A rafter cripple is one technique for repairing a roof frame that has suffered damage to the point of splitting. Cripples are typically cut from the same size dimensional lumber as the rafter they are cut to patch. A vertical brace is cut to lift the rafter into position and add support from the bottom edge. Cripples must be cut long enough to be attached to sound rafter material on either side of the split. The cripple is then attached alongside the damaged rafter to act as a splint.
Measure the damaged section of the rafter that you intend to repair. Cut a piece of lumber the same width and thickness as the rafter, 02-by-4 or 2-by-4, 24 inches longer than the damaged section of the rafter.
Cut a piece of 2-by-4 lumber to fit from the floor of the attic to the roof. Stand it upright with its bottom end on the floor and one wide face against the rafter in the centre of the damaged area. Mark the top of the joist, or the floor, and the bottom of the rafter on the 2-by-4. The joist is the horizontal framing member supporting the ceiling of the floor below. In floored attics it will not be visible. Cut the piece on the lines you just marked so it fits snugly between the joist, or floor, and the rafter.
Set the base of the cut 2-by-4 on the top edge of the joist. Tap the angled top into place under the bottom edge of the rafter with a hammer. Tap the brace toward the outside edge of the roof until the bottom edge of the rafter is straight and the split in the rafter closes up.
Nail on an angle through the top and bottom ends of the 2-by-4 brace into the joist and the rafter. Position the cripple alongside the rafter, with its top edge against the bottom face of the roof. Center it on the damaged area. Nail through the cripple into the rafter with 16d nails, one every 6 inches.