Common Rafter Terminology

Common Rafter Terminology

LOR= Length Of Rafter, this is the mathematical length, from the center of the ridge to the outside of the wall; in most cases in this manual tails are added later.

TLOR= Total Length OF Rafter, this is the total length from center of ridge to the facial cut.

LL= Line Length, also called LOR

HAP= Height Above Plate, this is the vertical measurement from the corner of the birds mouth to the top edge of the rafter. This measurement is important because wood has depth and since mathematical calculations are based on straight lines Hap should be kept consistent if possible.  Differing HAP measurements have to be accounted for when laying out and cutting rafters.

Run= ½ the span, this is the measurement most rafters are based upon.

Span= 2 x the run, the measurement from one side of the building to the other.

Shortening= the horizontal distance the rafter is shortened at the top to account for the thickness of the ridge board.   It is sometimes called the setback which is the lines you draw to accomplish shortening.

Hypotenuse = the long side of a right triangle                                      

Adjacent Side = Run                                                        

 Opposite Side = Rise                                             

/ = divided by

There is a relationship between the sides of a right triangle and the interior angle A.

Sine = opposite side divided by the hypotenuse side  or sine = o/h

Cosine= adjacent side divided by the hypotenuse side or cosine = a/h

Tangent = opposite side divided by the adjacent side or tan = o/a or rise divided by run

Arc = when ever arc is put in front of a trig function like arcsine it refers to the inverse of that  function. You may also see INV on some calculators.

Learning this limerick will help. Sine, Cosine, Tangent; Oscar / Has : A / Harry : Old / Animal

                                                                         Sine = O/H : Cosine = A/H : Tangent = O/A

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