METAL BUILDING SYSTEMS:REROOFING AND RENOVATIONS OF METAL BUILDING SYSTEMS.pdf
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1、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 responsibi
2、lity 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
3、 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
4、 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 n
5、ew 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 du
6、ties 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, pr
7、otecting 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
8、 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
9、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 c
10、auses 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 period
11、s 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 pene
12、trations. 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 p
13、romptly, 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 mec
14、hanically 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 boa
15、rds 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 popu
16、lar 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
17、 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
18、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 s
19、tructure, 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 b
20、elieve 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 roofi
21、ng 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 f
22、rom 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 t
23、o 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 deck
24、ing, 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 satura
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