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Porch Roof Beam Span Calculator

How To Calculate Rafter Length

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This rafter length calculator lets you calculate roof truss dimensions using two slightly different sets of variables. It all comes down to what information you already have. Usually, that is either the pitch or the rise of the roof. What is more, the formula used to calculate roof truss dimensions is one you probably know very well – it’s the exact same formula you use to describe a right triangle!

The formulas we used are the following:

  • For calculations based on the roof rise:
  • rise² + run² = rafter length² ,

    meaning that:

    rafter length = .

  • For calculations based on the roof pitch:
  • The formula used to calculate rafter length from roof pitch is, in fact, the same as the one above. The one difference is that we need to calculate the roof rise using the pitch first. So what you need to do first is to get the pitch, using the following formula:

    rise / run = pitch,

    expressed as percents, meaning that:

    rise = run * pitch.

    We can merge these two equations to get a single rafter length formula based on roof pitch:

    rafter length² = ² + run²

    rafter length = ² + run²)

    rafter length = run *

    If you’re unsure about the terms such as roof run and rise, refer to the illustration below:

    What Roof Pitch Is Walkable

    A roof pitch of 6/12 or less is considered easily walkable, and no extra precautions are required. A roof of pitch 7/12 is manageable, but caution should be taken. Pitches between 8/12 and 10/12 are also considered walkable, but it is not advisable and, if you have to walk on these roofs, extreme caution should be taken. Roofs any steeper than this are considered not walkable, and special equipment or scaffolding is required. Please do not take risks on roofs, as the fall can be fatal.

    Span Tables For Canada

    Knucklez said:i was just wondering if someone has a link for a clearly described span table for canada? i am thinking of an exterior porch roof for 40lb/^ft snow load and a 12′ span. want to know appropriate header size options using dimensional lumber. my guess is 2×10 sitting on top of a 6×6 PT post on each end, but wanted to confirm.thx

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    Using Floor Beam Span Tables

    Continuing on from Part 2: Wood Joist Span Tables of Residential Structural Design, we were just about to make our example house wider. Once we go beyond the allowable spans for floor joists , we will need some kind of support under these floor joists.

    This support could take the form of a structural wall. The wall could be either a properly sized structural concrete or concrete block wall or a wood framed wall. We’ll discuss these possibilities later. For now, we’ll look into supporting the floor joists with a floor joist beam.

    The widest span in the floor joist span table in Part 2 of this tutorial module showed that floor joists can span 17’2″ if they are 2 X 12s spaced 12″ o.c. . Let’s expand our house beyond that 17’2″ span capability to 24 feet wide. So the house dimensions will now be 24′ X 13′.

    It would be possible to still frame the floor in the same way as above but just run the floor joists in the opposite direction but for our example, we are going to keep the floor joists running in the same direction. Now at 24 feet, we are beyond the span capabilities of our table in our previous example. So we will have to place wood floor beams across the width of the house to support the floor joists. See the picture below for a plan view of how this will look.

    What Roof Pitch Is 30 Degrees

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    A 30° roof pitch is roughly the same as a 7/12 roof pitch. To convert from degrees to the American ratio:

  • Find the tangent of the angle, tan. This gives you the pitch of the roof.
  • Multiply the pitch by 12 to find the x in the ratio x/12.
  • Check your results with the Omni Roof Pitch Calculator.
  • Enjoy having one less thing to worry about.
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    Rafter Spacing For Porch Roof

    Rafter spacing for a porch roof depends on the distance the board must span, the loads it must carry, and the deflection factors, as well as wood species and grade. The design of the porch roof is a factor too. It could be a lean-to style, flat pergola style, gabled, or some other style or a mixture of styles. The roof may be closed to protect from the elements or finished with slats to protect from the sun but not rain, so purpose affects spacing too.

    Roofs in warm climates may only need to support light loads so may be further apart, but winds may be a factor that affects structural requirements. In regions that experience heavy snow loads, span and spacing may have other restrictions and need to be closer together. The roof finish may also affect the spacing between roof joists. Typically, porch roof rafter spacing is either 16 or 24 on-center.

    How To Use The Roof Truss Calculator

    Our tool has two applications. You can use it either as a rafter length calculator, and get the dimensions of your trusses estimated, or as a roof truss count calculator, which will also allow you to estimate the roof truss costs and even include the price of installation.

    To start your calculations, you need to choose from the two options mentioned above. In the field above the illustration, pick either “rafter length” or “truss count”.

  • If you picked rafter length:
  • Start by deciding what you want your calculations to be based on. You can choose the rise of the roof , or the roof pitch . If you want to learn more about the way it’s calculated, check out our roof pitch calculator.

