Even experienced roofers can sometimes make mistakes in estimating the amount of roofing material that will be needed for your roofing installation. There are unscrupulous contractors who will overestimate roofing material. Being able to measure your own roof from the ground can save you hundreds or thousands of dollars. By using this roof measurement guide you can arrive at an estimate to determine how much roofing material you will need and compare it to your roofing quote.
Roofing Material is Measured in Squares
Roofing materials are measured in squares. This simplifies things by keeping the numbers smaller. One square equals an area of 100 square feet (10 feet x 10 feet). So instead of saying your house requires 2500 square feet of material, you would just say 25 squares of material are needed.
• Measuring overall area
• Determining roof pitch
Taking a ground measurement
The only tools you will need is a measuring tape. A 100 foot tape will make the job easier, as will having a helper to hold one end of the tape while you record the distance.
Start at one corner of your house and determine how much overhang your roof has. Your measurements should be taken as closely as possible directly under the outside of the overhanging roof. Work your way around the house recording the measurements from each corner to the next. You will find that a simple sketch of the house outline will be helpful.
Measurement of area is found by multiplying length by width. Most houses are not constructed of one simple square or one rectangle. They are however, almost always made up of a combination of rectangles. Take a look at your sketch and reduce the outside perimeter to simple rectangles, multiply the length by width of each and then add the sums together. This will give you the total number of square feet of area beneath your roof. Divide this by 100 and you will have the number of squares for your area.
Example: Your house may be 50 feet long and 30 feet wide. This would be length (50 feet) x width (30 feet) = 1500 square feet. Let us suppose you have a garage or another room that sticks out 20 feet from your house and is 25 feet wide. Again you would multiply length (20 feet x width (25 feet) = 500 square feet. Adding the sums 1500 + 500 = 2000 square feet. We divide that by 100 to get 20 squares.
Determining Roof Pitch
Without an instrument to measure roof pitch, you will need to estimate. To make things simpler we will divide roof slopes into 3 categories and assign each a multiplier:
This would be a roof with a pitch of 3:12 to 5:12, which means the roof rises 3 to 5 feet for every 12 feet of base horizontal length. This type of roof is normally considered walk able to make it easier to install. The multiplier would be 1.15 to 1.25. (More on what we do with this number later).
This would be a roof with a pitch of more than 5:12 up to 9:12, which means the roof rises up to 9 feet for every 12 feet of base horizontal length. This type of roof would require special equipment and installation procedures, making it more expensive as it is not walk able. The multiplier would be 1.25 to 1.4
Roofs in this category have slope that is greater than 9 feet of rise for every 12 feet of horizontal run. Multiplier variance can range from 1.4 for a simple gable roof to 1.7 and greater for a high pitch roof with many dormers and valleys. For these purposes, you can use a 1.5 multiplier for a steep slope with a large amount of dormers and valleys and get in the ballpark for an estimate. This type of roof is the most difficult and some roofers are not equipped to install them.
Using the Multiplier
Example: Let us use our earlier example of a house 50 feet long and 30 feet wide with a garage that has a base area of 2000 square feet or 20 squares. We will assume that the pitch is 4:12 and is relatively simple with 3 end walls and 2 valleys. Since this would fall under the low slope category we decide to use an average of 1.2 as a multiplier factor. Therefore 20 squares x 1.2 = 24 squares of roofing material needed.
You can see that your final estimate will be just that – an estimate. If your roof is relatively simple as the large percentage of roofs are, you can get quite an accurate estimate of the amount of material you will need. Armed with this knowledge, you can feel more assured that your roofing quote is from an experienced and reputable contractor.