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CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     The present application is a continuation-in-part of application Ser. No. 10/378,103, filed Feb. 27, 2003. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to fasteners employed to fasten a covering material to an underlying substrate. More particularly, the invention relates to a plastic/metal composite stress plate with a fastener for fastening a membrane, such as a roof membrane or roofing insulation, to a roof deck, a wall, concrete, stone, plaster, or wood.  
         [0004]     2. Reported Development  
         [0005]     Fasteners are conventionally employed in the building industry for fastening or clamping a flexible membrane, such as an insulation membrane to a substrate, such as a roof deck. The fasteners typically comprise a large head portion and a shank portion. In use, the shank portion is driven through the membrane into the underlying substrate to anchor the fastener thereinto, while the head portion holds the membrane against the substrate and prevent removal thereof by wind lift. The undersurface of the head portion is typically provided with gripping means so that the membrane is prevented from moving or sliding under the fastener. The gripping means are designed not to penetrate the membrane in order to prevent atmospheric moisture from entering into the substrate through the holes which tend to be made by the gripping means. It is also important that the gripping means are spread/distributed in the undersurface of the head portion of the stress plate in order prevent tearing of the membrane. Conventional fasteners are illustrated by the following references.  
         [0006]     U.S. Pat. No. 4,787,188 discloses a stress plate for securing a roof membrane to a roof deck. The stress plate is circular having a top surface and a bottom surface with a central circular opening for receiving a screw for fastening the stress plate over a roof membrane and to the roof deck. The stress plate is equipped with four gripping prongs of triangular shape which are circumferentially spaced from each other by 90°.  
         [0007]     In use a first membrane is applied to a roof deck surface, then the membrane is secured to the roof deck surface with the stress plate and the screw. A top sheet or membrane is lapped over the first membrane to cover the stress plate and welded to the first membrane. The four gripping prongs in the stress plate grip the first sheet and hold the same on top of the roof deck without tearing.  
         [0008]     U.S. Pat. No. 5,049,018 discloses a fastener for gripping a substrate material. The fastener is of a unitary piece comprising a head portion, a shaft portion, and a hook portion at the end of the shaft portion, wherein the hook portion has an outwardly and upwardly extending resilient end portion. The end portion has an end surface which provides gripping contact with a wall of a hole in a substrate into which the fastener is inserted.  
         [0009]     U.S. Pat. No. 5,163,798 relates to a fastener assembly which is employed to secure plies or membranes of roofing, felt and paper to prevent the materials from being blown off the base roofing material before the base material is sufficiently hardened.  
         [0010]     The assembly comprises a fastener and a retainer plate. The assembly includes a fastener plate which defines a substantially rectangular opening. The fastener includes a head and a pair of legs which are integrally hingably connected to the head. The legs have a contoured distal portion and an angular side configuration so that at least one of the legs is forced apart as the fastener is driven into the base material.  
         [0011]     We have observed that under windy conditions the prior art fasteners need improvement in securely holding a flexible membrane on a substrate without the gripping means penetrating the flexible membrane, and without tearing the flexible membrane.  
         [0012]     Accordingly, an object of the present invention is to provide a new and improved stress plate with a fastener to allow attachment of one or more flexible membranes to an underlying substrate without tearing the flexible membrane or allowing it to slip out from under the stress plate.  
         [0013]     In another aspect, the present invention relates to a method for securing thermoplastic roof membranes to a roof deck using a stress plate and fastener and fusing overlapping portions of two roof membranes to provide a waterproof covering over a roof deck.  
         [0014]     Asphalt roof membranes to prevent moisture from entering into an underlying roof deck are being replaced by thermoplastic sheet materials which offer a superior, longer-lasting roof at a lower cost. In the process of installing thermoplastic sheet materials over a roof deck, the typical steps are as follows. A first sheet is laid adjacent to the lower edge of the roof and running parallel thereto. Fastening means, such as batten bar or a line of stress plates are positioned neat the upper edge of the first sheet. Fasteners are inserted through the batten bar or stress plates and into the roof deck to securely hold the first sheet to the roof deck. Then a second sheet is laid over the roof deck in a marginally overlapping relationship with the first sheet. The second sheet also overlaps the batten bar or the line of stress plates. The overlapping edge area of the second sheet overlaps the area of the first sheet on both sides of the batten bar or line of stress plates. A weld is then applied between the first sheet and the second sheet resulting in the fusion of the two sheet on both sides of the batten bar or the line of stress plates. The weld is applied by the use of a welding machine or tool which softens the thermoplastic sheets and, after cooling, solidifies and forms a continuous sheet. The steps are repeated until the roof deck is completely covered by the thermoplastic sheets.  
