I AM PLANNING TO BUILD AN ADDITION TO MY A FRAME. MY HOUSE WAS BUILT IN ABOUT 1960. 3 1/2 X 5 INCH BEAMS WERE SET UP LIKE TRIANGLES ON FOUR FOOT CENTERS. STRIPS 2 FEET WIDE AND ABOUT 4 INCHES THICK WERE NAILED PERPENDICULAR TO THE BEAMS. THE DOWNSIDE OF THESE STRIPS, WITHOUT MORE, FURNISHED THE INSIDE OF THE HOUSE. REGULAR SHINGLES WERE NAILED DIRECTLY TO THE UPSIDE OF THESE STRIPS.