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RELATED APPLICATIONS 
     This application is a continuation of copending U.S. patent application Ser. No. 08/960,166 filed Oct. 27, 1997 which is a continuation of U.S. patent application Ser. No. 07/924,738 filed Aug. 4, 1992 abandoned, and provisional application Serial No. 60/133,244 filed May 4, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to roof vents, and more specifically to passive attic vents for use with tile roofs. 
     2. Description of the Prior Art 
     Energy efficiency is a serious consideration in building design and construction. Many building codes require builders to minimize energy requirements to maintain comfortable living spaces. One of the most common energy losses in a home is due to heat transfer through the attic. In warm climates, heat builds up in the attic from solar energy incident on the roof or from heat transfer up from the living space. If the attic is allowed to become too hot, the installed insulation becomes ineffective and the attic heat is transferred to the living space below. In colder climates, moisture builds up in the attic, robbing the insulation of much of its R value. Consequently valuable heat in the living space is conducted out through the attic. 
     Early efforts at minimizing energy losses through the attic focused on the insulation between the living space and the attic and ignored the effects of the heat and/or moisture build-up. As insulation improved a point was reached where more insulation was not necessarily better or possible due to space limitations. Gable vents and dormer type passive ventilation systems have been incorporated to ventilate the attic. These ventilation devices conduct excess heat and or moisture out or the attic thus maintaining the efficiency of the installed insulation. However, both gable and dormer ventilation systems are clearly visible and often require extensive architectural manipulation to minimize their esthetic effect on the structure. 
     In geographical areas that are typically warm and dry such as the American Southwest, many homes have low pitch, hip roofs which have no gables, and dormers may have a significantly effect the aesthetics of a design if improperly located or too numerous. Therefore, these systems have proven to be inadequate. In colder and or wetter climates such as the Eastern United States, snow buildup, or driven snow or rain counteract the conventional passive ventilation devices and usually block the vents and or reintroduce more moisture than was originally present thus minimizing the benefit of the vents. 
     Passive attic vents which attempted to camouflage their appearance have been marketed in recent years. These camouflaged vents are generally a closed device made for direct conduction of air from the attic or waste vents and are often made of plastic or other material amenable to mold manufacturing. The direct conduction or one-piece construction may limit air flow and may provide a direct path for moisture such as driven rain or snow into the attic thus minimizing the benefit of the vent. To improve the conventional ventilation technology it is necessary to understand clay or concrete roof construction. 
     A roof is designed to shed rain and snow and shield the living space from sun. A roof is composed of structural elements to support its weight and form a slope to assist in shedding rain and snow. 
     The first structural element is the roof rafter  8  or truss which creates the basic slope of the roof as shown in FIGS. 7 and 8. Secured on top of the rafters or trusses, such as rafter  8 , is a layer of wood  6 , such as planks, plywood or oriented strand board (OSB). Nailing plywood  6  to the roof rafters forms a sloped diaphragm or structural layer D. 
     Structural layer D forms a very strong structural element and is likely to leak only along the seams between sheets of plywood  6  if left as the complete roof. However, wood requires frequent attention and treatment to retain its weather resistance, and thus is not a good long term roof material. 
     Plywood  6  is usually covered with lapped layers of roofing felt  4  or paper or other suitable material which is treated with tar and or other chemicals to render it water resistant. The lapped layers of felt  4  may become sealed together by the heat on the roof and form a true water proof membrane or layer and could be used for a roof topping. However conventional roof felt or paper such as felt  4  is fairly fragile and susceptible to damage from sun or wind. If left unshielded in the sun it would dry and crack in a short time and thus is inadequate as a lone weatherproofing material. 
     By covering felt  4  with a layer of material resistant to sun and other weather effects, felt  4  may be protected from direct solar radiation and may produce a weather-tight roof. Layer  2  may be composed of asphalt shingles, wood shingles, clay tiles, concrete tiles, metal tiles or similar conventional materials. In this example, layer  2  is composed of interleaved clay tiles such as cap tiles  2 C and pan tiles  2 P. Battens, such as batten B, may be used as securing sites for metal, clay or concrete tile roofs. 
