One square of asphalt shingles weighs about two pounds. So, a typical roof with 20 squares of asphalt shingles would weigh about 40 pounds. Similarly, one square of architectural shingles weighs about three pounds.  So, a typical roof with 20 squares of architectural shingles would weigh about 60 pounds. However, one square of slate shingles weighs about four pounds. So, a typical roof with 20 squares of slate shingles would weigh about 80 pounds.