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: 
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. provisional application Ser. No. 60/619,482, filed Oct. 15, 2004, which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to construction, and in particular to a system for constructing an exterior surface, such as a roof or siding, with individually-mounted facing units.  
         [0004]     2. Description of the Related Art  
         [0005]     A wide variety of construction materials are available for accommodating the requirements of particular structures and building sites. The considerations involved in selecting, designing and specifying such materials include aesthetics, material cost, installation cost, weather resistance, maintenance, thermal resistance and availability. Surfaces of structures are often covered with decorative and protective facing units, which typically provide weather protection on exterior walls and roofs. For example, facing units used for roofing materials include composition shingles, wood shingles, wood shakes, cement tiles, clay tiles, metal panels and slate. A well-known technique for achieving positive, gravity drainage on sloping roof and vertical wall surfaces is to overlap the facing units. For example, each course of facing units can overlap the course directly below it. In this manner the upper facing unit edges are covered. Facing unit installation patterns are commonly laid out to achieve sufficient overlaps to provide adequate moisture protection. Moreover, the weight of the facing units can be used to advantage for holding down the courses that they overlap, whereby uplifting wind forces are resisted.  
         [0006]     In addition to roof constructions, the same or similar facing units are commonly used for vertical wall surfaces. Exterior walls can be finished with materials having similar characteristics to roofing materials. Such dual-purpose construction materials can be used for achieving desired aesthetic effects, such as matching roof and exterior wall surface appearances.  
         [0007]     A facing unit construction should facilitate installation, minimize the dead load of the resulting faced structure, be cost-effective, provide weather protection and utilize the facing unit materials efficiently. Heretofore there has not been available a facing unit construction with the advantages and features of the present invention.  
       SUMMARY OF THE INVENTION  
       [0008]     In the practice of the present invention, a facing unit construction is provided, which includes an underlayment subsystem, spacers mounted on top of the underlayment subsystem and clip assemblies for securing the facing units in an overlapping configuration. The spacers include longitudinally-extending ribs for aligning the adjacent facing units. The clip assemblies comprise base clips for engaging the facing unit upper edges and wire hooks for retaining the facing units. The construction is adapted for facing roofs and walls with various facing units, which can comprise different materials, shapes and sizes. The construction can be preconfigured for desired aesthetic results, including random and other layouts with different sizes of facing units. Different configurations for the spacers and clip assemblies can be utilized as needed to accommodate particular constructions with various facing unit materials.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a perspective view of a facing unit construction embodying the present invention.  
         [0010]      FIG. 2  is a cross-sectional view thereof, showing a roof construction application.  
         [0011]      FIG. 3  is an enlarged, perspective view of a center channel spacer.  
         [0012]      FIG. 4  is an enlarged, perspective view of a clip assembly.  
         [0013]      FIG. 5  is a perspective view of a facing unit construction comprising an alternative embodiment of the present invention  
         [0014]      FIG. 6  is a perspective view thereof, showing a roof construction application.  
         [0015]      FIG. 7  is a cross-sectional view thereof, showing a roof construction application.  
         [0016]      FIG. 8  is an enlarged, perspective view of a hook thereof.  
         [0017]      FIG. 9  is an enlarged, perspective view of a spacer thereof.  
         [0018]      FIG. 10  is a perspective view of a facing unit roof construction comprising an alternative embodiment of the present invention.  
         [0019]      FIG. 11  is a perspective view of a facing unit roof construction comprising and alternative embodiment of the present invention.  
         [0020]      FIG. 12  is a perspective view of a facing unit roof construction comprising an alternative embodiment of the present invention.  
         [0021]      FIG. 13  is an enlarged, perspective view of an alternative embodiment clip assembly with a snow guard.  
         [0022]      FIG. 14  is an enlarged, perspective view of an alternative embodiment clip assembly with a snow guard extending from a facing unit hook thereof.  
         [0023]      FIG. 15  is an enlarged, perspective view of an alternative clip assembly with double hooks.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0000]     I. Introduction and Environment  
         [0024]     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure.  
         [0025]     Certain terminology will be used in the following description for convenience in reference only and will not be limiting. Said terminology will include the words specifically mentioned, derivatives thereof and words of similar meaning.  