         [0015]     It has been observed that stress plates having a high profile create bumps in the weld area. It has also been observed that stress plates made of metals do not adhere to the second or overlapping thermoplastic sheet thereby creating bubbles in the weld. Accordingly, it is another object of the present invention to provide a low profile stress plate which substantially remains in the plane of the thermoplastic sheets and which is coated with a thermoplastic material so that the second or overlapping sheet is also welded to the stress plate.  
         [0016]     U.S. Pat. No. 6,640,511 discloses an anchor plate with a fastener. The plate has an elevated bonding platform and a countersink. The bonding platform is coated with a heat activable adhesive. The other top surfaces are not coated with the adhesive.  
         [0017]     The reference uses an electromagnetic induction heater for attaching a thermoplastic membrane to the anchor plate and the underlying roof structure.  
         [0018]     Attaching thermoplastic membranes to a roof surface by using electromagnetic induction heat is cumbersome.  
         [0019]     These and other aspects will be addressed as the description of the invention proceeds.  
       SUMMARY OF THE INVENTION  
       [0020]     In the device aspect, the present invention comprises two non-integral components: a metal stress plate and a fastener. The stress plate has a top surface and a bottom surface and includes an opening in its center portion to allow a fastener, such as a screw, therethrough for attachment of the stress plate to an underlying substrate, such as a roof deck. The opening may be circular or rectangular. The top surface of the stress plate is coated with a thermoplastic coat, such as polyvinyl chloride, thermoplastic olefins, chlorinated polyethylene, chlorosulfonated polyethylene, nylon and ethylene propylene diene rubber. The stress plate has a low height profile so that on installation a thermoplastic sheet will not produce bumps therein. To achieve this objective, the total height of the stress plate will be of from about 0.026″ to about 0.100″ and preferably about 0.050″ to 0.070″. It is to be noted that the metal and thermoplastic components are integral with each other in the stress plate, and that the circumferential portions form ridges or protuberances designed to provide strength and adhesive properties to the thermoplastic sheets when laid over the stress plate.  
         [0021]     In the method aspect, the present invention comprises the steps of: 
        a) laying a first thermoplastic sheet or membrane on a portion of the roof deck;     b) laying a line of stress plates near the edge of the thermoplastic sheet or membrane parallel to the edge;     c) securing the thermoplastic sheet or membrane on the roof deck by inserting fasteners through the stress plates, the first thermoplastic sheet or membrane and into the roof deck;     d) laying a second thermoplastic sheet or membrane on the roof deck in an overlapping relationship to the line of stress plates and the first thermoplastic sheet or membrane; and     e) applying heat and pressure to the overlapped portion of the thermoplastic sheets or membranes on both sides of the line of stress plates and over the line of stress plates thereby causing a fusion of the thermoplastic sheets or membranes and to the line of the stress plates.        
 
         [0027]     The heat weld may be accomplished by a welding machine known in the art, such as described in U.S. Pat. Nos. 4,259,142, 4,289,552, 4,440,588, 4,533,423, 4,861,412, 4,894,112, 5,110,398, and 5,935,357. However, we prefer to use a welding machine which is disclosed in U.S. Pat. No. 6,536,498, and is incorporated by reference in its entirety,. The welding machine comprises a pressure roller and a heating element for a welding apparatus for producing a weld simultaneously on each side of a line of fasteners and over the fasteners. The pressure roller is integral with an axle designed to be connected to a driving means at one end thereof, at the other end of the axle the pressure roller comprises a distal end, a proximal end and a center portion which defines a groove between the proximal and distal ends. The groove of the pressure roller carries an elastomeric cushion designed to smoothly ride over a line of fastening means.  
         [0028]     The pressure roller is used in combination with a heating element containing a blower which forces heated air through a nozzle. The nozzle having an outlet therein comprises three portions: two large opening portions and a restricted opening portion therebetween. In use, the large opening portions allow delivery of the major portion of the hot air produced by the heating element while the restricted opening still allows delivery of sufficient amounts of the heated air to soften the overlapping portions of the thermoplastic sheet over and under the fastening means.  
