## How to calculate rafter length?

The formulas we use are the following:

1. For calculations based on the roof rise:

`rise² + run² = rafter length² `,

meaning that:  `rafter length = √(rise² + run²)`.

Pitch – Is the incline of the roof represented as the ratio of rise to the span (twice the run). For example, if the roof rises 4 feet and the span is 16 feet (twice the run), then the pitch is 4/16, or 1/4.
But they do tend to space rafters in one of several industry-standard increments, typically either 12, 16 or 24 inches apart. These spacing intervals are measured “on center,” meaning that the measurement is taken from the center of one rafter’s horizontal surface to the center of the next rafter.
Maximum allowable span for a 2×6 rafter:- the maximum allowable span for 2×6 rafter is 16 feet 5 inches when spaced 12 inches apart from centre, 14 feet 8 inches at 16″ OC & 12 feet 8 inches at 24″ OC with best quality No.

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The length of the rafter is given from the long point of the ridge cut at the top down along the top edge the board to the same plumb cut on the rafter’s notch. The next result is the run. This is the horizontal measurement from the outside of the wall to the inside of the ridge.
Rafter span: this is the distance between the supporting wall and the centre of the roof measured horizontally. The span is NOT the distance between the supporting wall and the peak of the roof measured along the diagonal. Rafter: This is the lumber that runs from the centre (peak) to the supporting wall.
2-grade 2×6 joists can span up to 10 feet 9 inches from beam to beam when spaced the standard 16 inches apart with a maximum live load of 30 inches per square foot. In comparison, No. -1 grade lumber can span slightly further to 10 feet 11 inches under the same parameters.

In general, a 2×8 will span 1.5 x 8, so 12-feet. Based on all factors though, a 2×8 joist span is 7′-1” to 16′-6”, and a rafter 6′-7” and 23′-9”. In this guide, we’ll explain what span means in terms of construction, factors that impact the span, and how much weight a 2×8 can support.

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