     Layer  2  sheds the majority of rain and snow and is generally impervious to long term weather effects. Layer  2  does have many small openings and spaces between the tiles or other elements, thus felt  4  remains as the waterproof layer and sheds any water or snow which passes through layer  2 . 
     Referring now to FIG. 8, conventional camouflaged vents, such as vent  7 , provide a direct and closed conduction path P for attic air or waste vent air. In a passive ventilation system, the volume of air conducted via path P is limited by the cross sections at opening O and inlet I and the temperature differential between the air AI in the attic and air AO outside the attic. To permit adequate attic ventilation, many conventional vents, such as vent  7 , will be needed. Due to the directness of path P, wind driven rain or snow may be blown into opening O and travel directly into the underlying attic space bypassing tile layer  2  and water proof felt layer  4 . 
     Due to the complex shapes required, conventional camouflaged vents, such as vent  7  are often fabricated from moldable materials such as plastics. Plastic permits a vent to survive moisture yet may not be as durable as conventional roofing materials due to the effects of solar radiation and/or airborne chemicals. 
     What is needed is a new roof system incorporating an improved passive ventilation system that can be simply manufactured from highly durable material and will not affect the appearance of a building design if used in adequate numbers to properly ventilate the attic and or rafter spaces, and is useable on many roof configurations and with many types of conventional roofing materials. 
     SUMMARY OF THE INVENTION 
     The present invention provides a new roofing system that incorporates an open attic or rafter space ventilation technique. The new roofing system includes solid conventional roofing materials such as clay or concrete tiles combined with two or more primary vents conducting air through the structural layer and the water resistant membrane. 
     Thus, in a first aspect, the present invention provides a ventilated roof comprising a roof structural layer through which air is to be ventilated; a primary vent disposed in the structural layer to provide an air flow passage therethrough having a first venting performance; a plurality of tiles mounted on the structural layer to form a tile layer thereover and arranged to provide air flow passages between adjacent tiles having a combined second venting performance; and a secondary vent disposed in the tile layer to form an outer roofing layer therewith and having an air passage therethrough with a third venting performance smaller than the first venting performance, the outer roofing layer being in air flow communication with the primary vent to provide a venting air flow passage for venting said air. 
     In another aspect, the present invention provides a method for ventilating a roof comprising the steps of providing a roof structural layer through which air is to be ventilated; selecting a primary vent having a first venting performance; mounting the primary vent in the structural layer to provide an air flow passage therethrough; selecting a plurality of tiles; arranging the tiles on the structural layer to provide air flow passages between adjacent tiles; mounting the tiles on the structural layer to form a tile layer thereover having a combined second venting performance; selecting a secondary vent having an air passage therethrough with a third venting performance smaller than the first venting performance; and mounting the secondary vent in the tile layer to form an outer roofing layer therewith in air flow communication with the primary vent to provide a venting air flow passage for venting said air. 
     In yet another aspect, the present invention provides a ventilated roof comprising a roof structural layer through which air is to be ventilated from an attic; a primary vent disposed in the structural layer to provide an air flow passage therethrough having a first venting performance; a plurality of tiles mounted on the structural layer to form a tile layer thereover and arranged to provide air flow passages between adjacent tiles having a combined second venting performance; and a secondary vent disposed in the tile layer to form an outer roofing layer therewith and having an air passage therethrough with a third venting performance, the outer roofing layer being in air flow communication with the primary vent to provide a venting air flow passage having a fourth venting performance greater than the second venting performance for venting the air from the attic. 
     In a further aspect, the present invention provides a method for ventilating a roof comprising the steps of providing a roof structural layer through which air is to be ventilated; selecting a primary vent having a first venting performance; mounting the primary vent in the structural layer to provide an air flow passage therethrough; selecting a plurality of tiles; arranging the tiles on the structural layer to provide air flow passages between adjacent tiles; mounting the tiles on the structural layer to form a tile layer thereover having a combined second venting performance; selecting a secondary vent having an air passage therethrough with a third venting performance; and mounting the secondary vent in the tile layer to form an outer roofing layer therewith in air flow communication with the primary vent to provide a venting air flow passage having a fourth venting performance greater than the second venting performance for venting said air. 
