Remove the roofing material directly behind the chimney to approximately 18 inches from the rear face of the chimney and to approximately 4 to 6 inches past each side to expose the wood roof sheathing beneath, then set the removed roofing material aside for optional reuse once the cricket is installed.
Measure the horizontal width of the rear side of the chimney.
Layout a triangle on a piece of 3/4-inch plywood with a base matching the full width of the chimney face determined in Step 2 minus 2 inches and a center height of 8 inches. Example: For a chimney that is 36 inches wide, the base of the plywood triangle will be 34 inches wide.
Cut out the triangular piece of plywood drawn in Step 3 with a circular saw.
Apply a 1/2-inch bead of construction adhesive around the perimeter edges of one side of the plywood piece cut in Step 4, then place it vertically against the back side of the chimney with the longest side down and resting where the back of the chimney meets the roof line. Make certain the short point of the triangle is centered on the chimney, then press the plywood firmly against the chimney surface to spread the adhesive.
Secure the plywood piece placed in Step 5 with three 2-inch screw-shank concrete nails into a grout joint for a masonry chimney or three 10d box nails into the framing for a wood-framed chimney. This is the vertical cricket support block.
Measure the dimension of the top edge of the cricket support block installed in Step 5, from the upper center point to right side end point. Example: For our 34-inch wide cricket support in Step 3, the top edge measurement would be 18 3/4 inches.
Layout a right triangle on a piece of 1/2-inch plywood, making one side of the 90-degree angle the measurement determined in Step 7 and the other side 12 inches. Connect the end points of both measurements to complete the triangle shape. Example: For a 36-inch wide chimney, the triangle plywood piece would have a 12-inch side and 18 3/4-inch side at 90 degrees to each other. The remaining side of the triangle would be 22 1/4 inches long.
Cut out the triangle plywood piece laid out in Step 8. This will be the right side of the cricket’s top platform.
Apply a continuous 1/2-inch bead of construction adhesive to the top right side surface of the cricket support block installed in Steps 5 and 6.
Place the triangular piece of plywood cut in Step 8 in the space behind the chimney, resting the point of the 90-degree angle on the top point of the cricket support block installed in Steps 5 and 6, with the long side of the 90-degree angle against the chimney and resting on top of the cricket support.
Attach the plywood piece to the cricket support block and the side resting on the roof sheathing with screws spaced approximately 6 inches apart.
Repeat Steps 7 through 12 for the left side of the cricket platform, keeping the 12-inch sides of the two pieces against each other, then seal the perimeter edges and ridge joint of the cricket with exterior-grade silicon caulking.
Cut a length of 3-inch metal “L” flashing to the width of the chimney, then measure the center point of the flashing piece and use tin snips to cut a square the line the edge to the bend on one side only. Example: For a 36-inch wide chimney, the flashing piece will be 36 inches wide, with the cut placed at 18 inches. The cut allows the flashing to be bent over the ridge of the cricket.
Center the flashing piece cut in Step 14 over the ridge of the cricket, with the center cut side vertical, and press the ends down onto the cricket surface to crease a center line.
Remove the flashing piece bent in Step 15 and apply a continuous 1/2-inch bead of construction adhesive to the back of the vertical sections of the flashing, then press the flashing against the chimney surface to spread the adhesive. Secure the flashing in place with a single roofing nail at each end, then place a dab of silicon caulking onto the nail heads to waterproof them.
Cut a single piece of asphalt felt wide enough to cover the cricket platform and extend onto the roof surface at both sides approximately 6 inches. Tack in place with roofing nails.
Install the roofing over the cricket and flashing, making certain to lap the ridge line of the cricket to insure water drainage.
Apply a continuous bead of silicone caulking around the top edge of the metal flashing installed in Step 16 and the existing chimney flashing around the perimeter of the chimney structure to insure a complete watertight seal.