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CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is entitled to the benefit of Provisional Patent Application Ser. No. 60/591,164, filed 2004 Jul. 26. 
    
    
     SEQUENCE LISTING 
     Non-Applicable. 
     BACKGROUND 
     1. Field of Invention 
     This invention relates to an aerodynamic means that mitigate wind generated vortices and uplift loads on the roof perimeter area of a building, in a simple, effective, and economical way, applicable for both new constructions and retrofits of existing buildings. 
     2. Discussion of Prior Art 
     The previous and present roof construction practices normally lead to a roof perimeter configuration that tends to generate corner-edge vortex and subjects the roof perimeter area to severe uplift and high risk of wind damage. Structural methods have been used to mitigate the risk of wind damage. For example, builders may use stronger fasteners or smaller spacing between fasteners for roof cover and deck in the roof edge and corner area, and use “hurricane straps” in lieu of toenails to tie down the roof framing to the wall structure. Some aerodynamic methods have been recommended. Banks et. al. described in U.S. Pat. No. 6,601,348 (2003) various types of wind spoilers raised above the roof plane that function to mitigate edge vortex formation. However, the apparatus is rather complicated in shape and structure, and is susceptible to wind damage itself because the raised structure subjects itself to accelerated airflow across the roof edge. In U.S. Pat. No. 4,005,557 (1977), Kramer et. al. described conceptual designs for a roof wind spoiler system used strictly near roof corners. The limited breadth of the apparatus impedes its effectiveness and causes higher wind loads along the neighboring segments of roof perimeter, which the apparatus does not extend to. Its design is also only suitable for flat roofs. Ponder disclosed in U.S. Pat. No. 5,918,423 (1999) a wind spoiler ridge cap that is designed for protecting roof ridges, while this present invention deals primarily with roof perimeters. The structure disclosed herein is continuous along a roof edge or at least substantially extends from the roof corners towards the middle part of a roof edge. The designs are suitable for both sloped and flat roofs. The examples given hereafter are particularly suitable for roofs that have roof cover extending outwardly beyond the roof deck boundary and subjecting itself to accelerated upward flow deflected by the wall directly below. 
     In U.S. Pat. No. 6,606,828 of this applicant et al., a series of roof edge configurations are recommended for use to mitigate vortex and high uplift in flat-roof perimeter areas, where the concept is one of coordinated exterior curvature design for a roof edge system. The present invention discloses a distinct roof edge apparatus that utilizes overhung plates that preferably have face perforation and/or outer edge serration. 
     SUMMARY OF THE INVENTION 
     This invention discloses an aerodynamic means that mitigate wind generated vortices and uplift loads on the roof perimeter area of a building, in a simple, effective, and economical way, applicable for both new constructions and retrofits of existing buildings. This is achieved by using an elongated device generally having an angle-like cross-section and being attached along a roof edge. The elongated device, which can be formed from sheet materials, is generally positioned in such a way that the open side of the angle faces outwardly and downwardly, with one side of the angle generally vertical and the other side generally horizontal. The generally vertical side is normally attached to an existing fascia or bargeboard, while the generally horizontal side extends and overhangs outwardly. The overhung portion is preferably made air-permeable and/or has a zigzag outer edge. The permeability provides a pressure equalizing effect while the outer edge serration provides a flow disorganizing effect, each of which prevents or interrupts the vortex from formation along a roof perimeter. Such a roof edge device is generally referred to as roof edge windscreen in this application. The specific configurations recommended herein pertinent to this invention are primarily applicable for edges of gable, hip, gambrel, mono-slope and flat roofs where no perimeter draining device, such as gutter, or edge flashing is installed. It is prudent that modifications be made according to the spirit and principles of the present invention when other types of roofs or roof edge constructions are encountered. 
     OBJECTS AND ADVANTAGES 
     Accordingly, several objects and advantages of the present invention are: 
     to provide roof edge devices which shield roof edge details from upward airflow, wind-driven rain and wind pressure; 
     to provide roof edge devices which suppress edge vortex formation and reduce wind loads on roofing materials, roof decks and framing in the roof perimeter areas; 
     to provide roof edge devices which reduce wind uplift loads generally on a building structure that are transferred from the roof; 
     to provide roof edge devices which reduce vortex scouring of roofing materials, such as asphalt shingles, roofing tiles, paver etc, and prevent them from becoming wind-borne missiles injuring people and damaging adjacent building envelopes during severe wind events; 
     to provide roof edge devices which stabilize wind flow over the roof and minimize cyclic loads on roof components resulting from recurring winds, reducing the chances of damage due to material fatigue; 
     to provide roof edge devices which prevent rainwater from being driven sideward and upward by wind turbulence and pressed through the gaps between roofing material and roof deck, and into the inner space of the roof assembly, during wind/rain events; 
     to provide roof edge devices which possess the desired aerodynamic performance while maintaining an aesthetic and waterproofing functionality under both extreme and recurring weather conditions. 
