Determine The Roof Slope Multiplier
Note: If you already know the roof slope in degrees, then all you have to do to find the roof slope multiplier is find the secant using a scientific calculator.
For example, if the roof slope angle is 45°, then sec = 1.414213. That will be your roof slope multiplier.
So, to proceed.
Once you havethe slope of the roof expressed in standard pitch, you can consider yourself in possession of the lengths of the a and b sides of a right triangle. Now you need to pull out your trusty Pythagorean Theorem and figure out the length of the hypotenuse.
A good calculator can be found at web2.0calc.com.
As Im sure you remember, a² + b² = c². So, if your slope is 6:12,
6² + 12² = c²
36 + 144 = c²
13.416 = c
So 13.416 is the length of the hypotenuse.
Now that youhave the length of your hypotenuse, 13.416, you want to think of that as the length of a line on the actual sloped roof surface over a horizontal distance of 12 inches. You want to find out how much longer that line is than the 12 inches it covers.
You will express this difference as a number, called the roof slope factor .
Once you know the roof slope multiplier for any particular slope, you can use it to find the actual area of any size roof with the same slope.
So in this case, for every 12 inches the roof goes across horizontally , while rising 6 inches , theres actually 13.416 inches of roof .
If you divide the 13.416 inches by the 12 inches , that gives you 1.118.
So What Is A Roofing Square
A roofing square is a measurement, but it has nothing to do with square feet, meters, yards, or miles. A roofing square is commonly used by roofing companies and is roughly equivalent to 100 square feet. This measurement does not change whether it is a square of shingles or a square of tarp. This is because roofing squares are used to make calculating the cost of materials and labor easier than if you had to use square feet or yards.
Why Should I Measure My Own Roof
If you plan on reroofing your own home then you will need to calculate materials so you wont waste your own time and money. If youre like most people, youll need to hire a professional.
Knowing your numbers will help you figure out what roofing materials are within your budget and having your square footage handy is a great way to protect yourself from inflated quotes.
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How Do You Calculate Roof Tile Area
How to measure your roof. Take the roof width and multiply it by the slope height. This calculation will give you half the roof area, so then simply multiply by 2 to give you the area of both sides. You now have the area of your roof in square metres.
How big is a plain tile?
Technical
below 90° at 90° 
Also, How many plain tiles are in a square Metre?
Standard plain roof tiles typically measure 265x165mm. With a minimum headlap of 65mm and a maximum batten gauge of 100mm, these tiles will give a covering capacity of 60 tiles per metre squared. Roof tiles, however, come in a vast range of shapes and sizes, and roof structures can also vary greatly.
How much tile batten do I need? Deduct the length of the gutter overhang and the length of the nib from the length of the tile. For example, if the tile has a length of 16 inches, the overhang is 3 inches and the nib is 1 inch, the result is 12 because 16 3 1 = 12. This is the distance to the next batten.
How many concrete tiles are in a square?
Typically, 68 boosted tiles per roofing square are installed. They are often placed at the roof eaves and on caps.
17.8 tiles/m² 16.3 tiles/ m²  at 282 mm gauge at 307 mm gauge 
How To Measure Your Roof Area From The Ground
A more accurate way to calculate the area of the roof than using Google Earth is to get outside to estimate the roof pitch and the base area of the property. Using these two figures, we can get a good idea of the shingle roofs square footage and estimate your asphalt shingles needs and costs. This DIY technique can be helpful, especially if you are not comfortable getting on your roof, or if you have limited access to it.
Ground measures only work well for a gabled roof, since there are usually just two main rectangular pitches to calculate area for. For more complex roof shapes, like hip roofs, youll need to work with a roofer or measure from on top of the roof itself to calculate its total area.
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Calculating Rafter Length From Rise
After measuring the roof span, the next step in designing a gable roof is to determine the rise, based on desired roofing material and other design considerations. This determination also affects the length of the roof rafters. Considering the entire truss as a pair of backtoback, rightangled triangles allows you to base the calculations on the Pythagorean theorem, which tells you that a2 + b2 = c2, where a is the span, b is the rise and c is the rafter length.
If you already know the rise, it’s easy to determine the rafter length by simply plugging the numbers into this equation. For example, a roof that spans 20 feet and rises 7 feet needs rafters that are the square root of 400 + 49 = 21.2 feet, not including the extra length required for the overhangs.
