![]() When you choose the second option, you know that the rise (b) is equal to the square root of c squared – a squared, where c is the rafter length and a is the span. Take the time to set up a similar ratio or you can solve the Pythagorean Theorem equation. Now that you finally have the length of the rafter, you have two options for finding the rise. After plugging in the numbers, you get 1/40 = 1.06/x (x is the required rafter length). Now set up the following calculation: Base of triangle = base of roof. Suppose the base of your roof is measured as 40 feet. The length of the hypotenuse of this small triangle is therefore 1.06 ft. ![]() Because the length of the span and rafter are measured in feet, you’ll want to convert 12.65in to 1.06 ft. The length of the hypotenuse of this triangle is the square root of the equation a2 + b2 = 12(2) + 4(2) = 144 in + 16in = 12.65in. This is equal to a right-angled triangle with a base of 12 inches (one foot) and a rise of four inches. Don’t worry because that is still enough information to calculate the rafter length using a quick and simple ratio. ![]() If you don’t have the rise of the roof, you might know the pitch based on the manufacturers’ recommendations for the type of roofing you plan to use. This does not include the extra length required for the overhangs. For example, a roof with a span of 20 ft and a rise of 7 feet needs rafters that are the square root of 400 + 49 = 21.2 feet. Now imagine the entire truss as a pair of back to back right angles triangles.Īlready having the rise will make it easy to determine the rafter length by loading the numbers into the equation chart. This will also affect the length of the rafters. Rafter length from riseĪfter you calculate the roof span, next you’ll have to determine the rise which is based on the roofing material you choose and other design options. Pitches should in most cases never exceed 12/12 or else the roof is dangerous to walk on. For example, if you use a heavy load of asphalt shingles, a minimum pitch of 2/12 is needed for proper drainage. The best outcome of the pitch depends on the covering of the roof. A sample calculation would be: A roof that rises 1 inch for every 4 inches of its horizontal distance has a 1/4th pitch. The “pitch” (highest point) of the roof is an important ratio when measuring the truss. Keep this in mind as you are calculating the rafter length. The average roof will overhang the side walls of the home by just a small amount (12-18 inches). The rafters of the truss will be used as the hypotenuse (highest part of the triangle). The run forms the base of a right-angled triangle with the height that is equal to the rise of the roof. Half of that distance is referred to as the “run”. The roof span is the distance between the outsides of the walls that will support the roof. ![]() The first thing you’ll need to do is measure the “roof span”. We chose this equation because it allows you to reduce each truss to a pair of right angles triangles that are arranged back to back. This old equation that you may remember from middle school will finally come in handy to help you during your roof replacement journey. To calculate the measurements of this type of roof truss, it’s best to use the Pythagorean Theorem. The simplest and most easy to build roofs are usually “Open Gable Roofs”, and this is the type of roof that most homeowners have. Learning how to estimate roof trusses and their measurements may seem like a tough problem but we put together a formula to help you easily figure out your roof truss calculations. The truss is built by attaching the ends of the member to joints that connect to the intersections. The two parts of a roof truss and the “members” (straight lengths of wood) and the intersections that disperse the weight evenly down the length of each member. A roof truss is typically a triangular structure that is designed to distribute the weight of the roof. ![]() One example of a roofing part you’ll need to measure is a roof truss. Having this knowledge will help you determine where your money is going when you receive a cost estimate from the contractor you choose to work with. If you’re in the market for roof replacement, you’ll have to take the time out to educate yourself on a few different roofing components and their dimensions. When it’s time to replace your roof, the contractor isn’t the only one that has work to do. ![]()
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