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This application claims priority from U.S. Provisional Application Ser. No. 60/563,417 filed Apr. 19, 2004, which is hereby incorporated herein by reference. 

   BACKGROUND 
   The present invention relates to a device and a method to prevent oil canning of flat metal surfaces, such as metal roofs and siding. Oil canning is a well-known phenomenon in metal roofing and siding. It is a rippling or buckling of the sheet metal material and is found to some extent in most installations. It detracts from the aesthetic appearance of the roofing and siding. 
   SUMMARY 
   The present invention provides a device and a method for substantially reducing or eliminating the oil canning phenomenon in metal roofing and siding. 
   In one embodiment, a standard roofing or siding panel can be used. In order to prevent the oil canning, a spacer is placed between the metal panel and the substrate. This spacer applies a “bow” or camber to the flat surface, pre-stressing or pre-stretching the panel. The spacer is preferably made from a material which is softer than both the metal panel and the substrate over which the panel is installed. This allows the spacer material to “give” when a person walks on the flat panel, so there is no permanent kinking of the panel. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a broken away plan view of a metal roof made in accordance with the present invention; 
       FIG. 2  is a broken away section view along line  2 - 2  of  FIG. 1 ; 
       FIG. 3  is a broken away section view along the left-most end of the roof, schematically shown as being along line  3 - 3  of  FIG. 1 ; 
       FIG. 4  is a broken away section view along line  4 - 4  of  FIG. 1 ; 
       FIG. 5  is a view of a single metal panel, as seen from the same vantage point as  FIG. 3 ; 
       FIG. 5A  is a broken away plan view of the right edge portion of the panel of  FIG. 5 ; and 
       FIG. 6  is an enlarged, detailed view of the dotted area of  FIG. 5 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is a schematic view of a portion of a roof  10  made in accordance with the present invention. In this embodiment, a plurality of roofing panels  12  are assembled together and installed on a roof  10 . Each of the panels  12  defines two long edges  16 ,  18 , (See also  FIG. 5 ) and two short edges  20 ,  22 . Referring to  FIG. 5 , adjacent the first long edge  16 , a right edge rib  24  extends from the top short edge  20  to the bottom short edge  22 , and this rib  24  defines a notch  26 , which also extends the length of the panel  12 . A left edge rib  28  is adjacent the second long edge  18  and likewise extends from the top short edge  20  to the bottom short edge  22 . This rib  28  defines a projection  30 , which also extends the length of the panel  12  and which is designed to be received in the notch  26  at the right edge  16  of an identical adjoining panel  12  when the rib  28  of this panel  12  fits over the rib  24  of the adjoining panel  12  to lock the two adjoining panels  12  together in a water tight assembly, as is well known in the industry. 
   Referring to  FIG. 2 , the roof  10  includes a substrate  14  (such as plywood) onto which the metal panels  12  are secured via fasteners  32 , such as nails or screws, at their top edges  20 . These fasteners  32  may also be used to secure “Z” brackets  34  which hold the cap  36  in a water tight arrangement which protects the apex of the roof  10 , as is also well known in the industry. 
     FIGS. 1-5  also show a spacer or support  38 , which extends from the top edge  20  to the bottom edge  22  of its respective panel  12 . As seen in  FIG. 1 , each of the panels  12  has such a spacer  38 . Each spacer  38  is located approximately midway between the two long edges  16 ,  18  (See  FIG. 5 ) of its panel  12 . The spacer  38  may be continuous or may be made up of a plurality of discontinuous, aligned segments.  FIG. 4  shows that the spacer  38  may terminate just short of the lower edge  22  of the panel  12  to allow the bottom edge  22  of the panel  12  to be bent around a drip edge  40 , which is secured to the substrate  14  and lies between the panel  12  and the substrate  14 . A sealant  42 , such as caulking, may be added at the bend to help form a water tight seal. It is understood that the spacer  38  could extend all the way to the end of the panel  12 , but terminating this spacer  38  short of the bottom edge as shown in  FIG. 4  may facilitate making a tight bend around the drip edge  40 . 