     In a still further aspect, the present invention provides a ventilated roof comprising a roof structural layer through which air is to be ventilated; a primary vent disposed in the structural layer to provide an air flow passage therethrough having a first venting performance; and a plurality of tiles mounted on the structural layer to form a tile layer thereover and arranged to provide air flow passages between adjacent tiles in air flow communication with the primary vent to vent the air and having a combined second venting performance. 
     In yet another further aspect, the present invention provides a method for ventilating a roof comprising the steps of providing a roof structural layer through which air is to be ventilated; selecting a primary vent having a first venting performance; mounting the primary vent in the structural layer to provide an air flow passage therethrough; selecting a plurality of tiles; arranging the tiles on the structural layer to provide air flow passages between adjacent tiles in air flow communication with the primary vent; and mounting the tiles on the structural layer to form a tile layer thereover having a combined second venting performance. 
     In still another aspect, the present invention provides a ventilated roof comprising a first roofing layer having a primary vent through which air from an attic is to be ventilated, and a second roofing layer constructed from a plurality of similar roofing tile elements disposed over the first roofing layer and having an effective third vent in air flow communication with the primary vent to vent said attic, said effective third vent combining air flow passages between the tile elements. 
     And in yet another aspect, the present invention provides a method for ventilating a roof comprising the steps of selecting a first roofing layer having a primary vent through which air from an attic is to be ventilated; selecting a plurality of similar roofing tile elements; and disposing the tile elements over the first roofing layer to form a second roofing layer having an effective third vent in air flow communication with the primary vent to vent said attic, said effective third vent combining air flow passages between the tile elements. 
     Another aspect of the present invention combines new, easy to manufacture, unitary structural ventilation tiles or secondary vents into the roof shield layer over a water resistant roof layer. The primary vent or vents may be sized large enough to benefit from the secondary ventilation in addition to the primary, rafter space ventilation. 
     The new tile or secondary vent tile may be of hollow construction using durable materials such as steel, copper, aluminum, or any other suitable material. The secondary vent tile provides some secondary attic ventilation through the roof shield layer in addition to the primary ventilation provided by the permeability of the roof shield layer. The interaction of the one or more primary vents and the secondary vent(s) in the roof shield layer and the permeability of the roof shield layer generate greater air flow from an enclosed air space such as an attic or rafter space due to a given pressure or temperature differential than the calculated net free ventilation area (NFVA) of the primary vents would anticipate. 
     In another aspect of the present invention one or more secondary vents in the roof shield layer may be generally co-located with one or more primary vents in the weatherproof roof structural layer. 
     In another aspect of the present invention the unitary structural vent tile or hollow tile is easily manufactured and is as easily installed as a conventional roof tile. A structural vent tile or hollow tile according to the present invention may be made from a contiguous piece of material thus minimizing hand labor and resulting in greater manufacturing efficiency. 
     In another aspect of the present invention one or more primary vents may be located to maximize airflow from the attic and one or more structural ventilation tiles or secondary vents may be located to minimize visual awareness of their presence and/or provide adequate secondary ventilation and prohibit direct ingress of water, snow or other foreign material through the structural ventilation tile(s) and one or more primary vents into the attic. 