         [0029]     The stress plate of the present invention may be of circular, ellipsoidal, square, or rectangular configuration.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]      FIG. 1  is a top, perspective view of the circular plastic/metal composite stress plate with a fastener;  
         [0031]      FIG. 2  is a side elevational view thereof, all other sides being identical thereto;  
         [0032]      FIG. 3  is a top plan view thereof;  
         [0033]      FIG. 4  is a bottom perspective view thereof;  
         [0034]      FIG. 5  is a top perspective view of the ellipsoidal plastic/metal composite stress plate with a fastener;  
         [0035]      FIG. 6  is a side elevational view thereof;  
         [0036]      FIG. 7  is another side elevational view thereof;  
         [0037]      FIG. 8  is a top plan view thereof;  
         [0038]      FIG. 9  is a bottom perspective view thereof;  
         [0039]      FIG. 10  is a top perspective view of the square plastic/metal composite stress plate with a fastener;  
         [0040]      FIG. 11  is a side elevational view thereof, all other sides being identical thereto;  
         [0041]      FIG. 12  is a top plan view thereof;  
         [0042]      FIG. 13  is a bottom perspective view thereof;  
         [0043]      FIG. 14  is a top perspective view of the rectangular plastic/metal composite stress plate with a fastener;  
         [0044]      FIG. 15  is a side elevational view thereof;  
         [0045]      FIG. 16  is another side elevational view thereof;  
         [0046]      FIG. 17  is a top plan view thereof;  
         [0047]      FIG. 18  is a bottom perspective view thereof;  
         [0048]      FIG. 19  is a is a cross-sectional view illustrating the use of the stress plate and the fastener for attaching roof membranes to a roof deck;  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0049]     Reference is now being made to the drawings wherein like numerals represent like parts throughout the figures showing the various embodiments of the present invention.  
       First Embodiment  
     Circular Stress Plate with Fastener  
       [0050]      FIGS. 1-4  relate to the first embodiment of the present invention in which the circular stress plate is generally designated at  10  and the fastener is generally designated at  12 . The components are non-integral and, when put together, constitute the invention. The circular stress plate  10  has a round or rectangular opening  14  in its center portion through which the fastener is inserted when the stress plate is employed for attaching and firmly holding a roof membrane to an underlying roof deck. The stress plate has a circular body with a top surface and a bottom surface and characterized by 
        a) a diameter of from about 1″ to about 4″ or more, and preferably about 2″;     b) an opening  14  in its center portion the diameter of which is about 0.25″;     c) a first flat surface  16  surrounds the opening having a radius of about 0.25″ which is designed to receive the head portion  18  of fastener  12  without passing through said opening  14 ;     d) a second flat surface  20  adjacent to the circumferential edge  28  having a radius of from about 0.12″ to about 0.25″, and preferably about 0.20″;     e) a third flat surface  22  extends between the first flat surface  16  and the second flat surface  20  having a radius of from about 0.25″ to about 1.0″, and preferably about 0.50″;     f) connecting the second flat surface  20  with third flat surface  22 , a first circumferential portion  24  in the form of a ridge or protuberance extends above the second flat surface and towards third flat surface at an angle of from about 25° to about 70°, and preferably at about 45°; and     g) connecting the first flat surface  16  and the third flat surface  22  a second circumferential portion  26  in the form of a ridge or protuberance extends above the first flat surface and towards the third flat surface at an angle of from about 25° to about 70°, and preferably at about 45°.          
         [0058]     The distance between the first flat surface  16  and the third flat surface  22  is of from about 0.026″ to about 0.100″, and preferably about 0.050″ to 0.070″. The distance between the second flat surface  20  and the third flat surface  22  is also of from about 0.026″ to about 0.100″, and preferably about 0.050″ to about 0.070″.  
         [0059]     The composition of the stress plate consists of a metal, such as steel or galvanized metals, having a thickness of about 1 to 3 mm; and a thermoplastic layer  29  coated on and being integral with the top surface of the metal plate having a thickness of about 0.5 mm to 2 mm. The thermoplastic coat completely covers the top surface of the stress plate, namely the first, second, and third flat surfaces and the first and second circumferential portions which connect the first, second and third flat surfaces. The total height of the stress plate is preferably about 0.050″ to 0.070″.  
