Most roofing contractors typically measure a roof by calculating the area of a roof and then adding on 10% for possible waste. When roofing contractors are in high demand such as they are in Florida after a hurricane, this way of estimating a roof may be okay, but in a tighter market you may need to have sharper estimating skills in order to compete with your competitors and make a profit. Here I’ll show you how to get a quick, and highly accurate roof estimate on all types of roofs.
Before submitting a bid for any roofing job, you need to know what the materials will cost. In order to know how much materials you’ll need you must know the size of the roof. First you will need to measure the total area of the roof. Then you will need to measure the lengths of the ridges, rakes (gables), hips, valleys, & eaves. If the job is a new construction roof installation you may be able to get the measurements from the blueprints; however, if the roof is a roof replacement or roof repair then you will need to measure the roof yourself.
Many roofers start to measure a roof by measuring the eaves. On a gable roof you will only need to measure in one direction. On a hip roof you will need to measure in two directions.
After you have the measurements for the eaves, you will need to measure the width of the roof. To measure the width on a gable roof you will need to hook your measuring tape to one of the eaves and run it all the way over to the opposite eave. You will use the same method for measuring the width of a hip roof.
After you have measured the width of the roof you will need to measure the length of the roof. To measure the length of a hip roof you will need to hook your measuring tape up to the eave at the ridge rafter, run it up all the way up the ridge rafter, over the length of the entire ridge, and then back down the opposite ridge rafter down to the opposite eave. You can measure the ridge while you are getting the length of the roof. If you get the length of a gable roof by simply measure the ridge. On a gable roof the ridge and the length will be the same measurement.
After you measure the length of the roof you will need to measure the valleys and hips. You will need to get these measurements so that you know how much ridge cap (hip-covering) and valley metal.
When measuring a roof some measurements need to be more accurate than others. For example, you could be a little off with the measurement of the ridge, hip, or valley and it wouldn’t make much of a difference in your expenses. However, if you don’t get accurate measurements of the width or length of the roof it could cost you more in materials than you estimated if you underestimate, or it could cost you the job if you overestimate and therefore bid an overbid price. If you were to measure a roof 100 feet by 120 feet that was actually 101 feet by 121 feet the difference would 221 sqft which is 2.21 squares of roofing materials which could be several hundred dollars.
When measuring a roof you must ALWAYS draw the layout of the roof. Your roof layout sketch must include the dimensions, slopes, obstacles and penetrations, and any other unusual situations such as areas of soft spots, rotten wood, and anything other obstacles.
Once you have all your measurements you will use them to calculate your areas, slopes, angles, and allowance factors. Let’s go ahead and do an example of a simple roof.
How to Measure a Flat Roof
By looking at the roof plan above you can see the actual dimensions. A flat roof with this design is probably just about the easiest design to measure. Obviously not all roofs are going to be this simple, but this is at least a good start to learn how to measure a roof.
When measuring a roof like the one illustrated above you are going to need to divide it into rectangles as we did in Image 1.2. You will then calculate the area of the rectangles and add them together. This is known as the positive roof measuring method. So you will find the area of section A, B, & C, and the add them together to get the area of the entire roof. To find the area of a level rectangle you will multiply the length by the width.
Area of a level rectangle = Length x Width
The Positive Roof Measuring Method
The positive roof measuring method requires you to break the roof up into measurable sections. For image 1.2 find the area of all sections of the roof, and then add them together to find the total area of the roof.
Area of section A = 20′ x 60′ = 1200 sqft
Area of section B = 20′ x 40′ = 800 sqft
Area of section C = 20′ x 20′ = 400 sqft
Therefore, the total area of the roof plan above is 1200 sqft + 800 sqft + 400 sgft = 2400 sqft
The Negative Roof Measuring Method
To find the area of a roof using the negative roof measuring method you will extend the roof line past the roof to form a single rectangle. You will then subtract the rectangular areas that lie outside of the roof.
First you need to extend the roof lines to form a single rectangle.
60′ x 60′ = 3600 sqft extended rectangle
You then need need to find the area of the rectangles outside of the roof area. You can see in Image 1.3 that the areas outside the roof layout are sectioned off into section A and section B. We will find the areas of these sections and then subtract them from our extended rectangle.
Area of section A = 20′ x 20′ = 400 sqft
Area of section B = 20′ x 40′ = 800 sqft
The total area outside of the roof layout is 800 sqft + 400 sqft = 1200 sqft.
3600 sqft – 1200 sqft = 2400 sqft
You can see that using the negative roof measurement method gives us the same result as the positive roof measurement method.
How to Measure the Perimeter of a Flat Roof
The perimeter of a flat roof is the total length measured around the edges of the roof. The outer edges of most roofs are not simple rectangles and squares. Most roofs have recesses which complicate measuring the perimeter of a roof.
The formula to find the perimeter of a roof is:
L + W + L + W + R + R = Perimeter of a roof
2L + 2W + 2R = Perimeter of a roof
The formula for a roof without recesses is:
2(L+W) = Perimeter of a roof
Let’s try an example. Measure the perimeter of the roof in image 1.5.
The perimeter of the roof is:
2 x (25′ + 15′ + 5′) = 90 linear feet
The roof slope is defined by the rise (in inches) per 12 inches of run of the roof. For example, a “4 in 12 roof” is a roof that rises 4 inches for every 12 inches of horizontal run.
The formula to find the slope of a roof is:
Slope = Total Rise/Total Run
Roof Slope = Rise (in inches)/12 (inches per foot of run)
Roof Slope = (Total Rise/Total Run) x 12
Let’s use an example to find the slope of the roof in image 1.7.
Roof slope = 5’/10′ x 12 = 6 inches of rise over 12 inches of run.
