Typically, the rafter length is represented as the rise and the run ratio. Meanwhile, the rise is the vertical distance from the roof peak to the base of the roof. The run is the horizontal distance from the middle joist to the building. There are three ways to find the rafter length, as seen in the table below. This slope is also present in flat roofs however, the degree is much lower than in regular roofs. The pitch refers to the roof’s slope from the roof’s peak to the nearest wall. You have to fall back to the basics of Pythagoras theorem to calculate the rafter pitch or length. How Do You Calculate Rafters on a Flat Roof? Usually, it is best to refer to the building code specific to your geographical area to avoid confusion. They are common because they are more suited to carrying different load weights. The common classes, even for flat roofs, are C16 and C24. There are about 12 different timber classes, and they always have the letter C before the number. The wood species and grade are the two main factors when grouping timber under their respective classes. The acronyms for these grades are GS and SS. The strength grade for each species is either General Structure or Special Structure. Timber is obtained from different wood species, which include redwood, spruce, whitewood, Douglas fir, and other types of softwood. Roof load is measured in Pounds per Square Foot (PSF). In addition, equipment like elevators and plumbing can contribute to dead weight.ĭead load also covers load caused by winds or snow. This weight refers to ceilings, floors, staircases, pillars, and walls. On the other hand, the dead load refers to the weight that remains stagnant for a long period. The live load can go from as low as 10 pounds per square foot to 70 pounds per square foot. This actual value refers to all anticipated weight throughout the roof’s lifespan. The American Society of Civil Engineers (ASCE) recommends that the estimated live load value exceed the actual value. Live load refers to moveable loads that is, the load is not stationary and changes position on the roof.Įxamples of this load are human weight and other equipment. Regarding roofs, there are two main types of loads – live and dead loads. However, you have to pay attention to the type of wood.įor example, the measurement for a southern pine 12-inch 2×4 rafter is 10′-4″ while its redwood counterpart is 9′-4.” #2. However, the roofing engineers can adjust these measurements as they see fit.Ĭonsidering the factors that determine the size of the rafters, it is most common to use 16- or 24-inches sized rafters.Įach rafter size can work with these roof spans. The industry standard for roof rafter span is in four dimensions: 12, 16, 19.2, and 24 inches, respectively. These factors include the building span, the timber grade used to make the rafter, and the load the roof carries. Knowing the size of rafters suitable for your flat roof depends on certain factors. There are four main rafter sizes which are 2×4, 2×6, 2×10, and 2×12. What Size Rafters Should I Use for My Flat Roof?
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