METAL BUILDING SYSTEMS：REROOFING AND RENOVATIONS OF METAL BUILDING SYSTEMS.pdf

CHAPTER 14 REROOFING AND RENOVATIONS OF METAL BUILDING SYSTEMS 14.1INTRODUCTION Building reuse and rehabilitation grow ever so popular, and the volume of renovation work now rivals that of new construction. The principles of metal building systems such as cost efficiency and single-source responsibility are applicable not only to new construction but also to building renova- tion. As the first generations of pre-engineered buildings approach the ends of their useful lives, they can be rehabilitated or partially replaced with new metal building framing. Some components of metal building systems have their place in renovation of buildings constructed conventionally. A common problem facing the owners of low-rise buildings is leaky roofs. Whenever building renovation is mentioned, reroofing or roof retrofit immediately comes to mind. Accordingly, this chapter focuses first on renovations of both conventional and metal roofs using metal roofing. Modifications of exterior skin are addressed next, followed by reinforcement of primary and sec- ondary framing. The chapter concludes with a discussion of some difficult issues surrounding adap- tation of existing pre-engineered buildings to new conditions of service. 14.2ROOF RETROFIT WITH METAL BUILDING SYSTEMS 14.2.1The Troublesome Roofs No building problem seems to cause more aggravation than a leaking roof. Dealing with the occu- pants pointing to a leaka problem that is easy to spot but hard to fixis among the most frustrat- ing duties of the building owner. After a few rounds of thankless repairs, a total reroofing seems to be the only solution left. Not surprisingly, reroofing work makes up two-thirds of all roofing projects in this country. Why do roofs fail so fast? The roof is the hardest-working part of the building, protecting it from the blazing sun in the sum- mer, snow in the winter, and rain year-round. Every windstorm attacks the roof by first trying to lit- erally lift it off the building and then slammming it back. Ultraviolet radiation shortens the lives of unprotected single-ply membranes, slowly robbing them of elasticity and strength; it causes damage to other roofing types as well. Most conventional low-rise buildings have flat or nearly flat membrane or built-up roofs, a solu- tion that was popular until only a few years ago. A minimum pitch of 1?8:12 was considered ade- quate for drainage; it probably wasin theory. In real life, however, building foundations settle a 393 bit unevenly, roof beam elevations vary slightly owing to fabrication and installation tolerances, and beams and decking deflect under load. Also, point loading from HVAC equipment, suspended pip- ing, lights, and such causes some roof structural members to deflect more than others. All these fac- tors may result in the actual roof profile being far from the assumedwith some areas of the roof having no slope at alland lead to an accumulation of ponded water. Roofing not designed to be submerged for prolonged periods of time, like some built-up asphalt- based products, may slowly start to disintegrate and eventually leak. Other factors leading to roofing failures include local damage from careless foot traffic or equipment maintenance, clogged roof drainsagain resulting in pondingand poorly protected roof penetrations. The deterioration often starts at the flashing locations, expansion joints, and improperly fastened gravel guards. Regardless of the origin, roof leakage may result in saturation and ruining of fiber- glass insulation, staining of finishes, and corrosion of roof decking. If not addressed promptly, dam- age can progress to the point of making the roof unrepairable, leaving tear-down and replacement as the only solution. 14.2.2Reroofing Options One popular choice for reroofing of conventionally built roofs is the single-ply membrane, espe- cially of the lightweight fully adhered or mechanically fastened varieties. This material is not with- out drawbacks. To cover an old tar-and-gravel roof with a single-ply membrane, all gravel usually has to be removed. This messy operation, if not handled properly, may result in a badly gouged roof that needs to be overlaid with protection boards or even torn off completely. The roof slope, if pre- viously inadequate, can be changed only with expensive tapered insulation. And, as already men- tioned, in sunny locales solar radiation causes the unprotected membranes to fail rather quickly, ruling this system out. Another increasingly popular option is reroofing with metal. This solution offers numerous advantages. As discussed in Chap. 6, metal roofing comes in a variety of finishes including polyvinylidene-based coatings that are extremely durable and ultraviolet-resistant. Standing-seam roofing with sliding clips can better handle thermal expansion and contraction than membranes. Even with slopes as low as 1?4:12 for structural panels and 3:12 for architectural roofing, water