Except spray foam which starts in two mixable liquids, most roofs are measured in squares. One roofing square is 100 square feet. You do need some geometry to measure areas. The formula for a side of a triangle is area equals half the base times the height. This is useful for end gables which are already triangles. However most roof sections are four sided. So first you will need to determine the length of the angled side of the roof.
You could determine the roof area with just the floor plan but that would take some trigonometry. It could be easier to just clamber upon the roof and get the distance from eave to peak. To help out, divide up pieces of the roof into triangles or rectangles. The rules for these shapes are much easier to calculate. But it is never a bad idea to ask the Denver roofing contractor to check your figures.
To determine the roof slope, you need to break out that old formula from algebra – rise over run. So just take the height of your roof from eaves to peak and put that over the distance from the center of the roof to the eaves. This is your pitch. Another way to calculate it is with a level. Set a level of known proportions (best a one foot level) set one end on the roof and make it level. Then measure the distance to the roof on the other side. That measurement in inches is your rise over 12 for a one foot ruler. Otherwise just divide. If it was 3 inches that would be expresses3/12 or 1/4.
Once you find the total area in square feet, this is divided by 100 to find the number of squares of roofing material the Denver roofer has to order. They will need more the more angles on the roof because some pieces will have to be cut to fit. Extras are also important.