     FIG. 3  illustrates a typical installation of a roof  10  made in accordance with the present invention, particularly as it relates to finishing a longitudinal edge, such as the leftmost edge depicted in  FIG. 3 . A “Z” bracket  34  is attached to the left edge of the panel  12  (note that the left edge rib  28  has been cut off of this leftmost edge panel  12  to allow for the installation of the “Z” bracket  34 ). A seal  44  between the panel  12  and the “Z” bracket  34  prevents water migration past this joint. A drip edge  46  finishes off the edge, as is well known in the industry. Fasteners  48  are arranged at regular intervals along the right edges  16  of the panels  12 , attaching the longitudinal edges  16  of the panels  12  to the substrate  14 . The fasteners  48  go through the panels  12  via longitudinally aligned slotted openings  49  (see  FIG. 5A ) to allow for longitudinal displacement of the panels  12  due to normal thermal expansion and contraction. 
   In  FIGS. 2 and 4 , the spacer  38  is a rope-like material which is attached to the substrate  14  via fasteners  50 , which extend at intervals through the spacer  38  and into the substrate  14 . Once the spacers  38  are installed, the panels  12  are then installed over the substrate  14  and over the spacers  38 , and the panels are secured to the substrate  14  via the fasteners  48 , stretching or pre-stressing the panels  12  over the spacers  38  in order to induce a camber or slight outwardly convex bow on the substantially flat outer surface  51  of each panel  12  and a slightly concave bow on the substantially flat bottom surface  52  of each panel  12 . 
   In  FIGS. 5 and 6 , the spacer  38  is shown to be attached to the bottom surface  52  of the panel  12 ′ such that the spacer  38  need not be pre-installed on the substrate  14  prior to the installation of the panels  12 ′. In this case, the spacer  38  is a polypropylene rope that has been adhered to the underside of the panel  12 ′ with an adhesive  54  prior to installing the panel  12 ′. In this particular embodiment, the panels  12 ′ are twelve inches wide, and the polypropylene rope spacer  38  has a diameter of ⅜″-⅝″. In testing, it was found that a diameter of ½″ was very sufficient to prevent oil canning. Since the spacer  38  is in the center of the panel  12 , it creates a slope of approximately one-half inch per each six inches of width of the panel  12 , with the center of the panel  12  being approximately one-half inch higher than the edges, which lie at the level of the substrate  14 . 
   It is advantageous for the spacer  38  to be made of a material that is flexible and compressible, preferably more compressible than the material of the panels  12 ,  12 ′ and also more compressible than the substrate  14 . Then, during installation of the roof  10  or during follow-up maintenance, should somebody walk on the roof panels  12 ,  12 ′, the spacer  38  will compress and “give” a little, preventing unwanted kinking of the panels  12 ,  12 ′, and then it will return to its original shape, again providing a camber or prestress to the panel. 
   The spacer  38  may be adhered to the rear face  52  of the panel  12 ,  12 ′ by the installer prior to installation, or it may be a part of the original product made by the manufacturer. The spacer  38 , being a separate member from the rest of the panel  12 ,  12 ′, does not require a deviation from the smooth outer surface  51  of the panel  12 ,  12 ′ as would be required by an extruded rib. While a polypropylene rope was used in this example, other types of spacers  38  may be used. For example, the spacer  38  may be a foam bead that is adhered to the rear face  52  of the panel  12 ,  12 ′. While the spacer  38  is continuous, the spacer could be made up of a plurality of discrete, substantially longitudinally aligned members that would provide the same effect. While the panels  12 ,  12 ′ shown here have been described as roofing panels, the same procedure and construction would be used for metal wall panels, with the same result. 
   It will be obvious to those skilled in the art that modifications may be made to the embodiments described above without departing from the scope of the present invention.

Summary:
A device and method for securing a substantially flat sheet of metal (such as metal roofing or siding) to a substrate in a manner which avoids or reduces oil canning. A longitudinal spacer is installed between the metal surface and the substrate so as to induce a bowing or camber of the sheet of metal.