This article was co-authored by wikiHow Staff. It might trigger traumatic memories of high school math class, but the sheer bliss in knowing that you sized your rafters correctly is well worth it. Determine the span, L, of the tributary load (1/2 of the jack rafter span) at the top of the rafter: L = 1/2 (13.71 ft)* = 6.86 ft *The clear span does not include the wall thickness of 3. . I couldnt figure out a simple way to pinpoint the center of the valley line between the rafter beam and the cross-gable ridge. Next, set the framing square on top of the rafter beam and use a pencil to trace the outside edge of the framing squares tongue. This quick demo briefly shows how to use this construction math power tool to do all your design, estimating and jobsite calculations far more quickly, simply and accurately to improve your productivity and profitability. Lengths of roofing timber almost always bow slightly. Roof length - the distance from one gable-end to the other. It can be difficult to visualize rafter plumb and side cut angles and how the rafters all fit together. It is easily understood even for the everyday person that doesnt do this work for a living. RoofCalc.org is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com. Youll also need to purchase a copy of a roofing ready reckoner: a manual with rafter length tables. You can then plug the numbers into the Pythagorean theorem formula to solve for the hypotenuse, i.e., the length of the rafter. On this one, the top and bottom cuts are plumb cuts with single bevels. A Tale of Two Pitches: Exploring Off-Angle Roof Framing The roof rafters provide integral structural support to the roof. The Rafter Length Calculator Formula: Determine Rafter Size in Minutes. The other thing you'll need is theroof pitch/angle. A ready reckoner is pretty much a 'rafter length calculator' - a book full of tables for working out the rafter lengths quickly and easily. How do you calculate the length of roof rafters when building a roof? If you already know the rafter length and want to figure out any of the other dimensions, simply input the two you know, and you will get the third one. The top of this calculator has three fields labeled "Feet", "Inches", and "Fraction". The rafter length calculator lets you calculate truss dimensions using two slightly different sets of variables. % of people told us that this article helped them. While prices did come back down stabilized, we now have a a clear picture that in most cases, cost of framing lumber has about doubled between Dec 2015 and Dec 2022, which is a 15% annual increase! rafter length = (rise2 + run2). To calculate rafter length using pitch, follow these steps: Remember that pitch is the ratio between rise and run: pitch = rise / run, or rise = run x pitch Use the rafter length formula: rafter length = (rise2 + run2) Substitute the rise from step 1 into the rafter length formula: rafter length = ( (run pitch)2 + run2) Simplify the formula: Assuming a 4:12 pitch, you would need a rafter that is 6 feet 3 inches long. Next select a the pitch the jack rafter is on. As soon as you fill in all the variables, you will see the total expenses at the bottom of the calculator. This tool will give you total number of rafters needed for the job (including ridge boards), length of each rafter, length of lumber (boards) for each rafter, as well as total cost for all materials, presented in easy to understand form. Amazon, the Amazon logo, AmazonSupply, and the AmazonSupply logo are registered trademarks of Amazon.com, Inc. or its affiliates. Adding these two measurements to the original span (blue) will arrive at the total span for the second rafter length we need. Before you cut your boards, youll need to measure the width of your building and calculate the exact length of each rafter. If more are needed, then simply start over by entering the last measurement in the longest jack rafter field. 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. This will lengthen the total length of the rafter by 6 inches (15cm), bring the new total length to 45.81 inches (116.4cm). Pressing the Escape (ESC) button will close the modal and bring you back to where you were on the page. If your rafter length is calculated to be longer than 20 feet, then there are two options for you: 1) Most lumber yards and building materials suppliers sell an actual 2 x 12 by 24 feet boards for $45-55 each. Hence, we did not even fathom to include cost of installation / demolition work it is completely outside the scope of this tool. In retrofit applications, there are 100s of external factors that affect labor cost. When building a roof, how do you calculate the length of the rafters? The bridge measure of any jack is the same as the bridge measure of a common rafter having the same unit of rise as the jack. . Become a member and get unlimited site access, including the Just remember if it is different, then the measurements of the rafters will be different and will have to be handled by repeating the steps for this side. The common roof rafter is used to calculate the roof's height and where the ridge board should be installed. By signing up you are agreeing to receive emails according to our privacy policy. Your knowledge exceeds anything that I have came across, expecially since the start of the search engines on the web, in all of my 30 years of doing construction. Calculate and mark where the birdsmouth and tail cuts need to go. Rafters are commonly spaced about 24 inches (61cm) apart. Ps. The site owner may have set restrictions that prevent you from accessing the site. Include your email address to get a message when this question is answered. I marked measurements on the plan. You can calculate the roof angle by plugging arctangent (roof slope) into a scientific calculator. 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.