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How To Calculate Roof Rafters

What Angle Do You Cut Roof Rafters

How to calculate the length of roof rafters (captioned)

Step 1: MARK AND CUT THE FIRST RAFTER ENDS The ridge end of the rafter, and often the eve end, will need to be cut to the angle of the roof slope. The roof on this shed is a 4 in 12 slope which makes it a 18 and one half degree angle on the board ends. A rafter or speed square will have marks for both of these angles.

Adding The Overhang Onto The End Of The Rafters To Fix The Fascia Boards To

To work out the second length of the rafter so that it runs past the brickwork and can have a fascia board fixed to it, we need to work out the overhang measurement. This will ultimately define the width of the eaves soffit board as well, so it’s important to know what you are going to use for that.

When building a roof, in some instances I will work out the exact overhang needed for the fascia and soffit boards and add that to the rafter lengths already calculated. That way you can completely finish all the cutting down on the bench before taking them up and installing them.

Instances like that include things like sheds, dormers, timber framed and other structures where there’s little chance of getting it wrong .

If I’m cutting a roof where the wall plate or brickwork aren’t perfectly straight/parallel it can be better to leave the rafter lengths too long and then cut them off after the roof is installed. You can do this by using a string or chalk line to mark them. So in that instance, I work out roughly what the overhang will be at each end of the roof and make sure the rafter tails are long enough to allow for that before cutting the rafters to length.

If you are building an extension you will probably need to match it with the existing soffit size of the main building. Otherwise, if you are building new and will be using UPVC fascia and soffit you’ll want to know the sizes that it’s available in. For this example we’ll assume the soffit will be 250mm deep.

How Do You Layout Roof Rafters

Laying Out a Common Rafter To lay out a common rafter , you need the pitch of the roof , expressed in units of rise per foot of run. Step 1: Start at the ridge. Step 2: Plumb and seat cuts complete the birds mouth. Step 3: Flip the square to mark the overhang. Step 4: Trim the plumb cut to account for the ridge board.

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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 Hem-fir #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 14-0 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: Hem-fir #3
  • Roof Rafter Span: 14-0

Now lets find the minimum allowable rafter size given the above scenario:

Therefore given the above example, the minimum allowable rafter size for Hem-fir #3 lumber spanning 14-0 is a 2×10 since it can span 14-5. A 2×8 member for example cannot be used since it can only span 11-9.

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.

Measuring And Marking The Birdsmouth And Tail Cuts

How to calculate the length of roof rafters (captioned ...
  • 1Measure the diagonal of the rafter. Start this measurement from the top of the cut you just made. When the rafter is installed, this top/long end will be at the peak of the ridge beam. Hook your tape measure on to the rafter, and measure the diagonal rafter length that you calculated earlier. Use your pencil to mark the length directly on the rafter.XResearch source
  • The diagonal measurement is the full length of the rafter, minus the overhang. Measuring the diagonal will allow you to make the tail and bird’s mouth cuts in the correct locations.
  • In this example, the diagonal measurement that you found earlier was 39.81 inches .
  • 2Position the framing square to make the birdsmouth cut. Align the framing square so that the stair gauge that youve placed on the tongue of the framing square is on the pencil mark you made to indicate the full diagonal length of the rafter. Hold the long end of the framing square so that the other stair gauge is also resting against the rafter this will ensure that you mark the tail cut at the correct angle.XResearch source
  • Use your pencil to trace the full length of the tongue along the width of the rafter.
  • 3Determine the depth of the birdsmouth cut. This is relatively simple: measure the full width of the outside wall. For most buildings, the birdsmouth cut is 4 inches deep.XResearch source The width of the 2×4 used to frame the outer wall measures 3.5 inches , and the outside sheathing measures 12 inch .XResearch source
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    Roof Framing: Key Terms And Definitions


    The rafters are the diagonal members that directly support the roof deck.

    Ridge board

    The ridge board serves as a nailing surface for the opposing rafters and provides no structural support.

    Rafter tie

    The rafter tie resists the outward forces imposed on the load-bearing walls. As the rafters sit on top of the wall plates at an angle, they exert horizontal forces on the exterior walls. Rafter ties, which often double as ceiling joists, prevent these horizontal forces from causing the walls to “pancake.”

    Ridge beam

    Some framing systems use a ridge beam combined with gable-end posts instead of a ridge board, which provides structural support to the rafters. For roofs with less than 3:12 pitch, codes require ridge beams because they reduce the extreme outward forces on the walls. A ridge beam can eliminate the need for rafter ties, which is often paramount for the cathedral ceiling aesthetic.