         [0026]     Referring to the drawings in more detail, the reference numeral  2  generally designates a facing unit construction embodying the present invention. Without limitation on the generality of useful applications of the invention, the construction  2  shown in  FIG. 1  would be useful for constructing a roof or a wall. Suitable underlayment strips  4  can be provided as appropriate for the particular construction, taking into account environmental and building code considerations. For example, building paper comprising felt saturated with asphalt or a similar waterproofing material is commonly utilized as a secondary moisture barrier, with the external facing units comprising the primary moisture barrier. Various materials can be used for underlayment, including elastomers, PVC, HDPE, rubber, etc. The underlayment strips  4  can optionally be printed with suitable alignment marking  6 , which can assist in properly aligning the adjacent underlayment strips  4 . Indicia  8  can be printed thereon to indicate, for example, the sizes of facing units to be installed at particular locations. For example, slate facing units are commonly available with approximate widths of 8, 10 and 12 inches. The entire roof or wall can be preconfigured, e.g. with a suitable CADD (Computer Aided Drafting and Design) system, from which the underlayment can be printed with the appropriate alignment markings  6  and indicia  8 . Various aesthetic effects can thus be preplanned and achieved.  
         [0027]     Center channel spacers  12  ( FIG. 3 ) are mounted with suitable fasteners, such as nails  14 , staples or other mechanical fasteners at predetermined positions, which can be located by the alignment markings  6  and the indicia  8 . The spacers  12  are generally perpendicular to the underlayment strips  4  whereby the finished construction  2  will repel water with gravity. Each spacer  12  includes: inner and outer faces  16 ,  18 ; upper and lower ends  20 ,  22 ; opposite side edges  24 ; and fastener receivers  26 . A medial or center rib  28  extends between the ends  20 ,  22  approximately midway between the side edges  24 , projects outwardly from the outer face  18  and forms a drain channel  30  at the inner face  16  for directing water out from the facing unit construction  2 . Each spacer  12  includes side ribs  32  at the outer face  18 , which space respective facing units  10  slightly above the spacer outer face  18  and further provide longitudinal rigidity for the spacer  12  in conjunction with the medial rib  28 . The ribbed configuration of the spacers  12  thus minimizes moisture retention and facilitates air circulation for drying the facing unit construction  2 .  
         [0028]     Each facing unit  10  can be secured by one or more clip assemblies  34  ( FIG. 4 ), which can be generally located on the alignment markings  6  between respective spacers  12 . Each clip assembly  34  includes a base clip  36  with top and side flanges  38 ,  40  having notches  42 , and a bottom flange  44 . The base clip  36  can be fabricated, for example, by sheet metal stamping or by another suitable fabrication technique and includes retainers  46  and locating bosses  48  for attaching a wire hook  50  at a proximate end  52  thereof. The hook  50  includes a shaft  54 , which can be pre-bent outwardly for compression by the facing units  10  upon installation thereof. Each hook  50  terminates at a distal end  56  forming a hook-shaped receiver. Suitable fasteners, such as nails  14 , can be utilized for fastening the base clips  36  in place. The base clip top and side flanges  38 ,  40  and notches  42  are adapted for aligning the head of nail coil gun with the base clip  36 . For example, the base clip  36  can be configured to engage the nail gun head whereby a mounting nails  14  can be quickly and accurately driven through a base clip receiver  60  and into the underlying structure in order to expedite rapid installation of the clip assemblies  34 . The base clips  36  can also be preprinted with suitable alignment marks for use in visually locating a nail gun head in proper position on the base clip  36 .  
         [0029]     The construction  2  can comprise multiple courses  58 , which is typical of facing unit construction techniques. Each course  58  can comprise, for example, an underlayment strip  4 , center channel spacers  12 , clip assemblies  34  and facing units  10 . The construction  2  of the present invention facilitates installation course-by-course, starting with the lowermost whereby each successive upper course overlaps the one below it in order to facilitate gravity drainage.  FIG. 2  shows a facing unit roof construction  2  constructed according to this technique. The roof  62  includes sheathing  64 , which receives the underlayment strips  4 . A starter board  66  can be provided adjacent to an eave  68  for properly positioning the first (lowermost) course  58 . The underlayment strips  4  can be installed course-by-course, or the entire surface of a roof or wall can be laid out in advance. The spacers  12  and the clip assemblies  34  can then be installed and the facing units  10  placed thereover by inserting lower edges  70  thereof in respective hook distal ends  56 . Side edges  72  of the facing units  10  can be placed against the spacer center ribs  28  for proper spacing and alignment. The next course  58  retains the immediately preceding course  58 . Specifically, the base clip top flanges  38  overlie upper and edges of the facing units  10 , capturing same in conjunction with the hooks  50 .  
