How to calculate length of roof rafters and pitch of roof, hi guys in this article you know about length of rafters, span of roof, rise of roof, run of roof and pitch of roof and also know about maximum and minimum length of roof rafters.
A rafter is a structural component build of wood or steel that is used as part of a roof construction. Rafter runs from the ridge or hip of the roof to the wall plate of the external wall. Rafters are generally provide in series, side by side, providing a plateform to support roof decks and roof coverings.
Length of roof rafters depending on climatic condition, style, slope and type of structure. It is used over brick external wall, wall plate, over two support and over top of beam.
The first step for deciding to figure out length of roof rafters in conventional roof framing is to layout and cut a common rafter pattern. The length of rise and run of the common rafter will determine the height of the ridge board as well as the length of any hip / valley rafters that may be involved with a conventionally framed roof.
Conventional framing of roof rafters falls under the heading of rough framing carpentry. Anyone who has ever tried framing a roof can tell you it is a fine tuned skill that can only be acquired with experience.
Let us now discuss some common terms are used in figure out length of roof rafters.
● 1) Rafters of roof :- distance between the top ridge on roof or hip of the roof to the wall plate of external wall.
● 2) span of roof :- span of roof is the clear distance between the two supporting arc,beam, external brick wall or beam truss.
● 3) Run of roof :- in rafter roof framing Run of roof is horizontal distance from external wall outside plate to the point which are directly below centre of Ridge. In general term run of root is half of span of roof. Run of roof = span of roof/2.
● 4) Rise of roof :- it is vertical distance between top of ridge to the centre of clear span between two support.
● 5) pitch of roof :- it is the inclined angle formed between Rafters of roof and run of roof. Which of roof is calculated by number of inches it Rises vertically for every 12 inches in extend horizontally. for example a roof that Rises 6 inches vertically for every 12 inches of horizontal run. So pitch of roof is equal to = rise/run = 6/12.
There is no standard value of pitch of roof, its value is depending on varying condition like climatic condition sloping areas height of a structure, location, style and culture climate.
The most common pitch of roof ranging between 4/12 to 8/12 feet.
How to calculate length of roof rafters
Look at the figure, considering span of roof = 30 feet, pitch of roof = 5/12, so Run of roof is equal to half of span = span/2 =30/2= 15 feet.
And we know that pitch = Rise/Run = 5/12 = Rise/Run, hence Rise = (Run ×5)/12 = 15×5/12 = 6.25 feet, so Rise of roof = 6.25 feet.
Now how to figure out the length of roof rafters, by Pythagoras Theorem we know that c2 = a2 + b2, c = √ a2 + b2, we have a = Run = 15 feet, Rise = b = 6.25 feet and c = roof rafters = ?
Now Rafters = √ 15^2 + 6.25^2
Rafters length = √ 264.0625 = 16.25 feet
hance length of roof rafters = 16.25 feet.
Calculate Roof Rafters length using angle
Look at the given picture, we have Rise = 6 ft ,angle of pitch = 20° and length of roof = 30 feet.
In right angle triangle we have c = rafters = hypotenuse, a = opposite side = Run and b = adjacent side = Rise
We know sinθ = opposite/ hypotenuse
Cosθ = adjacent/ hypotenuse
tanθ = opposite/adjacent
Cosθ = b/c , c = b/Cosθ = 6/Cosθ = 6.38 feet
hance length of Roof Rafters = 6.38 feet.
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