     These and other features and advantages of this invention will become further apparent from the detailed description and accompanying figures that follow. In the figures and description, numerals indicate the various features of the invention, like numerals referring to like features throughout both the drawings and the description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an expanded isometric view of a section of roof according to the present invention; 
     FIG. 2 is a partially exploded section view of a roof according to the present invention; 
     FIG. 3 is a condensed isometric view of the roof of FIG. 1; 
     FIG. 4 is a detail view of a hollow ‘S’ tile according to the present invention; 
     FIG. 5 is an alternate embodiment of the tile of FIG. 4; 
     FIG. 6 is a detail view of a hollow ‘M’ tile according to the present invention; 
     FIG. 7 is an exploded end view of the component parts of a conventional tile roof; 
     FIG. 8 is a side view of the roof of FIG. 7 taken along X-X′; 
     FIG. 9 side view of a conventional closed system vent installed on a tile roof; 
     FIG. 10 is a perspective view of a secondary vent frame and caps, according to the present invention, installed on a portion of a roof; 
     FIG. 11 is a top view of a secondary vent frame and caps according to the present invention; 
     FIG. 12 is a bottom view of the secondary vent frame and caps of FIG. 11; 
     FIG. 13 is a cross-section view of the secondary vent frame and caps of FIG. 11 taken along  4 — 4 ; 
     FIG. 14 is a cross-section view of the secondary vent frame and caps of FIG. 11 taken along  5 — 5 ; 
     FIG. 15 is a cross-section view of the secondary vent frame and caps of FIG. 11 taken along  6 — 6 ; 
     FIG. 16 is a perspective view from below of the front cap corner of a secondary vent frame and cap according to the present invention; 
     FIG. 17 is a perspective view of a mounting location for a primary vent showing the hole marked on the roof; 
     FIG. 18 is a perspective view of a mounting location for a primary vent showing the hole being cut in the roof; 
     FIG. 19 is a perspective view of a mounting location for a primary vent showing the primary vent being prepared for installation; 
     FIG. 20 is a perspective view of an installed primary vent showing the relationship to a secondary vent according to the present invention; 
     FIG. 21A is a top view of a first element composing a flat structural vent after a first manufacturing step according to the present invention; 
     FIG. 21B is a top view of the first element of FIG. 21A after a second manufacturing step according to the present invention; 
     FIG. 22 is a top view of a second element composing a flat structural vent according to the present invention; 
     FIG. 23 is a front view of the element of FIG. 22; 
     FIG. 24 is a side view of the element of FIG. 22; 
     FIG. 25A is a top view of a first element composing an ‘S’shaped structural vent formed in three manufacturing steps according to the present invention; 
     FIG. 25B is a side view of the element of FIG. 25A; 
     FIG. 25C is an end view of the element of FIG. 25A; 
     FIG. 26A is an isometric view of the first manufacturing step of forming a booster according to the present invention; 
     FIG. 26B is an isometric view of the second manufacturing step of forming the booster of FIG. 26A; 
     FIG. 26C is an isometric view of the third manufacturing step of forming the booster of FIG. 26A; 
     FIG. 26D is an isometric view of the fourth manufacturing step of forming the booster of FIG. 26A; 
     FIG. 27 is a top detail view of the element of FIG. 25A; 
     FIG. 28 is a top detail view of the booster of FIG. 26A; 
     FIG. 29A is a top view of a first element composing an ‘M’ structural vent formed in three manufacturing steps according to the present invention; 
     FIG. 29B is a side view of the element of FIG. 29A; 
     FIG. 29C is an end view of the element of FIG. 29A; 
     FIG. 30A is a top view of a second element composing an ‘M’ structural vent formed in three manufacturing steps according to the present invention; 
     FIG. 30B is a side view of the element of FIG. 30A; and 
     FIG. 30C is an end view of the element of FIG.  30 A. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, new roof system  80  is shown according to a currently preferred embodiment of the present invention. Roof system  80  includes roof shield layer  82  and one or more primary vents such as primary vent  84 . Roof system  80  may be installed on any conventional water resistant roof layer such as roof structural layer  86 . Roof shield layer  82  may be composed of conventional, solid tiles such as tiles  91 , or a combination of solid tiles and hollow structural tiles such as tile  85 . Conventional tiles  91  may be produced from any suitable material such as clay concrete, slate, or metal. 
     Referring now to FIG. 2, attic air  88  may be drawn through one or more primary vents such as primary vent  84  by a pressure or temperature differential between attic or enclosed airspace  87  and outside airspace  89 . Once attic air  88  is drawn through primary vent  84  it enters a ventilation layer or twining chamber  90 . In a first embodiment of the present invention, outbound primary ventilation flow  94  passes through roof shield layer  82  as exhaust air  98  to complete attic ventilation. It should be noted that this discussion focuses on an outbound flow of air, but similar pathways and dynamics exist for an inbound flow of air. 