    Calculate Roof Rafters Length Using Angle

    Look at the given picture, we have Rise = 6 ft ,angle of pitch = 20° and length of roof = 30 feet.

    In right angle triangle we have c = rafters = hypotenuse, a = opposite side = Run and b = adjacent side = Rise

    We know sin = opposite/ hypotenuse

    Cos = adjacent/ hypotenuse

    Cos = b/c , c = b/Cos = 6/Cos = 6.38 feet

    hance length of Roof Rafters = 6.38 feet.

    How to calculate height of rise for pitch roof

    If you know about the pitch roofs angle in degrees, you can find the roof pitch by converting the angle in degrees to a slope, then finding the rise .

    slope can be figure out by finding the tangent of the degrees, assume roofs angle is 20°, such as slope = tan , so slope = tan 20° = 0.3639

    then multiply the slope by 12 to get the rise such as 0.3639 ×12 = 4.36

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    Setting Out Rafter Lengths With A Ready Reckoner

    To show how easy it is to use, let’s imagine the roof has a 3.265m total span from outside to outside of the wall plates, the ridge is 45mm thick and the pitch is 30°.

    First thing to do is to subtract the thickness of the ridge from the span of the roof so that the ridge will fit perfectly between the two common rafters. Next, divide the remaining measurement by two. This will give us the span for each individual roof rafter:

    3265 – 45 = 3220mm

    3220 / 2 = 1610mm

    Now, when we turn to the 30° page in the rafter ready reckoner and when looking up the common rafter lengths, one of the things it tells us is

    The length of rafter per meter of run is 1.1543. This means that for every 1m the rafter travels horizontally, the angled length of the roof will be 1.1543m.

    All we do now is use a calculator to multiply 1.610 by the measurement in the book:

    1.1543 x 1.610 = 1.858m

    So, the rafter lengths 1.86m long – Easy!

    Is A 3/12 Roof Pitch Good

    How To Calculate Gambrel Roof Rafters Lengths And Wall Heights Without Complicated Math Formulas

    Any roofs up to 3:12 are generally considered low slope. In the residential arena, we see lower slope roofs as part of the homes architectural design. Then, when you get into roofs of 3:12 or greater pitch, most other metal roofs can be used and that generally is the pitch requirement for asphalt shingles as well.

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    Determine The Rise And Runto Calculate The Area Of Your Porch Roof

    • Option 1: The no ladder option. Put the level against a rafter in your attic with the 12-inch mark at the bottom of one of the rafters. Measure from the end of the level up to the bottom edge of the rafter. If it measures 4 you have a 4/12 roof.
    • Option 2: Find a safe place to position a ladder next to your roof. . Pick a spot along the roof edge and hold the level until it reads “level”. Then measure down from that 12 inch mark you made straight down to the top of the roof. If it measures 6 you have a 6/12 roof, if it measures 8 inches you have an 8/12 roof.
    • Option 3: If you have a gable end on your house, place the level against the rafter on the roof’s overhang. Again, use the 12 inch mark on your level and measure up to the bottom of edge of that rafter.

    504 square foot porch

    What Is The Standard Pitch Of A Roof

    Roof pitch refers to the measurement of the slope of a roof and you express this as a ratio. Although theres isnt any standard pitch of a roof used on all kinds of sloped roofs, you can determine the range of pitches by using a roof angle calculator and by considering factors like the local climate and roofing materials.

    Express roof pitch as the ratio of the amount of the vertical rise of a roof or rise over the corresponding horizontal distance or run. You write the pitch assuming a run of 12-inches thus, the ratio would describe how much a roof rises for each foot of the run.

    As aforementioned, there is no standard pitch of a roof. But the most commonly used pitches range between 4/12 and 9/12.

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    How To Calculate Your Rafter Length

    For many homeowners, working out their roofs rafter length can be a bit of a challenge. Whether youre fitting new roof tiles, installing a new roof window, or planning a full DIY loft conversion, an accurate rafter length calculation could make or break your project. Unfortunately, whether due to lacking the right tools, working at height or your roof deck and coverings getting in the way, this is rarely as simple as it seems.

    With that in mind, the team here at Roofing Megastore has taken the liberty of doing the legwork for you. Not only will you discover a few new ways to calculate your rafter length, but weve also included some helpful tips and tricks, along with a bit of jargon-busting to make everything clearer. Just make sure youve got a calculator handy!

