Trench header ducts

Claim 1

IN A PREFABRICATED METAL DUCT HAVING A REMOVABLE TOP COVER AND ADAPTED FOR PERMANENT INSTALLATION OVER A SUBFLOOR WITH FILL THEREOVER TO THE LEVEL OF THE TOP OF THE DUCT COVER AND HAVING FLOOR FINISHING MATERIAL OVER THE DUCT AND FILL, THE COVER AND ITS PORTION OF FLOOR FINISHING MATERIAL BEING REMOVABLE FROM THE DUCT AS A UNIT FOR ACCESS TO THE DUCT INTERIOR, SAID DUCT HAVING A BODY COMPRISING A BOTTOM AND OPPOSED UPSTANDING PARALLEL SIDE WALLS, EACH SIDE WALL RECEIVING SUPPORTING MEANS AT EACH SIDE WALL COMPRISING A RAIL MEANS, A RAIL HAVING A Z PORTION SAID Z PORTION BEING ABOVE THE SIDE WALL, SAID Z PORTION PROVIDING A LOWER AND UPPER HORIZONTAL STEP LONGITUDINALLY OF THE DUCT, THE UPPER STEPS AT OPPOSITE DUCT SIDES BEING FURTHER APART THAN THE LOWER STEPS, AN OUTER ANGLE STRIP HAVING AN ATTACHING FLANGE AND AN OUTER FLOOR FINISH MATERIAL STOP FLANGE, THREADED MEANS DETACHABLY SEWCURING SAID OUTER ANGLE STRIP ATTACHING FLANGE ALONG THE TOP SURFACE OF SAID UPPER STEP IN FACE TO FACE RELATION IN EITHER OF TWO POSITIONS SO THAT SAID STOP FLANGE EXTENDS DOWNWARDLY OR UPWARDLY OF THE UPPER STEP, THE STOP FLANGE IN EITHER STRIP POSITION BEING LOCATED ALONG THE REGION WHERE THE Z PORTION DROPS FROM THE UPPER STEP TO THE LOWER STEP, A FLOATING INNER ANGLE STRIP HAVING A TEMPORARY BORDER FLANGE AND A WIDER FLOOR FINISH STOP FLANGE, SAID TWO FLANGES BEING OF EQUAL THICKNESS, SAID INNER ANGLE STRIP BEING SUPPORTED ALONG THE LOWER Z PORTION STEP BY HAVING ONE FLANGE FACE RESTING AGAINST THE Z PORTION STEP AND THE REMAINING FLANGE EXTEND VERTICALLY UPWARD ADJACENT THE Z PORTION DROP; THE INNER ANGLE STRIPS ON OPPOSITE DUCT SIDES HAVING THEIR SUPPORT FLANGES EXTEND TOWARD EACH OTHER, EACH INNER ANGLE STRIP HAVING EACH OF ITS FLANGES NARROW ENOUGH SO THAT IN EITHER POSITION, THE SUPPORT FLANGE FOR EACH INNER ANGLE STRIP WILL NOT COVER ALL OF THE Z PORTION LOWER STEP BUT WILL LEAVE A FREE GASKET ACCOMMODATING LONGITUDINAL REGION ADJACENT THE FREE EDGE OF THE INNER ANGLE SUPPORT FLANGE, GASKETS FOR SAID LONGITUDINAL FREE REGIONS ON SAID Z PORTION LOWER