     In this embodiment of the invention, roof shield layer  82  is formed of tiles or other suitable roof shield elements that are disposed with respect to one another so as to cover structural layer  86  and protect it from the effects of weather, and also to provide air flow passages between the tiles to allow air flow into and out of ventilation layer  90 . Thus, in one embodiment of the invention each tile  91  is disposed with the downslope edge overlying the upslope edge of the adjacent downslope tile, in a manner well known in the art, and spaced from the upslope edge of the adjacent tile by a sufficient distance to create an air flow passage therethrough. The distance between overlapping edges of adjacent tiles need not be great, as the combined effect of such air flow passages over the entire surface of roof shield layer  82  can be significant. Therefore merely providing cracks between overlapping tile edges may be enough to effectuate a combined venting performance sufficient to effectively exhaust any air  88  pushed through from, or sucked into, attic  87 . Additionally, roofs will typically not be sealed around their edges, and these edges will therefore also act as air flow passages to and from ventilation layer  90 . The term “venting performance” as used above and in the claims is understood to encompass any measure or definition of air flow, including but not limited to a measure of the effective or total cross sectional area, the effective air flow volume, or the effective air flow speed. 
     In another embodiment of the present invention, roof shield layer  82  may include hollow vent tiles such as tile  85  to improve the efficiency of ventilation. As air  88  is drawn out through primary vent  84  it may be diverted by tiles  91  or one or more diverters such as frame diverters  92  shown in FIGS. 15,  25 A and  25 C. Diverters such as frame diverters  92  divide attic air  88  into twining or primary flow  94  and secondary flow  96 . Primary flow  94  circulates within ventilation layer  90  and is exhausted as exhaust air  98  through the cracks or openings provided over the entirety of roof shield layer  82 , as detailed above. With reference once again to FIG. 1, secondary flow  96  is directed through any generally co-located secondary structural tiles such as tile  85  to exhaust through the sides as side air  99 , through the front as front air  95 , or through the top as top air  97 . Thus the total air exhausted from attic  87  may be expressed as TA and is shown in equation  100 .                     100        T   A       =                  (     secondary                 flow                 96     )     +     primary                 flow                 94                   =                  (       top                 air                 97     +     front                 air                 95     +     side                 air                 99       )     +     exhaust                 air                 98                                      
     Referring now to FIG. 3, roof shield layer  82  is shown directly connected to structural layer  86 . Vent tile  85  may be located above any primary vent  84  as shown in FIG. 3 to prevent a broken tile directly above a primary vent from allowing water to pass directly through into enclosed space or attic  87 . Conventional tiles  91  adjacent to tile  85  are shown as clear to permit a view of the installed interrelationship between the elements of roof shield layer  82  and the elements of structural layer  86 . Primary vent  84  is shown installed directly below vent tile  85 . To exploit the ventilation efficiency of the present invention, vent tile  85  may be installed in any of the illustrated locations of row  102 , row  104 , or row  106 , and thus take advantage of the natural updraft created by rising attic air  88 , which will typically be warmer than outside airspace  89 . 
     Referring now to FIG. 4, a hollow tile such as tile  85  may be formed of two generally similar parallel surfaces such as upper surface  108  and lower surface  110  forming a hollow tile of generally similar size and shape to conventional roof tiles such as solid tiles  91 . Top air  97  may be allowed to escape through ventilation apertures  128  such as louvers, holes or other openings. Thus, it must be noted that due to the novel design of the invention, the venting performance (as measured by, e.g., the total effective cross-sectional area) of any such openings formed in all secondary vent tiles  85  installed in a roof according to the invention can be significantly smaller than the venting performance of all primary vents  84  formed in the structural elements  86  of the roof, and yet provide for adequate ventilation of all attic air  88  passing through the primary vents. This is a beneficial result of the use of air passages disposed between adjacent tiles which, as discussed above, can provide a substantial amount of air flow therebetween. Thus, exhaust apertures  128  formed in secondary vent tiles  85  according to the invention will provide a significantly smaller effective surface area exposed to outside airspace  89  than conventional roof ventilation systems that require vent openings formed in the outer surface of the roof shield layer that are approximately equal in total surface area to the primary vents in ventilating communication with the attic. This is advantageous because smaller outside apertures provide less opportunity for ingress of water, snow or other foreign material through the structural ventilation tile(s) and one or more primary vents into the attic. 