Determining The Ridge Beam Height
If you know the slope you want to use for your roof framing design, you can use it, along with the run , to determine the height of the ridge beam. For example, if the slope is 4 in 12, and the run is 12 feet, the ridge beam height will be 4 feet. The finished ridge beam height above the top of the wall will be plus the Y Height. The formulas are:
 M = / 12
 Z = M + Y
You can determine the Y Height by measuring the distance from the rafter’s bird’s mouth vertically to the top of the rafter . The formula for determining the finished height of the ridge beam is:

Ridge Beam Height above top of slab = height of wall + Z
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Measuring The Roofs Pitch
The incline or pitch of a roof can be easily measured with a level, tape measure, and a pencil. The roofs pitch is the number of inches the roof rises in 12 inches. Therefore, mark off 12 inches on the level and place it down horizontally against the roof rafter.
Next, measure vertically from the 12inch mark on the level straight up to the underside of the rafter, as illustrated below. That measurement is the number of inches the roof rises in 12 inches.
Some homes will have roofs with multiple pitches. Porches and dormer windows, for example, can have a different pitch from that of the rest of the house. For an accurate surface area measurement you should make note of these additional structures.
Using the table below, lookup the pitch/es and make a note of the number/s in the multiplier column. This multiplier number will be used to multiply the surface area calculation that SketchAndCalc will provide.
What Is A Roofing Square
Roof surfaces are measured in squares.
A roofing square is equal to 100 square feet of the roof.
To determine the number of squares on the gable roof example in this post, divide its total of 2400 square feet by 100 .
This means you would need 24 squares of shingles to cover that roof. Be sure to add 10%15% to all your material totals for trim allowance .
Pro Tip: Want to skip doing some math?Order a FREE RoofScopeX aerial roof reportto see the size of your roof in squares, as well as the slope of your roof. Keep the report on hand for future conversations with roofers or when reviewing estimates for your next roof replacement or repair.
Ready to get an estimate?
Search our list of independent roofing contractors in the Owens Corning Roofing Contractor Network to find the roofing professional thats right for you.
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Cost To Reroof A House
The total cost to reroof a house with new shingles is between $4,707 to $15,690 which includes old roof removal, new roofing materials, and professional installation. The cost of materials alone range from $3,240 to $10,800 depending on the size of your roof.
To remove the old shingles, expect to spend between $1,467 and $4,890 or from $125 and $500 per roofing square. Costs are adjusted based on the difficulty of removing and disposing of the existing type of roofing material. If your roofing professional discovers structural issues in the rafters after removing your old roof, you could have to pay an additional $1,000 to $10,000.
How To Calculate Roof Area
Before you begin work on any roofing project, its vital to have an accurate measurement of your roof area. This will allow you to work out everything from how many roof tiles you need to project cost and timeframe for completion. Thankfully, calculating your roof area is a quick and easy process once you know how. In fact, for the classic gable roof that most British homes have, you can work out the size of your roof with just two measurements.
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How Do You Measure Truss Size
Youll typically need a buddy to help you measure truss size. Place two ladders at each end of the area youll be measuring. While your assistant holds the other end, pull a tape measure along the top plate of the house to determine the length and write down the measurement in feet. To determine the width, repeat the process on the width of the top plate.
Start The Project With A Roof Area Calculator
Calculating the surface area of a sloped roof can be timeconsuming without the right tools. You want a roof area calculator that provides accurate estimates to help you order materials such as shingles, membrane, roofing, or ceramic tile.
Fortunately, if you know the roofs pitch, or have access to the homes attic to measure its incline you can arrive at an accurate roof area calculation using SketchAndCalc.
Roofing professionals can also use SketchAndCalc to provide fast accurate quotations over the phone. Experts recommend that a pitch measurement is taken before using a roof area calculator.
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How To Calculate Roof Truss Dimensions
Roofs come in many styles, but the simplest one to build not including flat or leanto roofs is probably the open gable. When properly constructed with the correct hardware, the trusses of an open gable roof evenly distribute the load of the roof and do not require any support other than the walls. To calculate truss dimensions, you can apply the Pythagorean theorem because each truss can be reduced to a pair of rightangled triangles arranged backtoback.
Sizing Roof Rafters Example 2
Now lets try another example, but this time a span will be given and you must select the correct size lumber to be used given the following scenario:
Question: When the ceiling is not attached to the rafters and where Hemfir #3 roof rafters are to be spaced 12 inches on center supporting a snow load of 30 psf and a dead load of 20 psf, what is the minimum allowable rafter size when required to span 140 in length?
First step, lets collect all the given information within the question:
 Ceiling not attached to rafters and a snow load of 30 psf: Use Table R802.4.1
 Roof Rafter Spacing: 12 inches
 Species and Grade: Hemfir #3
 Roof Rafter Span: 140
Now lets find the minimum allowable rafter size given the above scenario:
Therefore given the above example, the minimum allowable rafter size for Hemfir #3 lumber spanning 140 is a 2×10 since it can span 145. A 2×8 member for example cannot be used since it can only span 119.
To Learn more about the design and construction of wood roof framing, be sure to check out Section R802 of the 2018 International Residential Code .