  • If you picked truss count:
    • Input the roof length, the on-center spacing , and the roof truss calculator will return the number of roof trusses needed.

    • Our tool can also figure out the approximate roof truss cost. In the next fields of the calculator, decide whether you want to include the costs of installation in the calculations.

    • If you pick no, the only thing left to input is the cost of a single roof truss. The roof truss calculator will return the total expenses in the bottom-most field.

    • If you pick yes, you will also need to declare how much per time unit would the contractor installing the trusses charge you, as well as the expected duration of the installation. As soon as you fill in all the variables, you will see the total expenses at the bottom of the calculator.

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    Books & Articles On Building & Environmental Inspection Testing Diagnosis & Repair

    • Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews – InspectAPedia.
    • Manual for the Inspection of Residential Wood Decks and Balconies, by Cheryl Anderson, Frank Woeste , & Joseph Loferski, October 2003, ISBN-13: 978-1892529343,

    Sample Floor Beam Span Table

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    This table is simply a sample and may not be valid for your region.

    This span table excerpt shows two possible sizes of built-up floor beams . The full table shows more lumber sizes. It also shows the maximum that the beam can span for various numbers of such pieces of lumber built together . A 3-ply with 2 X 10s would mean that three 2 X 10s are joined together side by side with their wide sections running parallel to one another.

    The supported length shown along the left-most side of the table, is the total length of the floor joists to be supported on either side of the floor beam divided by two. In the case of our example house, the house is 24 feet wide so the house will require a total width of 24 feet of joists. Dividing by two gives us 12 feet of supported length. So we’ll read along the table row for 12 feet of supported length.

    Since the house is 13′ long, we are looking for a table entry for a built-up floor beam that can span 13 feet. We will also probably want the smallest built-up beam that will meet this requirement since this will be the least expensive one. Look at the table along the row for 12′ of supported length. You will see that a 4-ply, 2 X 12 can span a maximum of 13′ while supporting 12′ joists on either side of it.

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    What Is A Roof Pitch Multiplier

    A roof pitch multiplier, also known as a roof pitch factor, is a number that, when multiplied by the area covered by a sloped roof, gives the actual area of the roof. The math behind this number is that it is the square root of 2 + 1). Roof pitch multipliers are extremely useful in estimating the amount of material required to fit your new sloped roof, as conventional mathematics will leave you short!

    How Far Can A Double 210 Span Without Support

    The distance a double 2×10 can span without support depends on its species, grade, spacing on center, load parameters, wet or dry service condition, and its purpose. A doubled 2×10 joist has different spans and conditions than a double 2×10 beam, header, or rafter. A double #2 2×10 header spans from 3-7 to 9-2, depending on the number of stories and building widths, plus other factors. Its always best to check all factors that can affect the span, or consult a Structural Engineer.

    A double 2×10 beams span is dependent on species, grade, the spacing and span of the joists it supports, and the number of floors and load parameters. A 2 ply 2×10 beam of #2-Southern pine with a live load of 40PSF can span between 6-0 and 10-4 with a joist span between 6 and 18. One of DF-L, Hem-Fir, or SPF under similar conditions can span between 5-6 and 10-0. Shorter joist spans mean longer beam spans, as do better wood grades, but heavier loads result in shorter beam spans.

    Doubling or sistering a 2×10 joist is usually done to strengthen or repair an existing plank of the same dimensions, not extend its length. Joists may be doubled to turn an attic into a living area, to support a grand piano, or a hot tub. The species, grade, and spacing commonly affect the length, with a double #2-2×10 DF-L at 12 centers spanning 18.

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    Rafter Spacing For Metal Roof

    Metal roofing, typically steel or aluminum, comes in a variety of profiles and compositions and may be referred to as tin, steel, or corrugated metal. The width of the panel or sheet is commonly set by the manufacturer and the length by the ridge to eave measurement. However, the length can also be precut by the manufacturer and designed to overlap or interlock to keep the weather out.

    The rafter or roof joist spacing for metal roofing may need to be set to support rigid metal panels designed and engineered to fasten directly to the rafters. The rafter spacing will be set by the steel panel manufacturer to provide a structurally sound roof. Rigid metal panels are pre ordered and manufactured for the build, so lead time usually allows for rafter placing based on the panels.

    In most roofs though, the metal roofing is flimsier and is supported by and fastened to a plywood or OSB deck, or to strapping fastened to the rafters. The rafters are commonly spaced at 16 centers when sheathed to provide the greatest support. Rafters that are strapped are either at 16 or 24 centers with strapping placed every 8 to 12 on-center to provide support. The spacing of the straps usually depends on the size of the metal roofing being attached.

