Their purpose is to implement Boolean Logic. You might notice the correspondence to Binary Semantics and thus Binary (number system) logic & mathematics. Special fun: OR and XOR (exclusive OR).
Geographical Considerations: While most contractors will price their asphalt shingles roof replacement services within $4.00 to to $7.00 per square foot or $400 to $700 per square range, there will be significant geographic variations.
Asphalt is usually used for shingles because it is affordable and the shingles are relatively easy to install. Many homeowners choose to install asphalt shingles on their own, which saves the cost of installation. One square of asphalt shingles usually costs between $50 and $150, as of June 2011. Metal roofing gives you more durability, but this shingle material costs more, ranging anywhere from $115 to $650 per square. Tiles offer a distinct look on the roof, but they also come with a high price tag.