         [0030]     The finished construction provides a uniform, finished appearance with the clip assemblies being concealed except for the hook distal ends  56  and the center channel spacers  12  being concealed except for the center ribs  28 . The exposed parts of the facing unit construction  2  can be suitably finished for aesthetics and compatibility with the facing units  10 , which normally dominate the finished appearance of the construction  2 . For example, the spacers  12  and the clip assemblies  34  can comprise various plastics and metals, which can be tinted, painted, anodized and otherwise finished in order to achieve the desired visual effect.  
         [0031]      FIGS. 5-9  show a facing unit construction  102  comprising an alternative embodiment of the present invention, which is likewise adaptable for roof covering, wall siding and other structures. Alignment markings  106  with indicia  108  are provided for guiding the installation of hooks  110 . Spacers  112  can be installed over the hooks  110 , for example with mechanical fasteners such as staples  114 . The facing units  10  are captured by the hooks  110  and by the overlaps of the next-higher courses  116 .  
         [0032]      FIG. 10  shows another alternative embodiment facing unit construction  122  with spacers  124  overlying hooks  126 , which are fastened to the underlying structure along alignment marking strips  128 .  
         [0033]      FIG. 11  shows another alternative embodiment facing unit construction  132  with modified clip assemblies  134  including modified hooks  136  including hook-shaped distal ends  138 , which are received in hook receivers  140  formed in modified facing units  142 .  FIG. 12  shows another alternative embodiment facing unit construction  152  with mechanical fasteners, such as nails  14 , placed within fastener receivers  156  and attaching the facing units  142  to the underlying structure. The fasteners, i.e. nails  14 , are substantially concealed whereby this construction is particularly advantageous for siding and roofs wherein concealed fasteners are preferred. The faster receivers  156  can be prefilled with a suitable caulk or sealant  158 , such as an elastomeric or silicone-based compound.  
         [0034]      FIG. 13  shows an alternative embodiment clip assembly  162  with a combination hook and snow guard  164 , which functions to minimize snow and ice masses sliding along the finished roof.  
         [0035]      FIG. 14  shows another alternative embodiment clip assembly  172  with a snow guard  174  mounted on the end of a hook  150 , such as that described above.  
         [0036]      FIG. 15  shows another alternative embodiment clip assembly  182  with a pair of hooks  184 . A base clip  185  includes top and bottom flanges  186 ,  188  and side flanges  190 . The top and side flanges  186 ,  190  included notches  192 . The clip assembly  182  is adapted for placement approximately midway along a respective facing unit upper end  74 . The modified clip assembly  182  accommodates installation of the facing units  10  with a single clip assembly  182  on each.  
         [0037]     One or more layers of underlayment can be provided for protection from the elements. For example, the underlayment strips  4  can form a continuous layer and between-course underlayment strips  76  can be provided in addition for overlapping placement between respective courses  58  ( FIG. 2 ).  
         [0038]     It is to be understood that the invention can be embodied in various forms, and is not to be limited to the examples discussed above. Other components and configurations can be utilized in the practice of the present invention. For example, although the constructions described above are particularly suitable for constructing slate roofs, wall siding and other constructions utilizing various facing unit materials can be constructed according to the present invention. Alternative facing units include shingles, shakes, tiles and panels, which can comprise wood, concrete, ceramic, plastic, metal, etc. Still further, various fasteners including nails, staples, screws and adhesives can be used for mounting, securing and attaching the various components of the facing unit constructions. Suitable sealants, caulks and adhesives can be utilized in the receivers, around fasteners and between the different components as necessary for achieving a weatherproof construction.

Summary:
A facing unit construction is adapted for mounting facing units on a structure, such as a roof or a wall. The facing units can comprise various suitable materials, including slate, wood, tile, metal, concrete, ceramic and plastic. Center channel spacers are mounted on the structure and provide outwardly-projecting ribs for positioning the facing units and inwardly-open channels for draining water from underneath the facing units. Clip assemblies are also mounted on the structure and include flanges for capturing the facing unit upper edges and hooks for receiving the facing unit lower edges. The hooks can also extend through receivers in the facing units and can be provided with snow guards. The clip assembly base clips can be configured for automatically aligning nailing equipment. The facing units, spacers and clips form courses on the structure, which overlap for gravity discharge of water.