can drain faster than in nearly flat roofs. With steeper slopes, as recommended in Chap. 6, the roofing should perform even better. The required slope can be accomplished by erecting a light-gage frame- work on the old roof. The total weight of metal roofing and the new framework usually does not exceed 2 to 4 lb/ft2, placing this system among the lightest available. Quite often this small additional load can be safely taken by the existing roof structure, while the heavier systems such as built-up roofing would over- stress it. If the existing roof structure has no excess capacity at all, a system of beams or trusses span- ning between the new stub columns on top of the existing building columns can be erected. Some experienced architects believe that properly designed and constructed metal roofs will last 40 years.1While metal roofs may initially cost more than the competing systems, their exceptional durability combined with ease of maintenance often make metal a winner in life-cycle cost compar- isons. Being recyclable, metal roofing wins on an environmental scorecard, too. Metal roofing is very useful in circumstances requiring a replacement of the existing slate or tile roof supported by an aging, and undersized, roof structure. Such roofs can benefit from a metal Bermuda roofing with the slate, shake, or tile profile, or from a PVDF-covered metal shingle prod- uct designed to closely resemble the traditional materials. 14.2.3Tear off or Re-cover? A decision on preserving the existing roofing versus removing it often hinges on a level of moisture in the existing roof system. That the previous leaks caused some water to get into the roof insulation is clear; the question is only, how much water? Reroofing over existing roofing and moist insulation can 394CHAPTER FOURTEEN invite several problems. In addition to the already mentioned problems of diminished insulation per- formance and corrosion of the existing decking, entrapped moisture can cause offensive smell and growth of mold and mildew,resulting in serious indoor-air quality problems. Also,retrofit fasteners that penetrate the moist space might eventually corrode and undermine the integrity of a newly constructed roof system. The degree of water saturation can be determined by a moisture survey performed by the design professional or by a specialized consulting firm. The latter may give more reliable results, because specialized firms are likely to employ such advanced testing methods as infrared thermography, capacitance, and nuclear back scatter.2The survey produces a rough outline of the areas containing wet insulation and determines the degree of saturation, which can be confirmed by taking a few insu- lation cores. Only then can the magnitude of the problem be rationally assessed. A common solution to the problem of entrapped moisture is to install several “breather” vents and hope that the moisture escapes prior to the final enclosure. This approach works only for very modest moisture levels, however. If the existing roofing and insulation are totally saturated with water from frequent leaks, venting through a few holes might not be adequate, especially when structural decking or a vapor barrier restricts the downward moisture migration. Studies indicate that it would take 30 to 100 years for the insulation to dry out in such circum- stances, even with the vents installed.2A better course of action is to remove the roofing and the wet insulation. Tobiasson3notes: “In most cases, wet insulation should be viewed as a cancer that should be removed before reroofing.” He points out that every inch of saturated insulation can add up to 5 lb/ft2to the dead loada significant amount. A moisture survey that indicates numerous areas of wet insulation is to be taken seriously: a complete tear-off might be the only prudent option left. A survey of the roof structural decking is also extremely helpful. The persistent leaks might have led to a widespread decking corrosion beyond repair. Similarly, a presence of some potentially cor- rosive roof components could have degraded the deck. Phenolic-foam roof insulation produced in the late 1980s until 1992 is a case in point. It has been reported that this type of insulation, when wet or damaged, can contribute to corrosion of metal deck, sometimes to the point of making it unsafe to walk on. A replacement of roof decking is a serious matter since it opens the inside of the building to the elements and affects the operations. There are arguments against a complete roof tear-off, the most obvious being high cost. The bill for a disposal of the removed materials, perhaps containing hazardous waste such as asbestos roof- ing felts, could also be significant. The pros and cons of the two approaches require careful consideration. Curiously, the 2 billion ft2of reroofing work performed annually in this country are evenly divided between the tear-off and re-covering.2Of course, when the local building code prohibits the addition of another roofing layer, the decision on tear-off versus reroof comes easily. 