    How To Calculate Truss Count And Roof Truss Cost

    roof truss calculator metric roof truss calculator metric ...

    As you know, figuring out rafter length and other truss dimensions is not the only thing you can use this tool for! Our tool calculator can also help out with planning out your roof project by estimating the truss count and showing you the costs you can expect for the materials and their installation. To do so, it uses the following equations:

    • truss count = /24) + 1,

    rounded up to the closest integer .

    To calculate the costs, we use the following two formulas:

    Including installation costs:

    • total costs = truss count * single truss price + cost per time unit of work * duration of work

    Excluding installation costs:

    • total costs = truss count * single truss price

    If you’re interested in more details about calculating roof truss count, **visit our roof truss calculator. **

    If you’re unsure about the terms such as the on-center spacing, refer to the illustrations below:

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    Making The Birdsmouth And Tail Cuts

  • 1Saw along the line you marked for the birdsmouth cut. Be very precise with this cut since, unlike the plumb cut, you cant saw all the way through the board. Carefully follow the lines youve marked for the tail cut so that it maintains the specific measurements and angles.XResearch source
  • For added stability when making the birdsmouth and tail cuts, use C-clamps to clamp the rafter to a workbench or sawhorses.
  • 2Saw along the line you marked for the tail cut. Using your handsaw or circular saw, cut precisely along the line that youve marked for the tail cut. The excess rafter material will fall off as you complete this cut, leaving you with the finished rafter.XResearch source
  • 3Repeat the process for your other rafters. At this point, youve cut 1 roof rafter. Depending on the size of your structure, you will need to cut several more. All roof rafters should be identical, so follow the same steps to measure the plumb cut, tail cut, and birdsmouth cut.XResearch source
  • The number of rafters needed will be determined by the length of your roof. Rafters are commonly spaced about 24 inches apart. Refer to this chart for more detailed rafter-spacing information: .
  • Keep in mind that youll always need an even number of rafters.
  • How To Cut Roof Rafters

    This article was co-authored by wikiHow Staff. Our trained team of editors and researchers validate articles for accuracy and comprehensiveness. wikiHow’s Content Management Team carefully monitors the work from our editorial staff to ensure that each article is backed by trusted research and meets our high quality standards.There are 16 references cited in this article, which can be found at the bottom of the page. This article has been viewed 306,861 times.Learn more…

    If youre framing a gable roof on a new house, or building a shed or even a doghouse with a gable roof, youll need to cut a number of roof rafters. The roof rafters provide integral structural support to the roof. Before you cut your boards, youll need to measure the width of your building and calculate the exact length of each rafter. When you cut the rafters, youll need to make 3 separate cuts: the ridge cut at the top of the rafter, the birdsmouth cut where the rafter meets the buildings wall, and the tail cut, at the base of the rafter.

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    Basic Roof Framing Instructions

    The first step in conventional roof framing is to layout and cut a common rafter pattern. The rise and run of the common rafter will determine the height of the ridge board as well as the length of any hip / valley rafters that may be involved with a conventionally framed roof. Conventional framing of roofs falls under the heading of rough framing carpentry. Anyone who has ever tried framing a roof can tell you it is a fine tuned skill that can only be acquired with experience.

    Many a carpenter has been frustrated by this seemingly dying art. Even though conventional framing has been around for centuries, with the advent of trussed roof systems, the demand for pro roof cutter’s has decreased. The modern day truss manufacturer can build them to match nearly any conventional roof. But not always.

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    How To Calculate And Layout Hip And Valley Roof Rafters Using Math Formula And Framing Square

    Rafters can save you on delivery costs and require minimal space for assembly. Furthermore, individual framing members weigh less than fully built trusses, which means you need less muscle for the job. Rafters also allow for maximum attic space once the structure is finished.

    The roof framing process can get complicated for the uninitiated when it comes to complex roof shapes like the hip, mansard, and gambrel. However, building rafters for basic types of roofs like a gable or shed is relatively straightforward with some geometry and carpentry know-how.

    This article provides step-by-step instructions for measuring, cutting, and erecting basic gable roof rafters.

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    Calculating Rafter Size Using Your Rise And Run

    For this first method we have to take a look back at our GCSE maths, specifically geometry. As seen below, the rise, run and rafter length of our roofs actually make up the three sides of a right angled traingle. So, if we know the length of two sides of this traingle there, we can use Pythagoras Theorem to work out the length of our rafters:


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