Remove the old siding from the gable. Clear the gable wall of all nails and trim. Install Hardie trim where the gable meets the eave. Leave a 1/8-inch gap between the eave and the Hardie trim to allow for caulking after the siding has been installed.
Mark the top edge of the first course of Hardie board siding. Transfer the mark to the other side of the gable wall with the line level and a pencil. Snap a chalk line between the two points as a guide for the first course installation.
Measure down a board width minus 1 1/2 inches from the chalk line and place a mark on both sides of the gable wall. Snap a chalk line between the marks. This chalk line will be the top of a spacer block to set the angle of the Hardie siding. If you are continuing the gable siding from a lower wall, the spacer block will not be required on the gable.
Nail the Hardie board onto the gable. Keep the nails 3/4 of an inch from the edge and two inches from each corner. Hardie board is durable, but the edges and corners are fragile and can crack if the fasteners are placed too close. Place one nail per stud. Add additional studs at the gable ends if the nails do not reach the original framing stud.
Place a cut piece of Hardie board along the gable wall and over a length of Hardie siding. Where the cut piece crosses the full length, draw a line. Repeat the process for the other side of the gable. Measure the distance between the gable's eaves, deduct 1/4 of an inch and cut the siding to length.
The pitch of the roof will remain constant and you can use the cut pieces as a pattern to cut the rest of the Hardie siding. To use the cut pieces, you will need to snap a chalk line for each course of Hardie board siding and measure for each cut.
Caulk all the seams in the Hardie board siding to prevent water from penetrating and damaging the siding, the sheathing and the structural wood studs of the house. Let the caulk set for the recommended length of time. Prime and paint the Hardie gable siding to seal the board.