       Second Embodiment  
     Ellipsoidal Stress Plate with Fastener  
       [0060]      FIGS. 5-9  relate to the second embodiment of the present invention in which the ellipsoidal stress plate is generally designated at  10 ′ and the fastener is generally designated at  12 ′ The two components are non-integral and, when put together, constitute the invention. The ellipsoidal stress plate  10 ′ has a round or rectangular opening  14 ′ in its center portion through which the fastener is inserted when the stress plate is employed for attaching and firmly holding a roof membrane to an underlying roof deck. The stress plate has an ellipsoidal body with a top surface and a bottom surface and is characterized by: 
        a) a larger diameter of from about 1″ to about 4″ or more, and preferably about 2″;     b) a smaller diameter of from about 0.5″ to about 2″, and preferably about 1.5″;     c) an opening  14 ′ in its center portion the diameter of which is about 0.25″;     d) a first flat surface  16 ′ surrounds the opening having a larger radius of about 0.25″ and a smaller radius of about 0.15″ which is designed to receive the head portion  18 ′ of the fastener  12 ′ without passing through said opening  14 ′;     e) a second flat surface  20 ′ adjacent to the circumferential edge  28 ′ having a larger radius of from about 0.12″ to about 0.25″, and preferably about 0.20″, and a smaller radius of about 0.15″ to 20″;     f) a third flat surface  22 ′ extends between the first flat surface  16 ′ and second flat surface  20 ′ having a larger radius of from about 0.25″ to about 0.75″, and preferably about 0.50″ and a smaller radium of about 0.15″;     g) connecting the second flat surface  20 ′ with the third flat surface  22 ′, a first circumferential portion  24 ′ in the form of a ridge or protuberance extends above the second flat surface and towards the third flat surface at an angle of from about 25° to about 70°, and preferably at about 45°; and     h) connecting the first flat surface  16 ′ and the third flat surface  22 ′, a second circumferential portion  26 ′ in the form of a ridge or protuberance extends above the first flat surface and towards the third flat surface at an angle of from about 25° to about 70°, and preferably at about 45°.          
         [0069]     The distance between the first flat surface  16 ′ and the third flat surface  22 ′ is of from about 0.026″ to about 0.100″, and preferably about 0.050″ to 0.070″. The distance between the second flat surface  20 ′ and the third flat surface  22 ′ is also of from about 0.026″ to about 0.100″, and preferably about 0.050″ to about 0.070″.  
         [0070]     The composition of the stress plate consists of a metal, such as steel or galvanized metals, having a thickness of about 1 to 3 mm; and a thermoplastic layer  29 ′ coated on the top surface of the metal plate having a thickness of about 0.5 mm to 2.0 mm. The thermoplastic coat completely covers the top surface of the stress plate, namely the first, second, and third flat surfaces and the first and second circumferential portions which connect the first, second and third flat surfaces. The total height of the stress plate is preferably about 0.050 to 0.070″.  
       Third Embodiment  
     Square Stress Plate with Fastener  
       [0071]      FIGS. 10-13  relate to the third embodiment of the present invention in which the square stress plate is generally designated at  30  and the fastener is generally designated at  32 . The components are non-integral and, when put together, constitute the invention. The square stress plate  30  has a round or rectangular opening  34  in its center portion through which the fastener is inserted when the stress plate is employed for attaching and firmly holding a roof membrane to an underlying roof deck. The stress plate has a square body with a top surface and a bottom surface and is characterized by: 
        a) a larger diameter of from about 1″ to about 4″ or more, and preferably about 2″;     b) an opening  34  in its center portion the diameter of which is about 0.25″;     c) a first flat surface  36  surrounds the opening having a radius of about 0.25″ which is designed to receive the head portion  38  of the fastener  32  without passing through said opening  34 ;     d) a second flat surface  40  adjacent to the parametrical edge  48  having a radius of from about 0.12″ to about 0.25″, and preferably about 0.20″;     e) a third flat surface  42  extends between the first flat surface  36  and second flat surface  40  having a radius of from about 0.25″ to about 0.75″, and preferably about 0.50″;     f) connecting the second flat surface  40  with the third flat surface  42 , a first circumferential portion  44  in the form of a ridge or protuberance extends above the first flat surface and towards the third flat surface at an angle of from about 25° to about 70°, and preferably at about 45°; and     g) connecting the second flat surface  40  and the third flat surface  42 ′, a second parametrical portion  46  in the form of a ridge or protuberance extends above the second flat surface and towards the third flat surface at an angle of from about 25° to about 70°, and preferably at about 45°.          
         [0079]     The distance between the first flat surface  36  and the third flat surface  42  is of from about 0.026″ to about 0.100″, and preferably about 0.050″ to 0.070″. The distance between the second flat surface  40  and the third flat surface  42  is also of from about 0.026″ to about 0.100″, and preferably about 0.050″ to about 0.070″.  
