half diagonal thickness of ridge
use the 17" on your rafter square
3' 4"
They each have their place, but on the same style roof, trusses would be stronger.
One square covers 100 square feet, so to cover 1900 square feet of roof you would need 19 squares. However, you also need "starter shingles" and "hip and ridge" shingles, plus you need to have a few extras because of waste from cutting angles. Starter shingles go around the perimeter of the roof as a bottom layer for the first course of shingles. Hip and ridge shingles go, as you might expect, on the hips and ridges. In order to know for sure the exact quantity you would need for the entire roofing job you have to measure all of these areas.
A hip roof, or hipped roof, is a type of roof where all sides slope downwards to the walls, usually with a fairly gentle slope. A square hip roof is shaped like a pyramid. Hip roofs on rectangular houses will have two triangular sides and two trapezoidal ones. A hip roof on a rectangular plan has four faces.
A cripple jack rafter is a jack rafter which touches neither a ridge rafter or a top plate. Cripple jack rafters are often categorized as either valley cripple jack rafters or hip cripple jack rafter.
A Jack rafter is a rafter that has the pitch of the roof cut onto one end with a 54 degree angle this sort of rafter is one of several that fit onto a hip ridge. To cripple a piece of timber means to cut it partway through in order to straighten it if this is done to a wall timber it will need to be splinted to restore the strength of the stud
Acommon rafter will normally be a 2 x 6 with a 2 x 8 ridge. Extreme pitches or larger rafters will require a larger ridge.
The ridge board is typically the same dimension as the rafter or slightly smaller, depending on the design of the roof. It serves as a structural element at the peak of the roof, providing support for the upper ends of the rafters. In most cases, the ridge board is made from a 1-inch thick material, while the width may vary but often matches the rafter's width for a flush fit. This configuration helps to create a stable framework for the roof structure.
The run is on 17 not on 12 like you would use for a rafter.
Rake overhangs are not as critical to vent as are the rafter spaces above the living areas, but for the sake of appearance, the entire ridge is usually covered with venting.
It is true that no ridge board is needed if the pitch is steep and rafters are directly opposed. This is the only time a ridge board is not needed.
knee extension and hip flexion
The figures on the outer edge of the curve are degrees. Use these to set the angle/pitch of the roof. The figures on the inner edge of the curve are a ratio of the length of common rafter per foot or metre of run or span of roof. Take half the span of the roof in feet OR metres and multiply by the figure for the pitch of your roof this will give you the length of your rafter from birds mouth to the centre of the roof. Then make a deduction of half the thickness of the ridge board. at right angles to the bevel . The easiest way to use the bevels/angles from the square is to place the square on a piece of ply or board and mark all the bevels/angles onto the board and then use a sliding bevel/bevel square to apply the bevels/angles to the rafters and other members of the roof. No 1= Plumb & Seat cut for the Hip No 2=Plumb & Seatcut for Common Rafter No 3=Edge cut for the Hip (Mitre) No 4=Edge cut for Jack Rafter No 5=Side cut for Purlin No 4A=Edge cut for Purlin (Mitre) No 6 Lip cut to Purlin .This is the sloping cut which goes under the bottom edge of the Hip where the Purlin abutts to the Hip Rafter
First figure out why the rafter rotted ! It also depends on the construction methods the builder used. Some framing methods use "outriggers " to attach the barge rafter. You can tell by the nail heads in the side of the rafter. If there are a series of nails equally spaced doing down the rake of the rafter , most likely it is nailed to outriggers. You will have a couple of nails then a few feet on down you will have a couple more nails. Newer homes usually don't have outriggers. Your barge rafter is nailed to your roof decking. So the decking actually holds it up. It will be nailed also to your ridge at the top and to the sub-facia on the bottom. The barge rafter also serves as the sub-facia. You will have to remove this in order to get to the barge rafter. Usually the facia is a cedar board.
The highest point, the peak of each gable or hip.
6' minus half the thickness of the ridge plus overhang ie soffitt...