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How to use Roof Pitch Multiplier when Calculating Roof Size.

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Image of roof measuring using roof pitch

Why you should use Roof Pitch Multiplier when measuring a roof:

Measuring a roof can be both easy and very difficult. Of course it mostly depends on the roof type, but even a simple roof can be measured in many different ways. Most accurate way would be to climb up on the roof and physically measure it with a tape measure, recording the length of the gables, ridge, and possibly hips / valleys, etc.

If the roof is on a simple, one story ranch type house, with a rather shallow slope, it would be easy to do so. But what if your roof is three stories above the ground and it has a 12 pitch, and best of all it is a slate roof (which is extremely slippery). I would not climb on that roof without a full fall protection system already set up for me, and even then, I would only climb that roof if staging was already installed.

Roof pitch multiplier make your job so much easier, because you can get nearly exact roof measurements by just standing on the ground.

When and who can use Roof Pitch Multiplier

Measuring the roof with pitch multiplier can be be done by both home owners and roofing contractors alike. If you are a homeowner, you can quickly measure your roof to see how much your roofing contractor is charging you per roofing square (100 sq. ft.) If you are a roofing contractor, you can use this method to quickly estimate roof size, calculate your costs, amount of roofing materials you will need for the job, and provide the homeowner with a roofing estimate, without taking much time, which greatly increase the probability of a sale – remember, people are impatient and want answers now.

Roof Pitch Multiplier Table:

The table below was calculated using simple Pythagorean Theory, which we describe in our roof pitch guide. Basically we did the homework for you. Basically, each roof pitch measurement corresponds to a multiplier next to it.

3 pitch – 1.03 4 pitch – 1.06 5 pitch – 1.085
6 pitch – 1.12 7 pitch – 1.16 8 pitch – 1.21
9 pitch – 1.25 10 pitch – 1.31 11 pitch – 1.36
12 pitch – 1.42

How to use pitch multiplier:

To accurately calculate a roof, you will need to measure roof length and width, and already have the roof pitch measurement. Once you have all that, multiply the roof length by roof width, and then, multiply the result by a pitch multiplier.

For example, we have a roof which measures 56 x 29 feet, and has a roof pitch of 8. Here is the math:

56 x 29 x 1.21 = 1965 square feet or 19.65 roofing squares.

This roof size will be the same, whether you have a hip or a gable roof. However the roofing cost and the amount of roofing materials will be higher for a hip roof, because of increased waste and more labor involved in hip roof installation.

You can also use the pitch multiplier to calculate the length of the gable / rake. Just take the width of the house, multiply it by pitch multiplier and divide by 2. Using the example above, here is the math:

(29 x 1.21) / 2 = 17.5″ – length of each gable

It’s that simple – you can measure your roof in a matter of minutes and even without a ladder. Here are some things you should know when measuring a roof:

Always add overhangs to your roof measurements. Some homes have overhangs as wide as 2 feet or more. On an average sized roof, this can add a few roofing squares to your roof size, and then you would be thinking that your contractor is overcharging you. If you are a contractor, you could be installing a roof for free, if you make a mistake like that.

A real example of mis-measuring a roof: We estimated a roof, and provided a home owner with a preliminary price quote by email, base on roof information, roof size and pictures. The roof measured about 1500 square feet. When we came out for an estimate, and physically measured a roof, it came out to be about 1900 square feet. Turns out the home owner have us a measurement of outside walls of the house, and the roof had 2.5 feet wide overhangs all around. The difference was four squares or almost 25 percent, so we had to bump up the price. We still installed that roof, and the homeowner is now one of our best references. However, telling them the price went up by a $2000 from a preliminary roof estimate was … intimidating.

Written by admin

November 30th, 2010 at 6:33 pm

Roof Pitch

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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:

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 in 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.

Calculating roof pitch:

In the first method of calculating roof pitch, everything is simple. Let’s assume that in our first method of measuring the roof pitch, we measured that the rise of the roof is 13 inches for every 24″ run. Therefore, speaking the “roof pitch” language, it is 6.5″ over 12″ – well that is our roof pitch – 6.5 in 12.

Second method is both easier and more complicated at the same time. Because the ultimate goal is to calculate roof area, we won’t really need to calculate the roof pitch (unless you need it), as now we can calculate the actual roof size.

Now that you have the measurements needed to calculate the roof pitch, we will take a quick journey in time to the high-school geometry class, and refresh our knowledge of Pythagorean Theory which says that the sum of squares of two shorter sides of a triangle, equals the square of the longer side, or A^2 + B^2 = C^2.

In the second method of measuring roof slope, our measurements were 27 feet base with 6 foot rise. Because it is essential two equal triangles, we need to divide it in two – what we get is a right-angled triangle wit 6′ and 13.5′ sides. Using Pythagorean theory, we calculate the hypotenuse or the gable side to be about 14.77 or to make it easier, 14 3/4 feet long. Now we can calculate the roof pitch, which will be a lot of fun, since our base is not 12-based number.

The formula for calculating the roof pitch is as follows:

First, to get the roof pitch, we need to bring all dimensions to a 12 base (because the roof pitch is calculated in 12 base). We take run and rise (13.5 and 6), divide them by 13.5 and multiply by 12. We get a 12″ run with 5.3333′ or 5′ 4″ rise – well – that is our roof pitch – 5’4″ in 12′.

How to calculate roof size using roof pitch (without using Pythagorean theory). Here is where roof pitch comes in handy – there is a table of multipliers for each roof pitch, which were derived using the Pythagorean theory:

Roof Pitch Multiplier Table:

3 pitch – 1.03 4 pitch – 1.06 5 pitch – 1.085
6 pitch – 1.12 7 pitch – 1.16 8 pitch – 1.21
9 pitch – 1.25 10 pitch – 1.31 11 pitch – 1.36
12 pitch – 1.42

We excluded roof multipliers for roof pitch below 3 and above 12 for the following reasons. With roof pitch under 3, you can easily walk on the roof to get exact measurements, as that is usually a flat roof. Also the multiplier is so close to one, that it does not really change the total roof size unless your roof is something like a 100×100′. The roofs with roof pitch over 12 are not very common, and besides – you can now calculate the roof pitch yourself.

The above roof pitch multipliers can be quickly used to calculate the length of a gable side of the roof when you know the roof pitch and the total width of that side. All you need to do is take 1/2 the width, and multiply it by the corresponding pitch multiplier. Same method can be quickly used the get the total roof area of a gable roof. For example, your roof footprint is 24×36′ with 7 pitch. The roof area will be 24×36 = 1224 multiplied by 1.16 = 1420 sq. ft.

Written by admin

October 25th, 2010 at 1:31 pm

Posted in Roof pitch