14.2.4The Issue of Design Responsibility Who determines whether an existing roof is structurally capable of carrying the extra load, however modest, from reroofing? As we discussed earlier, the manufacturers of metal building systems are unwilling to get involved beyond the design of metal components; they normally disclaim any responsibility for evaluation of the existing roof structure and its capacity to support additional loads. Hire a local structural engineer to analyze the existing roof structure, they suggest. The problem is, the engineer can readily check only the average uniform load capacity of the roof, a computation usable only if the new roofing is simply laid on top of the existing. Any change in slope, however, requires a new (“retrofit”) framework supported by some discrete columns that will transmit concentrated, not uniform, loads to the existing structure. At the evaluation stage, the engineer often has no way of knowing which manufacturer will be selected to do the work and what the column spacing will be. If the manufacturer is already on board, so much the better. If not, the engineers can select one of the popular systems, such as the one described below, base their analysis on that system, and require the contractor to adhere to it. Or, they can assume the worst-case scenario and use a rather REROOFING AND RENOVATIONS OF METAL BUILDING SYSTEMS395 expensive approach of requiring the new trusswork to bear only on the existing columns, bypassing the existing roof framing altogether. Alternatively, they might require that the new supports be spaced so closely as to approximate a uniform loadnot the most cost-effective solution, either. To make matters even more complicated, any significant change in the roof slope will increase the vertical projected area of the roof and result in a larger design wind loading on the building. Now, the whole buildings lateral load-resisting system may have to be rechecked, involving the engineer even deeper into the project. To be ready for such complications and be able to make educated design decisions while prepar- ing the construction documents, specifying design professionals are wise to learn about the available types of support framing for metal reroofing. 14.2.5Structural Framework for Slope Changes The proposed reroofing framework has to provide the same level of strength and rigidity as any other metal roof structure. In practice it means that the spacing of the new (“retrofit”) purlins is probably limited to 5 ft or so (Fig. 14.1). A closer, perhaps half as wide, spacing is needed at the “salient cor- ner” areas near the eaves, rake, and ridge (for certain roof slopes), to resist the increased wind load- ing there. Similarly, purlin spacing is reduced in the areas of a potential snow drift accumulation. Whenever the existing roof structure stays in place, the new framework resists not only the wind and snow loads on the new roof but also the wind loads on the new gabled endwalls. Thus two kinds of bracing are required for stability: vertical, between the framing uprights, and horizontal, in the plane of the new roof, to act as a diaphragm. The diaphragm action can be provided by rod or angle bracing, by steel deck, or by certain types of through-fastened roofing (but usually not standing-seam roofing, as discussed in Chap. 5). Another important issue to consider is lateral bracing of the new purlins. When closely spaced and cross-braced, the framework verticals provide the necessary bracing, but this may not be the case when the supports are far apart. As pointed out in Chap. 5, the manufacturers design practices for lateral support of purlins vary widely and range from conservative to ignorant. To ensure a unifor- mity of design assumptions among the bidders, the owners requirements relating to the acceptable diaphragm construction and purlin bracing methods should be spelled out in the contract documents. To avoid a blow-off of the new metal roofing which in a sense acts as a giant sail erected on top of the existing building, proper anchorage into the existing structure is critical. The anchorage details should be designed by the metal system manufacturer and carefully checked by the engineer of record. The details should be custom designed for the actual existing roof structure, instead of showing the new fasteners terminating in a mass of concrete, an easy but useless “solution” submitted all too often. If the existing roof structural deckingnot just the roofingneeds to be removed for an easier attachment to the existing framing, or because of excessive corrosion, one should remember to replace it with a new decking or horizontal bracing to provide lateral support for the existing roof beams and to restore the existing roof diaphragm. The details of roofing for the new work, such as clip design, endlap fastening, placement of stepped expansion joints, and the like, remain the same as f