         [0080]     The composition of the stress plate consists of a metal, such as steel or galvanized metals, having a thickness of about 1 to 3 mm; and a thermoplastic layer  49  coated on the top surface of the metal plate having a thickness of about 0.5 mm to 2.0 mm. The thermoplastic coat completely covers the top surface of the stress plate, namely the first, second, and third flat surfaces and the first and second parametrical portions which connect the first, second and third flat surfaces. The total height of the stress plate is preferably about 0.050″ to 0.070″.  
       Fourth Embodiment  
     Rectangular Stress Plate with Fastener  
       [0081]      FIGS. 14-18  relate to the fourth embodiment of the present invention in which the rectangular stress plate is generally designated at  30 ′ and the fastener is generally designated at  32 ′. The components are non-integral and, when put together, constitute the invention. The stress plate  30 ′ has a round or rectangular opening  34 ′ in its center portion through which the fastener is inserted when the stress plate is employed for attaching and firmly holding a roof membrane to an underlying roof deck. The stress plate has a rectangular body with a top surface and a bottom surface and is characterized by: 
        a) a larger diameter of from about 1″ to about 4″ or more, and preferably about 2″;     b) a smaller diameter of from about 0.5″ to about 2″, and preferably about 1.5″;     c) an opening  34 ′ in its center portion the diameter of which is about 0.25″;     d) a first flat surface  36 ′ surrounds the opening having a larger radius of about 0.25″ and a smaller radius of about 0.15″ which is designed to receive the head portion  38 ′ of the fastener  32 ′ without passing through said opening  34 ′;     e) a second flat surface  40 ′ adjacent to the parametrical edge  48 ′ having a larger radius of from about 0.12″ to about 0.25″, and preferably about 0.20″, and a smaller radius of about 0.15″ to 20″;     f) a third flat surface  42 ′ extends between the first flat surface  36 ′ and second flat surface  40 ′ having a larger radius of from about 0.25″ to about 0.75″, and preferably about 0.50″ and a smaller radius of about 0.15″ to 20″    g) connecting the second flat surface  40 ′ with the third flat surface  42 ′, a first parametrical portion  44 ′ in the form of a ridge or protuberance extends above the first flat surface and towards the third flat surface at an angle of from about 25° to about 70°, and preferably at about 45°; and     h) connecting the second flat surface  40 ′ with the third flat surface  42 ′, a second parametrical portion  46 ′ in the form of a ridge or protuberance extends above the second flat surface and towards the third flat surface at an angle of from about 25° to about 70°, and preferably at about 45°.          
         [0090]     The distance between the first flat surface  36 ′ and the third flat surface  42 ′ is of from about 0.026″ to about 0.100″, and preferably about 0.050″ to 0.070″. The distance between the second flat surface  40 ′ and the third flat surface  42 ′ is also of from about 0.026″ to about 0.100″, and preferably about 0.050″ to about 0.070″.  
         [0091]     The composition of the stress plate consists of a metal, such as steel or galvanized metals, having a thickness of about 1 to 3 mm; and a thermoplastic layer  49 ′ coated on the top surface of the metal plate having a thickness of about 0.5 mm to 2.0 mm.  
         [0092]     Softer metals such as copper and aluminum may also be used, however, the thickness of the stress plate should be larger to provide sufficient integrity to the stress plate. The fastener is typically a screw of 2 to 3 inches long having threads thereon.  
         [0093]     The thermoplastic coat completely covers the top surface of the stress plate, namely the first, second and third flat surfaces and the first and second parametrical portions which connect the first, second and third flat surfaces. The total height of the stress plate is preferably about 0.050″ to 0.070″.  
         [0094]     In all of the embodiments of the present invention the ridge or protuberance connecting the flat surface serves the dual purpose of providing strength and rigidity to the stress plate, and firmly grips the second thermoplastic membrane in the marginal overlapping of the first thermoplastic membrane.  
         [0095]     The low profile of the four embodiments of the present invention insures that when a second membrane covers the line of stress plates, no objectionable bumps are created, to wit, the stress plates substantially remain in the plane of the membranes.  
         [0096]      FIG. 19  is a cross-sectional view illustrating the use of the stress plate and the fastener for attaching a roof membrane to a roof deck. Lower thermoplastic membrane  50  is positioned over insulation  52  which is over the roof deck surface  54 . The fastener  56  is then inserted through stress plate  58 , insulation  52 , and into roof deck  54 . Upper thermoplastic membrane  60  is then lopped over the marginal portions of the lower membrane covering the stress plate  58 . Upper membrane  60  is secured to the stress plate and the lower membrane by welded seam  62 .  