     Further objects or advantages are to provide roof edge devices which protect a roof edge from wind and rain damage, and which are still among the simplest, most effective and reliable, and inexpensive to manufacture and convenient to install. These and still further objects and advantages will become apparent from a consideration of the ensuing description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  schematically illustrates the cross-sectional view of one of the preferred basic configurations formed with sheet material, as being installed on an overhung gable end of a roof as an example. 
         FIG. 1B  shows a similar configuration as being installed on a non-overhung gable roof edge as an example. 
         FIGS. 1C and 1D  are isometric views showing examples of face perforation and edge serration. 
         FIGS. 2 and 3  schematically illustrate alternative cross-sectional shapes for the screen portion of the roof edge windscreen. 
         FIG. 4  exemplifies a configuration for roof edges with wrapped-down roof covering. 
         FIG. 5  illustrates an example of configurations for eave edges where significant rainwater run-off is expected. 
         FIG. 6  illustrates an alternative example of configurations for eave edges where significant rainwater run-off is expected. 
         FIG. 7  illustrates an optional serrated edge configuration where the sawtooth-like elements or projections bend alternatively upwardly and downwardly. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A roof edge windscreen is generally an elongated assembly that is disposed longitudinally in parallel with, and attached to, a roof edge.  FIG. 1A  shows a cross-section view for one of the preferred configurations of the present invention, a roof edge windscreen  110  being installed on a gable-end overhang  10  of a roof structure. A typical roof overhang is a portion of a roof structure that extends substantially outwards beyond the outer surface  21  of a supporting wall  20  of a building. The gable-end overhang  10 , along with such associated components as roof covering  11 , deck  12 , rafter  13 , fascia board  14 , soffit board  15 , lateral framing member  16 , and aesthetic trim members  31  and  32 , are prior arts. They are included here merely for illustration of their relationships with the roof edge windscreen  110  that is the subject matter of this invention. The apparatus can also be used for non-overhung roof perimeters, for example, on a non-overhung gable-end  17  as depicted in  FIG. 1B . Moreover, although many of the embodiments in this application are exemplified with gable edges, the present invention is applicable on other types of roof edges. Specific examples include, but not limited to, gable, hip, gambrel, mono-slope, and flat roof edges. For roof edges where certain rainwater runoff is expected, such as the eave edges of gable and hip roofs, this invention is also applicable where roof edge windscreens will replace rainwater-draining devices such as gutters as described later in this application. 
     The roof edge windscreen  110 , exemplified here as made of sheet material, consists of a screen portion  111 , an intermediate channel portion formed by segments  113   a  and  113   b , and lower mounting portions  115   a  and  115   b , along with an optional drip edge  117 , adjoining consecutively. As exemplified in  FIGS. 1C and 1D , the screen portion  111  preferably has face perforation  112  ( FIG. 1C ) or outer edge serration  114 , or has both ( FIG. 1D ). 
     Herein the perforation  112  is made with a plurality of through-holes on the sheet material. The specific layout, number, shapes and sizes of the distributed through-holes are not of primary significance, as long as the overall porosity resulting from the face perforation is in a preferred range approximately between 25% and 75% to provide desired air-permeability. This helps equalizing pressures on the opposite sides of the screen and suppresses the forcing mechanism for vortex formation along the edge. In  FIG. 1D , in addition to perforation, edge serration is made with a zigzag or wavy outer edge of the screen portion  111 , which disorganizes the flow shear layer over the edge and prevents vorticity embedded in the shear layer from forming a concentrated vortex. While larger sizes are preferred for the projections and notches to provide deeper serration or indentation, their specific layout, number and shapes are not of critical significance. Square, semi-circular and semi-elliptic shapes etc., for example, in addition to the triangular shape shown herein, are all permissible without compromising the functionality described herein. It is also allowable that the perforations, projections and notches have varying shapes and sizes in the same assembly. The choices may be made in combination with aesthetic considerations. 
     Thus the function of face perforation and edge serration is to disrupt the formation of the roof edge vortex that would otherwise cause severe uplift loads and scouring on the roof surface.As illustrated in FIGS. lA and  1 B, the screen portion  111  is disposed with its inner side in close proximity to the outer edge  19  of the roof covering  11  and is extended generally outwardly. Various modifications to the configuration of the screen portion  111  shown in FIGS. lA and  1 B are permissible. For example, as illustrated in  FIG. 2 , the screen portion  211 , or its outer segment, may curve outwardly and upwardly for roof edges where no significant rainwater runoff is expected, to the extent that such configurations are not expected to cause debris clogging and accumulation along the roof edge. As illustrated in  FIG. 3 , the screen portion  311 , or its outer segment, may also curve outwardly and downwardly. Furthermore, as an option for serrated edge configuration, the sawtooth-like elements or projections can bend alternatively upwardly and downwardly, see  FIG. 7 . These alternatives may be considered in conjunction with the aesthetic aspect of a building. 
