How Do We Calculate Roof Materials?

Roofing is sort of a science; mostly math. Because the roof is on an angle we need to know something more than just the square footage of the footprint. The other factor we need to know is what is called the "slope".

Slope is expressed in a ratio of X:12 where X is the inches of rise per 12 inches. So a flat roof would be 0:12 (read zero in twelve) and a high pitched roof would be 15:12. Most modern roofs are around 8:12 or 10:12 and that means we need to use the slope factor to determine the amount of roofing materials required to fully cover. Side note: you may here the word "squares" used when speaking about roofing because that is the was we price and purchase the materials. One square equals 100 square feet.

Factor This

Let's use a very simple roof with two gabled ends for our example. Let's say the roof is 50 feet long and 30 feet wide. Simple math (length x width) tells us the foot print (area of the ground covered by) this roof is 1500 square feet. We get this by the math of 50x30=1500 and those are square feet. However, if we ordered only 15 squares of roofing materials we would only be able to roof about 80% of your house. How can this be? It's because we did not use the slope factor of 1.2019 for the 8:12 pitch roof. This means the roof area, because it is on an angle, requires 20% more material than if it were flat. Therefore we need a base figure of 1813.5 square feet of roofing. Still, this isn't quite enough.

When roofing some of the shingles are used in part and not whole. It's due partially to the staggered application of the shingles. The industry standard "waste" factor for roofing is 10%. Also, we needed to calculate using the slope factor first before allowing for the 10% waste. Now we need 1813.5 plus 10% or 1994.85; almost exactly 20 squares - which is what we would order for your roofing.

There are other components to a roof than just shingles. There is also the underlayment (either tar or poly), starter shingles, roof vents, vent caps, hip, ridge, valley, flashing, drip edge, ice & valley shield, and pipe boots just to name a few. If you're roofing for yourself you also need to make sure and purchase plastic cap nails, roofing nails (we use coil gun nails), caulk, and spray paint.

We are GAF Certified!

Ask your DCI salesperson to see a home already roofed by DCI. We also have sample boards available for you to peruse for your color selection. Contact us today to have someone come give you a price for roofing your home.

Leave a Reply

Your email address will not be published. Required fields are marked *