    How Far Can A 210 Span

    Deck beam span calculator canada

    The distance a 2×10 can span depends on the wood species, grade, moisture content, spacing, and load parameters, along with other factors. Using it for a beam, floor joist, header, ceiling, or rafter also affects the maximum span. Doubled, tripled, or thicker laminations of 2x10s increase the load capabilities and the span distances too. Span values are drawn from the 2021 IRC, but it is advisable to check with your local building department or a Structural Engineer.

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    How To Use Pitch Calculator

    To estimate roof area, enter width and length , and either Rise or Pitch second value will auto-calculate. Click calculate. The results will pop up below the calculate button, and will include: Rafter Length, Total Size / Area, and Pitch.

    Using the diagram below, measure your roof from the ground, and enter building dimensions into the calculator #2 above. Enter the span , and the rise above the base line.

    To calculate your pitch and size, measure your roof and plug in your measurements in accordance with the diagram below. Enter EITHER Roof Rise or Roof Pitch.

    Beam Span Chart Table

    The span of a beam is dependent on a few variables: The grade and species of lumber, size of lumber and the load it carries. Fewer posts on upper-level decks are typically more desirable to the occupants and this drives the use of larger framing materials for longer spans. Beam span maximums are based on a maximum anticipated live load as well as other factors. Building codes for residential decks only require 40 psf in some areas, but check your local requirements to make sure you are aware of any additional local guidelines. In addition, many residents prefer the fell of a deck that is designed for higher loads. The longer the joist, the more area of deck the joist supports, and thus the beam supports more area as well.

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    Roof Rafter Span Table

    Span is typically measured horizontally along the rafter projection from the ridge to the outer edge of the supporting outside wall plate. It is also commonly expressed as a ratio of rise to run, or a fraction of the rise over the run. The following table compares the span for common #2 grade dimensional lumber of different species and the three typical spacings between rafters. The information collected is from Table R802.4.1 of IRC-2021 and the American Wood Council.


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  • Measure the run length it is the horizontal distance between the roof ridge and the wall of the building. Let’s assume it is equal to 6 m.

  • Measure the rise of your roof. Let’s say it is equal to 1.5 m.

  • Calculate the rafter length, substituting these values into the following formula:

    rafter² = rise² + run² = 1.5² + 6² = 2.25 + 36 = 38.25

    rafter = 38.25 = 6.18 m

  • Calculate the roof pitch as the proportion of rise and run:

    pitch = rise / run = 1.5 / 6 = 25%

  • Recalculate this value into an angle:

    angle = arctan = arctan = 14°

  • Finally, you can find the roof pitch in the form of x:12.

    x = pitch * 12 = 0.25 * 12 = 3

  • The pitch of your roof is 3:12. It can also be written down as 25% or 14°.

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    How To Calculate Truss Count And Roof Truss Cost

    As you know, figuring out rafter length and other truss dimensions is not the only thing you can use this tool for! Our roof truss calculator can also help out with planning out your roof project by estimating the truss count, and showing you the costs you can expect of 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:

    Steel Alternatives To Wood Beams

    If youre building a deck, wood isnt your only option for beams. Unlike wood, steel beams are not susceptible to the elements and wont warp over time. They make for a stronger, straighter, safer, and smarter alternative to wood substructures and may be more economical in the long-run.

    Steel framing materials, are machine-made, making each beam an exact replica of the beams on either side of it. Due to their strength, fewer beams or posts are needed. This means there are fewer holes to dig and fewer posts to look at from the ground floor of your home. This makes for a more attractive, unobstructed view from your front windows.

    Another benefit to steel framing is that the material is non-flammable and many insurance companies offer a discounted rate for steel deck framing.

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    Roof Rafter Spacing Code Requirements

    The International Residential Building Code , which is the basis of most building codes in North America, identifies the standard spacing between rafters as 12, 16, 19.2, and 24 on center. The most current IRC, published in 2021, along with previous editions, provides rafter span and spacing tables for different variables in Chapter 8.

    Roof rafter spacing requirements include rafter span, live and dead loads on the roof, deflection, wind and snow loads, wood species, grade, rafter size and thickness, and whether a ceiling will be attached or not. Additional factors include slope, eave and rake overhangs, roof tie-downs, rafter support bracing, collar ties, ridge connection, roof openings, ventilation, and roof finish. Live and snow loads vary from 10 to 70psf with dead loads of 10 and 20psf and deflections of L/=180 and L/=240.

    Another important factor is location. Code requirements vary based on different climate conditions in different geographic regions. So, it is always important to check with your local building department or other building experts for rafter or roof joist spacing requirements.


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