         [0000]     Wind Uplift Test  
         [0097]     Comparative wind uplift tests were conducted on a 2″ diameter circular composite stress plate versus a standard 2″ diameter circular metal plate without thermoplastic coating thereon. The wind uplift test measures the resistance of the roofing system to high wind currents. For example, a three second burst of wind at 175 miles per hour can exert a negative pressure of 90 pounds per square foot on the roof system.  
         [0098]     The composite circular stress plate consisted of a stainless steel plate coated with polyvinyl chloride; while the standard metal stress plate had no coating thereon.  
         [0000]     A) Composite Stress Plate  
         [0099]     The roofing system consisted of: a roof deck, an insulating layer placed on the roof deck, and a thermoplastic roof membrane placed on the top of the insulating layer. A line of composite stress plates was placed on the marginal area of the thermoplastic layer spaced 6″ apart from each other. The composite stress plates were then attached to the roof system by inserting the fasteners through the stress plates, the roofing membrane, and insulating layer and into the roof deck. A second roofing membrane was then placed on the first roofing membrane in a marginally overlapping position to the first roofing membrane and the line of stress plates. The overlapping portions of the first and second membranes were about 3″ wide. The welding was accomplished by subjecting the overlapped portions to heat, softening them to a weldable consistency and pressing them together by an apparatus having a heat and pressure means. After the overlapped portions of the membranes cooled, a solid seal was formed and the welded are was subjected to wind uplift test. The wind uplift test at 60 seconds showed 180 pounds of pressure per square foot.  
         [0100]     The testing was repeated using the same materials, conditions and processes except that the composite stress plates were spaced every 12″ apart from each other. The wind uplift test at 60 seconds showed 105 pounds of pressure per square foot.  
         [0000]     B) Standard Metal Stress Plate  
         [0101]     Wind uplift tests were conducted using the same materials, conditions and processes described in (A) above, except instead of the composite stress plate of the present invention a standard stainless steel stress plate was used. The stainless steel stress plates spaced every 6″ from each other showed a wind uplift at 60 seconds 150 pounds of plates pressure per square foot, and when the stainless steel stress plates were spaced 12″ from each other, the wind uplift test at 60 seconds was found to be 75 pounds of pressure per square foot.  
                                                                                     PARTS LIST                   First and Second Embodiments                Circular and ellipsoidal stress plates, generally designated   10, 10′           Fastener, generally designated   12, 12′           Opening in center portion of stress plate   14, 14′           First flat surface   16, 16′           Head portion of fastener   18, 18′           Second flat surface   20, 20′           Third flat surface   22, 22′,           First circumferential portion   24, 24′           Second circumferential portion   26, 26′           Circumferential edge of stress plate   28, 28′           Plastic coating on stress plate   29, 29′            Third and Fourth Embodiments                Square and rectangular stress plate, generally designated   30, 30′           Fastener, generally designated   32, 32′           Opening in center portion of stress plate   34, 34′           First flat surface   36, 36′           Head portion of fastener   38, 38′           Second flat surface   40, 40′           Third flat surface   42, 42′           First parametrical portion   44, 44′           Second parametrical portion   46, 46′           Parametrical edge of stress plate   48, 48′           Plastic coating on stress plate   49, 49′            Using the Stress Plate                Lower thermoplastic membrane   50           Insulation   52           Roof deck   54           Fastener (screw)   56           Stress plate   58           Upper thermoplastic membrane   60           Welded seam   62                      
 
         [0102]     Having described the invention with reference to its preferred embodiments, it is to be understood that modifications within the scope of the invention will be apparent to those skilled in the art.

Summary:
A two-piece fastener assembly for securing thermoplastic roof membranes to an underlying roof deck having: a stress plate and a fastener. The stress plate consists of a pre-fabricated metal layer and a thermoplastic layer which are integral with each other. The stress plate has top having three flat surfaces and two circumferential portions, forming ridges or protuberances connecting the flat surfaces. In a method aspect the steps of securing thermoplastic roof membranes to an underlying roof deck, includes: placing a first thermoplastic roof membrane on the roof deck; placing a line of stress plates on the marginal portion of the first thermoplastic roof membrane and securing the stress plates along with the first thermoplastic roof membrane to the roof deck by use of the fastener; placing a second thermoplastic roof membrane to overlap the line of stress plates and the marginal portion of the first thermoplastic roof membrane; and applying heat and pressure to the line of stress plates and the overlapped portion of the first and second thermoplastic roof membranes to fuse them together and provide a waterproof covering over the roof deck.