     The intermediate channel portion is formed by a generally vertical segment  113   a  and a generally inward and upward extending segment  113   b  that adjoin the screen portion  111  and the mounting portion  115   a  respectively, as illustrated in each of the preceding figures. The channel portion formed by segments  113   a  and  113   b  serves as both a draining device and a protection from upward flow and pressure for the underside of the overhung portion  18  of the roof covering  11 . Optional draining holes (not shown) can be used near the lower edge of the channel portion where segments  113   a  and  113   b  meet. 
     The roof edge windscreen  110  may be mounted on and secured to a roof edge with any appropriate means that does not negatively affect the functionality of the screen portion  111  or that of the intermediate channel portion formed by  113   a  and  113   b  described herein. A simple example is already illustrated in the preceding figures, i.e.  FIGS. 1 ,  2  and  3 . The mounting portions  115   a  and  115   b  are collectively conformed to the existing configuration of the roof edge and are attached to the side of the roof edge using fasteners  130 . Adequate aesthetic finishes and watertight sealing on the fasteners may be desired. Optional space washers (not shown) can also be placed between a mounting plate portion  115   a , or  115   b , and the trim member  31 , or fascia board  14 , at the location where a fastener is placed, to maintain a small gap for venting out moisture residing therein. In fact, any suitable mechanisms of similar functions may be used for mounting and securing the roof edge windscreen  110  onto a roof edge. The drip edge  117  is also optional. 
     The roof edge windscreen has at least three functions. The first is to suppress vortex over a roof edge. High uplifts and strong scouring that result from wind-induced edge vortex above the roof, are prime causes for wind damage to roof components. Secondly, it shields the underside of the protruding portion  18  of the roof covering  11 , such as an array of asphalt shingles or wood shakes, from upward flow and pressure that tend to peel the roof covering  11  upwards and away from other parts of the roof edge assembly  10 . The third function is to prevent upward flow-driven rain from being pressured into the roof structure through the unsealed gaps between the roof covering  11  and the roof components beneath it. 
       FIG. 4  provides an example for a modified roof edge windscreen  410  being installed on a roof edge where the roof covering  49  wraps downwards, most often seen with metal roof coverings, such as metal tiles, metal shakes and metal panels, as well as clay tiles in some instances. 
       FIG. 5  illustrates a roof edge windscreen  510  being used on an eave edge of a sloped roof where a draining device such as a gutter system is not being used. An outwardly and downwardly extending screen portion  511  is preferred to allow rainwater to shed off the eave, and drain partly through the distributed perforation and partly off the outer edge of the roof edge windscreen  510 . This is in fact a better draining scheme than allowing roof rainwater cascade down directly from the eave edge, which erodes sods, soils or aggregates around a building perimeter. 
       FIG. 6  shows an alternative, simpler configuration of roof edge windscreen  610  being used on an eave edge of a sloped roof where a draining device such as a gutter system is not being used. Herein the screen portion  611  extends inwardly, closely below the outmost portion of the roof cover  68 . This configuration has similar functions to the one depicted in  FIG. 5 . 
     A roof edge windscreen provides protection against wind and rain damage for a broad variety of roof constructions whenever the apparatus and its geometric relationship with the roof perimeter are configured in accordance with the spirit of this invention, as exemplified herein in the specification and governed in the appended claims. 
     Installation and Operation 
     An embodiment of this invention is a passive flow control device for roof edges. Once installed properly, it stays functioning in such a way that it mitigates vortex formation at a roof edge and reduces uplifts and vortex scouring on the roof perimeter area, whenever the wind blows towards a building bearing atop such roof edge devices, and requires no active operational intervention. 
     CONCLUSION, RAMIFICATIONS, AND SCOPE 
     It is apparent that roof edge windscreens of this invention provide advantageous devices for mitigating roof edge vortex and roof uplift, and are still among the simplest, most effective and reliable, inexpensive to manufacture and convenient to install. 
     Although the description above contains many specifications, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Various changes, modifications, variations can be made therein without departing from the spirit of the invention. Roof edge windscreens can be made of any reasonably durable material with any appropriate means of fabrication as long as a configuration according to the spirit of this invention is accomplished to support the described working mechanism and to provide the associated functionality. Various surface portions of a roof edge windscreen may also bear such surface details as corrugation or steps of adequate sizes, as opposed to perfectly smooth surfaces. Any appropriate conventional or new mounting method can be used to secure a roof edge windscreen to a roof perimeter without departing from the spirit of this invention. Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.

Summary:
An assembly attached to the roof perimeter to mitigate wind-generated vortices and uplift loads on the roof perimeter area of a building, applicable for both new constructions and retrofits of existing buildings. The assembly comprises an overhung screen portion preferably having face perforation and outer edge serration for equalizing pressure and disorganizing shear layer vorticity, and thus disrupting vortex formation. A roof edge windscreen is generally mounted onto an existing fascia or bargeboard. As an option appropriate for new constructions, it can also be mounted directly onto a framing member in place of fascia or bargeboard.