Fun Tip: To learn how to easily size ceiling joists, be sure to check out this POST.
Hopefully given the above examples you can find it easier to size wood roof rafters per the Tables within the International Residential Code.
In conclusion, lets take a quick second to answer some common frequently asked questions related to roof rafters.
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Are Tile Roofs Good
Tile is weatherresistant Clay and concrete tiles are known for their durability and ability to withstand high winds, hail, and even fire. In fact, it isnt unheard of for tile roofs to last upwards of 50 years. Tile roofs are pestresistant Unlike other roofs types, tile roofs are impervious to insect damage.
Cost To Install Plywood Sheathing Or Replace Roof Decking
The average cost to replace roof decking in a 2,400 square foot home is between $1,050 and $1,575. It will take approximately 75 sheets of plywood sheathing at an average cost of $14 to $21 per sheet. A smaller 1,100 square foot home will only need around 34 sheets for an average price of $600. For roof sheathing, plywood typically comes in sheets of 4 by 8 that measure either 11/32 or 3/8 thick.
Cost to Replace Roof Decking
Home Size 

$1,122 $2,414 
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Finding The Size Of The Roof In Squares
Roofing contractors estimate projects and materials by the square, so it is crucial to find this measurement to estimate the amount of material needed accurately. A square simply refers to a 10 x 10 square of roofing. This is simply 100 square feet of roof.
To find the squares, divide the overall roof area by 100 and then round up. For instance, if the roof is 1550 square feet, then it is 16 squares.
Will A Roofer Replace The Fascia And Soffit During My Roofing Project
You may or may not need the roof framing repaired and strengthened, sheathing replaced, or soffits, facia, flashing, etc. replaced at the same time as your roof. In most cases, it is unlikely that you will need an entirely new roof with everything replaced, but certain parts of your roofing system might need repairs made to prevent leaks after the new roof has been installed.
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Roof Dimensions Google Maps
Always remember that Google Maps are frequently tilted or tipped at a slight angle. Keep in mind that using Google Earth isnt going to give you a 100 percent accurate reading but it will give you a basic idea of the size of the roof for your calculations for the roofing materials.
How do I calculate the size of my roof?
lengthroofs
landmaptellareaGoogle Maps
Can Google maps show property lines?
can view parcel boundariesview property linesGooglemap view
Garage Roof Replacement Cost
Garage roof replacement costs $1,000 to $2,500 on average, depending on the size, pitch, and materials. Installing a new roof on a 280 square foot 1car garage costs $1,000 to $2,000, and a 400 square foot 2car garage runs $1,500 to $3,200, depending if it’s asphalt or metal.
Garage Roof Replacement Cost
The garage sizes used for the table above are:
 1 car – 12 by 12
 2 car 20 by 20
 3 car 36 by 24
The prices you see locally will be more or less depending on where you live, time of year, and the pitch of the roof. These prices would be the same if you were looking for the cost of a new roof for a detached house or workshop of a similar size.
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Wood Or Cedar Shake Roof Cost
The national average for wood or cedar shake roofing is between $12,000 and $16,000, or $800 per square. The anticipated life span for most wood shake roof installations is between 30 and 40 years.
Wood shake is available in a range of thicknesses, which will affect the price as well as the protection they offer. Additional treatments are available for a class C fire rating at $0.85 per square foot, and a wood shake roof will require maintenance to clean off debris before the rainy season starts to prevent mold and moss accumulating. While they are most commonly made from cedar, followed by spruce and treated pine, you can also get synthetic cedar shake like Brava Cedar Shake from Brava which have a 50year lifespan and requires no maintenance.
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Below, there is a simple triangle, and the math used to work out how to make a proper measurement. Once you work through the basic triangle, we will review a typical roof, and how to apply some simple math to calculate how much material you will need for a roof used in our hypothetical example.
1 roof square = 100 square feet The length times the height of a triangle is twice its area . So if you divide your answer of a product of length times height by two, you will get the area of a triangle. /2 = Area
New Shingle Roof 
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In Figure above: l=30 feet h=12 feet /2 = 180 square feet
Now that the basic concept has been covered, lets take a look at a more complex roofing system. In this overhead view you see both a hipend section, and gable ends. With a more complex roof such is this one, it is highly recommended that you make a basic sketch. By doing so, it will be easier to mark your measurements, and calculate the necessary materials for the project. For this example we will break this roof up into sections labeled A through E.
In the figure B. above, a sketched portion is a basic triangle. As we did in the first example, measure the length of the eaves, and the vertical line from the eaves half way point to the peak. Multiply these numbers, and then divide the answer by two./2 = 270 square feet
S1 = /2 = 135 square feetS2 = 50 × 15 = 750 square feetS3 = /2 = 135 square feet
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