     Referring now to FIG. 5, in an alternate embodiment of the invention two hollow cap tiles such as tiles  112  and  114  may be formed on a single ‘S’ shaped frame such as frame  116  by attaching, folding or otherwise forming caps  118  and  120  over ventilation access  122  and  124  respectively. In the tiles of FIGS. 5 and 6, the upper surfaces and the lower surfaces are separated and supported by spacers or tabs such as tabs  126 . In an alternate embodiment an ‘M’ style tile may be formed as shown in FIG. 6. A similar ‘flat’ hollow tile may be constructed using elements shown in FIGS. 21A-24. 
     Referring to FIG. 10, a section of pitched roof  11  near eave  60  is shown including a roof vent  10  according to another embodiment of the present invention. Pitched roof  11  is generally composed of a plurality of conventional tiles  21 , surrounded by edge tiles  13 , edge caps  15  and ridge caps (not shown). Roof vent  10  is in two parts, primary vent  40  (shown in FIG. 20) and secondary vent  12 . Roof vent  10  may be formed from any suitable metal such as aluminum, steel, or copper. In a currently preferred embodiment of the present invention roof vent  10  may be formed of  26  gauge galvanized steel. 
     Referring now to FIG. 11, secondary vent  12  may include one or more caps  14  attached to lower piece or frame  16 . Secondary vent  12  may serve as an alternate replacement for one or more conventional tiles  21  on pitched roof  11 . Different tile types and similar looking tiles from different manufacturers have different physical dimensions and may require a unique frame configuration for a precise fit between the tiles and frame  16 . Specific fit may be required between upslope edge  42  to upslope tile  21 U, pan flange  24  to pan  25 , and downslope edge  45  to downslope tile  13 D and cap flange  22  to cap  23 . Frame  16  may be formed to fit the contours and edge configuration of the field tiles  21  used. Frame  16  may be manufactured in any conventional manner. In a currently preferred embodiment of the present invention, and as shown in FIGS. 25A-25C, frame  16  is stamped from a single piece of material to fit precisely the field tiles  21  for which it is intended to be used. Frame  16  may include one or more pan areas  18  and a cap area  20  adjacent each pan area  18 . Viewed from above, pan areas  18  are concave and cap areas  20  are convex. As shown in FIGS. 26A-26D, the pan and cap areas may also be formed from a flat sheet of material such as sheet metal that is stamped into a concave or convex channel or trough, including any ridges or reinforcing ribs that may be formed in the pan or cap. The concave or convex channel defining the pan or cap, respectively, may subsequently be further shaped such as by bending to further define the desired pan or cap shape and assume the desired dimensions. Pan areas  18  align with individual pan tiles or with corresponding pan areas of field tiles such as pan areas  17  of FIG.  10 . Cap areas  20  align with individual cap tiles or with corresponding cap areas of field tiles  21  such as cap areas  19  of FIG.  10 . Secondary vent  12  is mounted with pitch axis  31  parallel to the pitch of pitched roof  11 . 
     Cap flange  22  is configured to fit underneath the cap of an adjacent field tile such as cap  23  as shown in FIG.  10 . Cap flange  22  may include one or more creases such as crease  30  to obtain a precise fit to an adjacent field tile. Cap flange  22  may also have one or more bevels such as bevel  32  to minimize interference with an adjacent field tile. Pan flange  24  is configured to mate with the pan of an adjacent field tile such as pan  25  as shown in FIG.  10 . Pan flange  24  may include one or more creases such as crease  28  (FIG. 12) to obtain a precise fit to an adjacent field tile. A plurality of ribs  26 ,  26 A and  26 B may be stamped into frame  16  for increased rigidity, as discussed above. In a currently preferred embodiment of the present invention ribs  26 ,  26 A and  26 B are parallel to upslope edge  42 . A hole  34  is included in each pan area  18  to accept a conventional fastener, such as a nail or a screw, to secure secondary vent  12  to a roof such as pitched roof  11 . 
