 2 inches above the roof surface. Overflow scuppers should not be placed on the north wall in cold climates as they can ice up; ideally, they should be in highly visible locations. If water is coming out, then the primary drains are not working and it is urgent to get the primaries operating quickly. One suggestion is to place the scuppers over the main entrance, which make them hard to ignore.
Selecting materials One of the most important aspects of any roof is selecting the right materials in the beginning. Keep these items in mind when designing/specifying a roofing system:
-Structural slope. Design the slope into the original structure. The additional cost is relatively small and the slope is there for the life of the building. Tapered insulation boards are available in almost all generic insulation types. These will allow development of slope or allow a change of slope as needed. The layout is usually best done by the company supplying the stock as it has specially developed computer software to aid in optimum layout.
-Lightweight insulating concrete. This can be used to develop slope over an existing deck or, in some cases, over an existing roof. Caution should be taken when using lightweight concrete to avoid creating a water trap in the roof system.
-Sprayed-in-place polyurethane foam. These systems are used as an insulation and to develop slope to drain in one application.
-Metal roof system or wood trusses. Convert the roof from low slope to a higher slope using either a metal roof system or wood trusses, sheathing and shingles. This has the advantage of changing a building's appearance, as well as encouraging slope to drain. A disadvantage is the creation of an attic space, which may make sprinklers necessary.
-Drains. Adding drains can be an easy and quick way to get rid of a ponded area, if the slope is already there. Examine planned or known drain locations, and make sure that they are located at low points and that distances between them are no more than 75 feet. Check slopes to obtain code-required 1/4:12. Drains should be installed at low points, clamped in place using proper under-deck clamping rings, and should have horizontal leaders connecting them to the down pipe to absorb any deck deflection.
Important details Often, problems on roofs are at perimeters and penetrations. The reasons for these problems relate to differential movement, stress concentration, abuse and accelerated weathering of the vertical surfaces.
Designing different material types, such as metal fascia, into built-up roofing can cause stresses capable of causing splits in the membrane. Flashings should shed water at the tops and tie into the membrane using a watertight design. The water-shedding design requires a proper counter flashing. From a maintenance standpoint, the counter flashing should be removable and replaceable so that flashing work can be done without causing damage to the counter flashing.
The National Roofing Contractors Association (NRCA) is a good source for information on standard details. Consult The Construction Details section of the NRCA Roofing and Waterproofing Manual on proper counter flashings and base flashings.
Regarding prefabricated metal fascia systems, include detailing that helps to keep them maintainable. Keep in mind that some prefabricated metal curbs with integral metal cants are not acceptable for warranty by some manufacturers. Most wood curbs are acceptable. To ensure proper execution:
-Design for the anticipated movement in metal components in the roof, and do not allow metal in the same plane as the roof. -Assure proper anchorage, particularly at roof edges, for restraint of the system. -Design additional protection into flashings where the potential for abuse exists, such as around roof hatches or doors. -Make sure units are high enough to allow for inevitable reroofing. Use round supports instead of I-beams to limit the number of pitch pockets.
Typically, roofs are not designed to be smoking lounges or storage areas for excess inventory; nor are they sunbathing areas. They often end up as all of these in addition to being work platforms for HVAC repair. A roof is not supposed to have high amounts of traffic unless it is designed specifically for it.
Therefore, rooftop traffic should be minimized as much as possible. The use of walkway systems can help, but if access is positioned properly and controlled, walkways are not as crucial. Products that will help you design a roofing system that will experience significant traffic:
-Walkway boards made from asphalt products and rubber products are common. Position the system between units and the access point.
-Locks used on doors or hatches to the roof. Alarms with a delay may be needed if the roof is used as a fire-escape route.
-Warning signs explaining possible warranty consequences of unauthorized damage should be posted. To make the aforementioned products work within the roof system, keep these things in mind: *Position access close to rooftop units needing service. *Use walk pads at access points and around units as a minimum. *Reinforce the importance of keeping unauthorized people off the roof.
Even though you want to keep roof traffic to a minimum, you have to be able to access the building's top to perform maintenance. Sometimes using a window is acceptable, but the window needs to be sized to allow a maintenance employee, as well as needed tools and materials, to pass through.
Rooftop hatches with permanent ladders are convenient; stairs and a door are easier; and ladder access is acceptable. When designing, look at each roof section and determine how to get to each area carrying a tool chest.
Manage your asset Roofs are an asset that must be managed to maximize value. If the roof is not maintained using proper inspections and a good recordkeeping system, it will be hard to make the roof last as long as it should.
Sell the district on planning a maintenance schedule for roofing systems. This is a set of components, each of which must work with the other components to keep weather out of the building. Interruption of a roof system results in interior or structural damage, disruption of occupant activities and possible loss of inventory. Maintenance should be semi-annual based on a set checklist that speeds inspections and prevents items from being overlooked.
The key is not necessarily how the roof looks today, but what has happened since the last inspection. The best way to know this is to have a strong, thorough record base so that even if inspectors change, the rooftop conditions will be observed.
