Or 22 in the in field, and the fraction ½, the 1 in the n field and 2 in the d fieldĮnter the ridge board the same way as the overhang using the appropriate fields.Īnd the 8 for the roof pitch has only one field. ¾ inch: the 3 here is the n value, the numerator, and the 4 is the d value, the denominatorįor the overhang 22 ½" can be added to the in field as 22.5 Otherwise, enter the 44 feet in the ft field If you know what 44' 6 ¾" is in decimal, you can add this in the building width ft field The roof pitch is 8:12 (it rises 8 inches in a one-foot run).
The overhang for the rafter is 22 and ½ inches, and you are using a 1 ½ inch ridge board. The building width is 44 feet 6 and ¾ inches Use this simple top cord truss calculator to estimate your rake (rafter) length for your next project’s roof sheathing requirements. If you enter a value for the ridge board, then the calculator will subtract half of this amount from the rafter length. If you add an eaves overhang dimension, then the calculator will add the amount the rafter sticks passed the wall to the rafter length. The calculation includes results for hip/valley factor, slope factor and the roof slope in degrees. So, if the math above is just a bit much for you, check out our below tool.Calculate the length of a rafter from the roof slope ratio of inches per foot and a building width measurement. Understanding how it all works make using trig a whole lot easier, and our right angle trig calculator just needs a couple of numbers plugging in while it does the rest. Calculating roof trusses trigonometry toolĪt Leach Roof Trusses, we like to make things as simple as possible, and our trigonometry tool is a real boon for a quick calculation. Knowing the rafter length means you can work out the rise (b) is equal to the square root of c2 -a2, where c is the rafter length and a is the span. Which we can convert to feet giving us a hypotenuse, and rafter length of 1.06 feet.The hypotenuse can be determined by calculating the square root of a2 + b2 = c2 so 144 + 16 = 160,.Imagine you need the pitch of the roof to be 4/12, which equates to a right-angled triangle with a base of 12 inches and a rise of 4 inches. If you’re not sure of the rise, you can use the pitch measurement in a similar way. Example: A roof span of 20 feet, with a rise of 7 feet needs rafters that equal the square root of 400 + 49, a value of 21.2 feet.Use Pythagoras’ theorem – a2 + b2 = c2, taking a as the span, b as the rise and c as the rafter length.Simplified maximum span tables for selected visual and mechanical grades of Southern Pine lumber in sizes 2×4 thru. Click here for background information regarding design values. The table states that 2 X 6 rafters spaced 16 inches on center (o.c) can span a maximum distance of 13 feet 5 inches. Ideal pitch varies according to roofing material NOTE: The span tables listed here are based upon design values for visually graded Southern Pine dimension lumber that became effective June 1, 2013. Looking through the center of the table, we can find a couple of options. Pitch – the amount of slope a roof has, often expressed as a ratio.Overhang – most roofs have an overhang of 12 to 18 inches which must be factored into calculation.Rafter – a beam of wood angled up from the ground that forms the hypotenuse in Pythagoras’ theorem.Run – half the distance of the span and the basis for the right angle triangle in Pythagoras’ theorem.Span – the distance between the outside walls that will support the roof.Knowledge is power, so let’s start with a few key terms you’ll come across when calculating roof trusses, and how they relate to Pythagoras’ Theorem. Each truss is essentially a pair of right-angled triangles placed back to back, and so understanding and applying Pythagoras’ theorem makes calculating roof trusses a straightforward affair. The roof trusses work to distribute the roof load and mean that all additional support comes from the roof. Pythagoras’ theorem is used when building an open gable roof (which just happened to be one of the simplest types of roof to build). Whether you’re a DIYer putting up a much-needed granny flat, an owner-builder creating your dream dwelling or are project managing a new residential or commercial development, our custom-built, expertly engineered roof trusses are an essential part of your renovation project.
At Leach Roof Trusses, we’ve been supporting all manner of builders on a range of projects since 1978.