     Referring now to FIG. 12, the underside of frame  16  is shown in more detail. Frame  16  includes a vent opening  36  in each cap area  20 . When installed on a roof near a primary vent, vent openings  36  are in ventilating communication with vent opening  46 . Each vent opening  36  is located between ribs  26 A and  26 B. 
     Where tile  85  is not composed of two generally similar parallel surfaces such as on secondary vent  12 , booster  38  may be attached to each pan area  18  adjacent edge  40 . Booster  38  is a spacer that compensates for the difference in thickness between field tiles  21  and frame  16 . Booster  38  may be formed and attached in any conventional manner to raise frame  16  above the roof battens such as batten B. Thickness compensating fingers  43  are formed along the downslope edge  45  of cap area  20 . Thickness compensating fingers  43  compensate for the difference in thickness between field tiles  21  and frame  16  to provide a seal against the top of a downslope field tile such as downslope tile  13 D. Wind clips  44  are attached to frame  16  to secure secondary vent  12  to lower course tiles  45  shown in FIG.  20 . 
     Referring now to FIGS. 13 and 14, ribs  26 ,  26 A,  26 B,  50  and booster  38  are seen in profile. Ribs  26  are shown as concave, but other configurations may be equally suitable. Rib  26 B is shown as convex, but other configurations may be equally suitable. Rib  26 A must be oriented concave up to minimize interference with caps  14  at shoulder  48 . Ribs  50  are shown as concave down, but other configurations may be equally suitable. Legs  52  are attached to frame  16  and to caps  14  to support caps  14  and maintain ventilating access  54  between frame  16  and caps  14 . Legs  52  may be attached in any conventional manner. 
     Caps  14  shield vent openings  36  from the weather and are attached to cap area  20  by any conventional means such as riveting or spot welding at shoulder  48  and legs  52 . Caps  14  include side hems  27 , a front hem  29 , and ribs  50 . In a currently preferred embodiment of the present invention, ribs  50  extend parallel to front hem  29  from one side hem  27  to the other side hem  27 . Side hems  27  and front hem  29  are included to improve the weather shielding efficiency of cap  14  without sacrificing ventilating efficiency. Ribs  50  and are stamped into caps  14  for rigidity. Front and side hems  29  and  27  may be made in any conventional manner such as cutting and bending. In a currently preferred embodiment of the present invention, front and side hems  29  and  27  are formed by stamping to increase the rigidity of caps  14 , and caps  14  are made in one standard size. A standard size cap  14  may be fitted to many different frames thus minimizing manufacturing and inventory complexity. 
     Referring now to FIG. 15, the uniform relationship between frame  16  and top surface or cap  14  is shown. Vent  10  serves dual purposes, ventilating attic  87  and protecting attic  87  from weather and pests. Vent opening  36 , vent opening  46  and attic opening  58  cooperate to conduct attic air  88  from attic  87 . A parallel top surface  85 T or caps such as cap  14  are attached to frame  16  as shields over vent opening  36  to prevent weather and pests from falling directly into attic  87 . Caps  14  also prevent direct solar irradiation of felt  4  or attic  87 . Vent openings  36  are covered by screen  37  to prevent entry into twining chamber  66  by pests larger than the screen openings. Baffles  55  shield vent openings  36  from wind driven moisture and particles, and extend along edges R and L. Baffles  55  are H high and they are folded up along angle A between 0° and 90° from vent opening  36 . In a currently preferred embodiment of the present invention, H is 0.25″ and angle A is 50°. Cap  14  includes side hems  27 , and a front hem  29  (shown in FIG. 16) to further shield vent opening  36  from entry of foreign matter. Side hems  27 , and front hem  29  extend from cap  14  to below vent opening  36 . 
