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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