So there are 6 inches of rise for 12 inches of run making this a “6 in 12 roof”
To find the pitch of a roof you would divide the total rise of the roof by the total span of the roof.
Pitch = Total Rise/Total Span
Slope = 2 x Pitch
Usually roof pitch is used by framers but sometimes that is all the roofer has. You can convert the roof pitch into the roof slope. Here is how.
(2 x roof pitch) = slope
multiply the slope by 12 and you have the roof slope.
Convert a 1/3 pitch into roof slope.
Slope = (2 x (1/3))
Roof Slope = (2/3) x 12 = 8
Therefore, the roof is an “8 in 12 roof”
How to Measure the Slope of a Roof
To measure the slope of a roof you will need a level and a measuring tape. You will mark your level 12 inches from one of the ends. Hold that end on the roof and bring your level up until the level bubble is centered. Now measure the distance between the roof and the 12 inch mark on your level. This measurement is the rise over 12 inches. If the distance between the 12 inch mark on your level and the roof is 5 inches then the roof is a “5 in 12 roof” if the distance is 4 inches then it is a “4 in 12 roof” and if the distance is 3 inches, then the roof is a “3 in 12 roof”.
Roof rafters are the inclined part of the roof frame that support the roof sheathing and decking.
Common Rafter – A common rafter extends perpendicularly from the outside wall to the ridge board.
Ridge Rafter – A rafter that runs parallel to the ridge rafter from the edge of an outside wall to the end of the ridge board.
Hip Rafter – A hip rafter runs diagonally from the outside corner of a building to the end of the ridge board.
Hip Jack Rafter – A hip jack rafter runs from the edge of an outside wall to a hip rafter.
Valley Jack Rafter – A valley jack rafter runs perpendicularly from the ridge board to a valley rafter.
Measuring Rafter Length
Take a look at image 1.11. The length of the eaves edge run horizontally level to the ground. Therefore you would be able to read the actual length directly from the plans. The same can be said for the ridge board as the ridge board runs horizontal and level to the ground. However, You would not be able know the exact length of the common rafters by simply looking at the plans because the roof is sloped. The plan length of the common rafter is know as the run of the rafter. The run of the rafter is the plan length measured from the eaves edge to the ridge. See image 1.12 for a better understanding.
To determine the rafter’s actual length from the plan view or from an aerial view you can simply use a roof-slope conversion chart shown in image 1.13. The conversion chart provides you with roof-slope factors that you can multiple the plan view length (the run) by the factor. For example, if you want to figure out the actual length of a common rafter with a plan length of 10′ for a “10 in 12” roof you would simply multiply the plant length by the “10 in 12” factor for common rafters. Therefore, the actual length would be 10′ x 1.302 = 13.02′.
The factors in column 3 of image 1.13 are use to find the actual length of valleys and hip rafters using the run of the valley rafters or hip rafters. The factors in column 4 of image 1.13 are used to find the actual length of the valley and hip rafters using the plan length of the rafter. The factors in column 3 and column 4 assume the hip and valley rafters are installed at 45 degree angles. Please refer to image 1.14 for a better understanding of the run of a hip or valley versus the plan length of a hip or valley. For common rafters and jack-rafters the plan-length and the run of the rafters are the same; however; this is not the case for hip and valley rafters as you can see in image 1.14.
common rafters: run is equal to plan length
hip & valley rafters: run is not equal to plan length
Let’s try an example. Look at image 1.15 Let’s assume the roof in image 1.15 is a 5 in 12 sloped roof. What is the actual length of the common rafters running from the eaves to the ridge rafter? As you can see the run is 10′ and since the roof slope is is a 5 in 12 the roof slope factor is 1.083. The equation for the actual length of the common rafter is:
Actual Length = run x roof-slope factor
so for our example,
10′ x 1.083 = 10.83 linear feet
so the actual length of the common rafters are 10.83 linear feet
What is the actual length of the hip rafters?
The run of the hip rafters is 10′ and the roof-slope factor for hip and valley rafters of a 5 in 12 roof when multiplying by the run is 1.474. So for our example the actual length of the hip rafters are 10′ x 1.474 = 14.74 linear feet.
What is the actual length of the valley rafters? You can see the the width of the dormer is 10′. Divide the width by 2 and you have the run of the valleys. 10’/2 = 5′ so the run of the valleys is 5 linear feet. Now lets find the actual length of valley rafters. The run of the valley is 5′ and the roof-slope factor for a 5 in 12 roof when multiplying by the run is 1.474. So for our example the actual length of the valley rafter is 5′ x 1.474 = 7.37 linear feet.
How to Measure the Perimeter of a Sloped Roof
The perimeter of a hip roof or a hip and valley roof run horizontally to the ground. Take a look at image 1.16 and image 1.17 for a visual example for better understanding. You can determine the length and width by simply using the dimensions from the roof plan. Therefore, you can find the perimeter of a hip roof or a hip and gable roof by using the same formula used to find the perimeter of a flat roof.
Perimeter = 2(L + W + R)
If the building has no recesses the the formula is:
Perimeter = 2(L + W)
Find the perimeter of the roof in image 1.15
Perimeter = 2(25′ + 30″) = 110 linear feet.
When you are trying to find the perimeter of a gable roof you must also factor in the slope of the gable. Since the gable edge (knows as the rake in roofing terminology) runs diagonal from the ground the plan length is not the actual length. If you were to use the plan length to determine the perimeter of a gable roof your measurements would be short resulting in underestimating the materials needed such as eaves drip, rafters, and fascia board. To find the perimeter of a gable roof your formula is:
Perimeter of Gable Roof = 2(Length + Actual Width)
Actual Width = (W x Roof-Slope Factor)
Thus, the formula for the Perimeter of a Gable roof is:
Perimeter = 2(Length + (Width x Roof-Slope Factor))