     Attic air  88  flowing through a passive vent such as vent  10  follows the same path whether from outside  65  into attic  87 , or from within the attic  87  to outside  65 , only the direction of flow changes. For the sake of simplicity, attic air  88  flow from attic  87  to outside  65  will now be described with the understanding that the present invention functions equally well conducting air in both directions. Air travelling through vent  10  must undergo a change of direction that helps to prevent foreign matter from entering attic  87 . As installed, vent opening  46  of primary vent  40  provides a convection driven ventilating channel through roof deck  56 . Primary vent  40  conducts air up from within attic  87  through attic opening  58  and vent opening  46  to twining chamber  66 . In twining chamber  66  attic air  88  is diverted by frame diverters such as diverter  92  into secondary flow  96  and primary flow  94 . Convection continues to drive secondary flow  96  up through vent opening  36  into ventilating access  54 . Secondary flow  96  in ventilating access  54  is then conducted up over baffles  55 . Once above baffles  55  the shape of vent cap  14  and hems  27  and  29  cause secondary flow  96  to change direction and divide and travel down beyond side hems  27  as side air  99  or front hem  29  as front air  95  to outside  65 . 
     Referring now to FIG. 16, thickness compensating fingers  43  and a wind clip  44  are shown in more detail. Thickness compensating fingers  43  may be formed by any conventional means, and in a currently preferred embodiment of the present invention thickness compensating fingers  43  are cut into downslope edge  45  of cap area  20  and folded. Due to the thickness disparity between frame  16  and adjacent field tiles  21 , thickness compensating fingers  43  are needed to provide a pest seal against the top of the down slope field tile  21  when pan flange  24  is fitted to the pan of an adjacent field tile such as pan  25  as shown in FIG.  10 . 
     In FIGS. 17-20 installation steps for roof vent  10  are illustrated as a general example. Referring now to FIG. 17, location  57  on roof deck  56  is selected for installation of roof vent  10 . Location  57  is marked to delineate where attic opening  58  will be cut. As shown in FIG. 18, saw  59  is used to cut attic opening  58  through roof deck  56 . In FIG. 19, sealant  61  is applied to bottom side  41  of primary vent  40 . Primary vent  40  is installed with bottom side  41  in contact with roof deck  56  and vent opening  46  in ventilating communication with attic opening  58 . As shown in FIG. 20, secondary vent  12  is then installed above primary vent  40  with vent openings  36  in ventilation communication with vent opening  46 . Vent opening  46  may be provided with screen  46 S for additional protection against introduction of vermin or debris through attic opening  58 . Fasteners (not shown) are attached through holes  34  into batten  70  to secure secondary vent  12 . 
     To maximize attic ventilation, roof vents  10  may be used in pairs. A pair of roof vents  10  may be located on a roof parallel to the rafters with a first roof vent  10  near the roof peak (not shown) and a second roof vent  10  near eave  60 . This configuration promotes passive air convection through the attic or rafter space as warm air rises through the first roof vent  10  cooler air is drawn into the attic or rafter space through second roof vent  10 . 
     Referring now to FIG. 21, in a currently preferred embodiment of the present invention a structural ventilation tile such as tile  85  may be formed of a single contiguous piece of material. 
     Having now described the invention in accordance with the requirements of the patent statutes, those skilled in this art will understand how to make changes and modifications in the present invention to meet their specific requirements or conditions. Such changes and modifications may be made without departing from the scope and spirit of the invention as set forth in the following claims.

Summary:
A ventilation system for an attic or rafter space that mimics the appearance of the roofing material and thus has little effect on the appearance of the building. In one aspect primary vents are installed in the roof deck below the roof tiles, and the roof tiles are mounted to as to provide air spaces in between them and thus allow air flow from the primary vents to the outside. In another aspect a secondary vent is constructed to look like the surrounding field tiles and is installed over each primary vent. One or more vent openings in the secondary vent and an opening in the primary vent conduct air between the attic or rafter space and the outside. The secondary vent has a frame with one or more vent openings and a cap covering each opening shielding the ventilating space. Frames are formed in one piece and are made to fit each different size and type of roofing tile. The caps and the frame are ribbed for rigidity. The caps are made in one size only to minimize manufacturing and inventory complexity, thus any cap may be fitted on any frame.