You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
     This invention relates to a sliding roof seam construction to accommodate differential expansion of adjacent metal panels. 
     A typical metal roof construction includes an array of metal purlins laid across structural beams. The purlins and beams are referred to below as the “substructure” of the roof. Most purlins have a “C” or “Z” cross-section with upper and lower flanges at either edge of a central web. “Z”-shaped members are popular because they have the advantage of being nestable. The roof substructure supports an array of interlocked metal roof panels, often with a layer of insulation beneath the panels. The roof panels are laid perpendicularly across the purlins so that, on a ridge roof, the panels extend from the ridge to the eave. Normally, the panels are fixed to the eave, and are supported in a way that lets them expand and contract lengthwise, yet prevents them from being lifted off the purlins by high winds. Widthwise expansion is not normally a problem, because the panels are typically corrugated; however, lengthwise expansion cycles can be considerable, inasmuch as ambient temperatures vary annually by as much as 100° F. The upper extreme is augmented by solar heating, which causes the to roof expand and contract considerably, even at constant ambient temperature, as the sun rises, sets and is hidden by clouds. For these reasons, roof panels must be connected to the substructure in a way that permits the panels to move considerably in the lengthwise direction. Usually the panels are fixed to the substructure at the eaves, and are permitted to expand toward the roof ridge, where their ends are covered loosely by a cap. 
     In a standard warehouse-style building with a rectangular footprint, the panels are all the same length, and as the panels are laid, their overlapped edges are tightly folded over to form a weather-proof structure. Various specialized roof seaming machines exist for this purpose. Panels of equal length expand and contract in unison as thermal variations occur. 
     When a roof has an inside corner, however, panels of one length are laid adjacent panels of a substantially different length, extending from eaves at different distances from the ridge. Such a roof is illustrated in  FIG. 4 . When the panels are heated or cooled, differential thermal expansion causes relative lengthwise movement in the panels on either side of the “shear line” SL. The edges of these panels cannot be seamed together without creating a danger of buckling or other structural damage resulting from differential thermal expansion. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to allow a construction in which adjacent roof panels of different lengths, or having offset anchoring eaves or different coefficients of thermal expansion, can be securely held on the roof without seaming the overlapping edges those panels. 
     This object is achieved by a sliding roof seam construction as described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings, 
         FIG. 1  is a perspective view from above of a roof seam embodying the invention; 
         FIG. 2  is a perspective view thereof from below; 
         FIG. 3  is an end view thereof, and 
         FIG. 4  shows a roof having an inside corner. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A portion of a roof seam embodying the invention is illustrated in  FIG. 1 . The roof panels  10  are supported by parallel purlins (or “roof substructure”)  12 , only one of which is shown. On a sloping roof, the purlins normally run parallel to the ridge of the roof. The purlins, in turn, are supported by structural members running from the cave to the ridge. 
     In the drawings, portions of two roof panels  10  and  10 ′ of different lengths are shown secured to respective eaves E 1 , E 2  ( FIG. 4 ) which are substantially offset from one another. Details of the eave construction are not important to this explanation. What is important is the way in which the panels  10  and  10 ′ are interconnected and supported on the purlins. 
     As  FIG. 1  shows, each panel has a broad central portion  20  bounded by raised side portions  22 ,  24  respectively having complementary edge structures  26 ,  28  respectively which are designed to be folded over to form a seam. Most of the panel joints on the roof are folded into seams, but between adjacent panels of different length (along the “shear” line) SL, the panel edges are left unfolded. To secure the panels to the purlins, and yet to allow the panels to move lengthwise with respect to one another on the shear line, expansion bracket assemblies are installed along the shear line. 
     The expansion bracket assembly  30 , best seen in  FIG. 2 , includes a center (or “base”) element  32  which is laid perpendicularly across and bolted to a purlin  12  (at attachment portion  32   a ), a first wing element  34  on one side of the center element  32  (i.e., at one side of distal portion  32   b ), and a second wing element  36  on the opposite side of the center element  32  (i.e., at the opposite side of the distal portion  32   b ). The first and second wing elements are substantially mirror images of one another. Each has a vertical segment  40  ( FIG. 3 ) which overlaps the center element  32 , a horizontal segment  42  extending outward from the vertical web, and an oblique segment  44  bent at an angle so as to conform to the shape of the corrugation on the roof panel. 
     Two slots  50 ,  52  ( FIG. 2 ) are formed in the vertical segment  40  of the center element  32 . The slots extend in a direction parallel to the panel edges and are sized so that the shaft of a rivet  54 , but not the head  58  thereof, can pass through either slot. The rivet also extends through a hole in the center element. The rivets are sized to hold the wing elements against the center element, but not tightly, inasmuch as the elements must be able to slide relative to one another in use. 
     To stabilize the bracket assembly for ease of handling, a nylon centering pin  60  is inserted at the factory through aligned holes which are formed, respectively, as the center of each of the elements. 
     During installation, an expansion bracket assembly is placed across each of the purlins with the bottom flange of the center element overlapping the top flange of the purlin. A fastener such as a self-drilling and self-tapping screw is driven through the overlapped flanges to secure the assembly to the purlin. The nylon centering pin, which prevents the wing brackets from shifting about during installation need not be removed by the installer. It shears off after installation, when thermal effects shift the roof panels with respect to one another. 
     Once the expansion bracket assemblies  30  have been installed along the shear line, the roof panels are placed on the roof, with their complementary edges  26 ,  28  overlapped in the usual way to form a seam. All roof joints except those along the shear line are folded together to form seams. Those on the shear line are left undeformed so that relative sliding movement can occur. 
     To connect the roof panels to the bracket, suitable fasteners  62  are driven through the raised side portions of the roof panels and the oblique webs of the wing brackets, where they overlap. The fasteners may be self-drilling, self-tapping screws. The screws are tightened sufficiently to prevent any movement between the panels and the underlying brackets. 
     If left unsealed, the joint along the shear line might provide a site for entry of cold air, rain water, dust or insects. To prevent leakage and to keep foreign material out of the joint, a cover  70  is installed over the undeformed joint. The cover includes a flexible seal  72 , for example a silicone membrane, which is secured to the respective side portions of the roof panels by a continuous adhesive sealant strip  74  ( FIG. 3 ) applied during installation. The membrane is flexible enough to accommodate the substantial anticipated lengthwise shifting of the panels on either side of the shear line. Preferably, the cover also includes a metal canopy  76  lying over the membrane and having the shape of an inverted “V” whose bend angle conforms to the roof panel corrugations beneath. The metal canopy, which is secured by screws  78  which are inserted through one side of the canopy and the underlying wing bracket, not only protects the membrane from damage but also presents a good finished appearance. However, the metal canopy is considered an optional feature and may in some instances not be necessary. 
     Details of the elements of the invention may vary. For example, the choice of materials, metal gauges, and the exact location and nature of the fasteners and pins which interconnect the various parts are a matter of design choice. Also, the invention might be used to compensate for differential expansion in a construction where panels made of different materials were laid side-by-side. Since the invention is subject to modifications and variations, it is intended that the foregoing description and the accompanying drawings shall be interpreted as only illustrative of the invention defined by the following claims.

Summary:
A sliding bracket assembly permits adjacent metal roof panels to move relative to one another to accommodate differential thermal expansion. The assembly includes a center element which is bolted across a purlin, and a pair of wing elements, one on either side of the center element. Roof panels are attached to the wing elements on either side. The wing elements can side lengthwise on the center element; thus the wing elements and their attached roof panels can move lengthwise relative to one another as the roof panels differentially expand and contract.