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ROOF SLOPE FACTORS

When Scaling a Roof Size From A Drawing, Multiply Roof Area x Slope Factor = Total Area


2/12 Roof Area x 1.0147
= Total Area
3/12 Roof Area x 1.031 = Total Area
4/12 Roof Area x 1.054 = Total Area
5/12 Roof Area x 1.083 = Total Area
6/12 Roof Area x 1.118 = Total Area
7/12 Roof Area x 1.158 = Total Area
8/12 Roof Area x 1.202 = Total Area
9/12 Roof Area x 1.250 = Total Area
10/12 Roof Area x 1.302 = Total Area
11/12 Roof Area x 1.357 = Total Area
12/12 Roof Area x 1.413 = Total Area

Valley & Hip Factors

1/12 1.4167
2/12 1.4240
3/12 1.4362
4/12 1.4530
5/12 1.4743
6/12 1.5000
7/12 1.5298
8/12 1.5635
9/12 1.6008
10/12 1.6415
11/12 1.6853
12/12 1.7320

 


How To Determine Roof Pitch or Roof Slope

Roof Pitch

You may often hear from a roofing contractor, an architect or a framing contractor / builder, a phrase “roof pitch”. Roof pitch is the most common way in US and Canada, to describe the roof’s basic quality – steepness. Roof can have many other attributes and shapes, but a roof pitch is often important in pricing a roof, as well as choosing the correct roofing materials.

Using a roof pitch, you can quickly and accurately calculate the roof size and amount of roofing materials needed to install a new roof. Most roofing contractors and estimators use roof pitch as the primary way to calculate roof area.

 

What is roof pitch?

Roof pitch or roof slope is a measure of roof steepness or incline, represented in inches rise of 12 inches run. For example a “3 pitch” or “3 in 12 pitch” or “3/12 pitch”, all imply that the roof rises 3 inches, for every 12 inches of it’s horizontal run.

Using a roof pitch, roof footprint, and basic geometry, you can quickly and with high degree of accuracy calculate the length of the gable sides, which will then allow you to get very precise roof area measurements.

The roof can also be measures in degrees. However, it is often difficult and unnecessary way to estimate a roof slope, while using roof pitch is simpler, faster, and often – more accurate. Nevertheless, our roof pitch calculator provides instant conversion from roof pitch to degrees and backwards, when you use it to calculate roof pitch.

 

How to measure and calculate roof pitch: 

calculating roof pitch using level and measuring tape:

   

  1. Measure 12 inches on the level. Mark the length with the marker. Many levels are equipped with a ruler on the side, but marking it will allow it to be more visible.
  2. Pull out your tape measure about 1 foot or so.
  3. Use your level to measure the roof run. While holding the tape measure with one hand, pick up the level with the other and place its bottom corner on the roof. Using the level's corner as your pivot point, hold the it parallel with the ground. Pivot it up or down until the horizontal vial's bubble is between the two lines.
  4. Measure the rise with the tape measure. Hold the level parallel to the ground and measure the distance from the roof's surface to the level's 12-inch mark. Make sure you rotate the tape so it is perpendicular to the level. Write down the rise. It is much easier to put the end of the tape on the roof and hold your hand up. If you decide to measure from the level down, you must add the length of the tape measure unit itself into the measurement.
  5. Calculate the roof's slope. The number you measured with the tape is the roof's rise -- the amount the roof's height rises over 12 horizontal inches. For example, if you measured 7 inches, the roof's ratio will be 7:12.

 

 

There are two distinctly different methods of calculation the roof pitch – one is more accurate, while the other is faster and easier. First method of calculating roof pitch involves climbing up on the roof, or at least the roof edge and actually measuring the roof pitch using a carpenter’s level and a tape measure. Second method is more of a guesstimating way of getting a quick idea of the approximate roof pitch, while standing on the ground. While not as precise, the second method allows for a quick calculation, when ladder is not available, or when you need to get/provide a quick roofing price quote / estimate or quickly estimate roof size.

 

Method One – calculating roof pitch using level and measuring tape:

For this most accurate roof pitch calculation, you will need a ladder, long enough to reach the roof edge, a 2 feet level, and a measuring tape. Use caution when climbing up the latter, and do not go onto a roof without adequate fall protection. We also recommend using a ladder stabilizer to prevent the ladder from sliding to the sides.

Once you reach the top of the ladder, position your level so that on end touches the roof surface, while the other end is in the air, and it is leveled. Now measure the distance between the roof surface straight down from the level’s opposite edge – refer to diagram below:


Method Two – Guesstimating roof pitch off the ground

This process is usually rather easy, fast and pretty accurate, but if you need to order roofing materials for a job, we still recommend getting a more accurate roof roof pitch measurement, such as in the first method.

The basic premise in guesstimating roof pitch, is knowing (measuring) the width of the roof on the gable side, as well as measuring the exposure of your siding. Most clapboard siding as well as vinyl siding has a 4″ exposure, before the next course starts. This means that every 3 runs of clapboard or vinyl siding, give you exactly one foot rise. You will need to calculate the number of runs on your siding from the base of the roof (horizontal line where the roof starts) to the tip of the roof. Most homes will have a 6″ rake board running along the roof line, so this will add about 7-9 inches to your roof rise.

Let’s look at an example in the diagram below. The width of the gable side is 24 feet, plus 3 feet of overhangs (1.5 feet roof overhang on each side). This gives us a total of 27 feet. The rise is 16 full courses of 4″ clapboard siding, which equals to 5′ 4″. Add another 8″ for the rake board, and we get a 6 feet rise.

Now we will have to do some math to actually calculate the roof pitch for both methods described above, but before we do that, I will quickly talk about measuring the rise if you have something other than a 4″ clapboard or vinyl siding.

If you have a brick exterior on your house, I would measure the distance of three courses of brick and then do similar calculations as in the example with clapboard siding. Same with a cedar shingles – measure exposure of 3 courses for more accurate total measurements of roof rise. If you have a vertical siding – well, get a ladder and make actual measurements.