Rafters - The Three Most Common Types

For any carpenter in residential construction that cuts roofs for a living, day in and day out, there are three types of rafters that he will constantly see. These are the common rafter, the hip/valley rafter, and the jack rafter.

The common rafter is the main rafter in any roof. The common rafter sits on top of the wall, its length meets the ridgeboard at the center of the building, making up half the span. The best example of this type of rafter is the gable roof. The gable roof is made up of nothing but common rafters. A gable roof is the easiest roof to cut and frame of any other type of roof. All common rafters are the same length from the plumb cut to the birdsmouth, and usually all have the same length tail.

The second most framed roof is the hip roof. The hip roof uses all three types of rafters mentioned here. The hip roof has no gable end to frame, but instead, has a roof that slopes to all 4 sides of the house. The main element in a hip roof is the common rafter. The amount of common rafters in a hip roof varies with the length and width of the building. For example, if a building has four equal sides (20x20, 24x24, etc.) the hip roof would resemble a pyramid and has four common rafters, one on each wall. The longer the length of the building the more common rafters there will be and the longer the ridgeboard will be.

What makes a hip roof? The hip rafter. If a building is square or rectangular there will be four hip rafters. The birdsmouth of a hip rafter sits right on the corner of the building. Its length slopes up at a 45 degree angle to where two commons meet the ridgeboard and make a 90 degree angle. The size of a hip rafter is determined by the size of the common rafter. If the common rafter is a 2x8, rule of thumb and most building codes call for the hip to be the next size up, in this case a 2x10.

A valley rafter has the same characteristics as a hip rafter except instead of originating at an outside corner, its birdsmouth sits where two walls create an inside corner. This situation is created when roofs run perpendicular to one another. These roofs slope to the valley rafter which bisects these roofs at a 45 degree angle.

Where there are hip or valley rafters, there are jack rafters. Jack rafters complete the framing on a hip roof from the hip to the top plate, starting a the common rafter down to the top plate or tail of the hip rafter. The tails of the jack rafter are cut the same as a common rafter. The plumb cut is the same pitch as a common rafter but cut at a 45 degree angle (compound angle) to match up with the angle of the hip.

With valley rafters, jack rafters usually start at a ridge and terminate at the valley. The cut at the ridge is a normal plumb cut. The cut at the valley is a plumb cut but the long point is at the bottom of the rafter and cut at a 45 degree angle (compound angle) to match the valley angle.

Mike Merisko (C) 2007

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