The SI unit for period is the second. For counts per unit of time, the SI unit for frequency is hertz (Hz), named after the German physicist Heinrich Hertz; 1 Hz means that an event repeats once per second.A previous name for this unit was cycles per second (cps).or each dot, f is the frequency in hertz (Hz) â meaning the number of times per second (i.e. cycles per second) that it flashes â while T is the flashes' period in seconds (s), meaning the number of seconds per cycle. Each T and f are reciprocal.
Photo: CertainTeed. Economical to produce, relatively easy to install and widely available, asphalt shingles are todayâs most popular roofing materialânot only because theyâre less costly than wood, wood shakes, tile, metal or slate, but also because their guaranteed life span pits them favorably against competitors.
Next is installation. The most common type of roofing material is asphalt shingles, which cost between $120 and $140 per square. Slate and clay tiles, wooden shakes and metal roofs are also options but come with higher prices. For example, metal roofing typically costs between $1,000 and $1,500 per square.