Common Rafter Layout Instructions

COMMON RAFTER: One running at right angles (90°) from plate to ridge. The common rafter will form the diagonal leg (hypotenuse) of a 90° triangle, with the rise and run forming the 90° angle of the triangle.

The diagram also shows correct points from which to measure. Study them and remember the picture when you are on the job. Where the arrows show | Rafter Length |, these are the lines to measure from. When your lumber is not straight, always put the crown or high side up when laying out any rafter. When laying out rafter (lets assume 5" rise), start at top end of rafter. Lay square on face of rafter, with "T" bar of square down over the edge of rafter. Pivot square to where number 5 on common scale lines up with same edge of rafter as pivot point. Keep pivot point tight against edge of rafter. Start your mark at pivot point, marking along top edge of square. This gives the top plumb cut, to fit against ridge.

Measure the rafter length along top edge of rafter. Mark another plumb cut same as above. This line represents outside wall of the building. (The same point from which you measured the width of the building). Add whatever length you want for a tail or eave to the rafter lengths given in the. Mark at end of tail on rafter is plumb cut, same as one at top end of rafter. The tails of the rafters may be cut on

The ground, or wait until rafters are all in place and mark the ends to a line and cut,whatever is the easiest. To get the Bottom or Heel cut.

VALLEY RAFTER: One running diagonally from the plate to the ridge at the intersection of gable extension with main roof.

HIP RAFTER: One running diagonally from the plate to the ridge.

Since both hip and valley rafters run at a 45º angle to the common rafter, they both represent the diagonal or hypotenuse of a right triangle; the three sides being the hip, plate and common rafter, or the valley, ridge and common rafter. Therefore, the cuts and lengths apply equally to hip and valley rafters.

You will notice the square has a separate Hip-Val Scale which must be used for either of these two rafters. But always use the same number on Hip-Val scale as you used on the common rafter scale,the number representing inch rise. The reason for the separate Hip-Val scale is that the hip and valley rafters run at 45º to the common rafter, and therefore must be longer. In, the hip rafter has a horizontal run of 17" to rise 12", while the common rafter rises 12" in only 12" of horizontal run. This requires a different angle for the plumb cuts. In, square is held on rafter and pivoted in the same manner as with a common rafter, but using the Hip-Val scale. If building is out of square, one

Hip will be cut a little shorter, depending on how great the error is. Keep longer corner at top end of hip up even with top of ridge. Keep ridge and hips well propped up until roof boards are nailed. Watch that you don't put a bow in ridge or hip while nailing other rafters to them.

To find intersection points of center of hips on ridge, leave ridge about a foot too long at point where both hips intersect the ridge. Take a regular length common rafter (such as used on main roof). Set bottom cut over edge of plate and in line with ridge. Make sure your walls are straight. Place top end of common rafter along side of the ridge, bringing top point of common even with top of ridge. Mark across top of ridge at this point. This mark is the center line of the two intersecting hips. The common rafter used to get this intersection point would be placed in the same position as the one in that comes in line with the ridge and runs underneath the little dormer on the 20'0" wall side. This way you know the rise of the hips will be the same as the rise of the common rafter on main roof. Leave the bottom ends of the hips (eave end) a little short so they will not interfere with lining up the facia boards at the corner.

JACK RAFTERS: One which does not extend from plate to ridge. Hip Jack, one running from plate to hip at 90º to plate. Valley Jack, one running from ridge to valley at 90º to ridge. Cripple Jack, one which neither touches the ridge nor plate, but runs from a hip rafter to a valley rafter at 90º to the ridge.