Things You'll Need
Metal flashing and counterflashing
A roof cricket has nothing to do with insects, but everything to do with leakproofing around a chimney or similar vertical opening. A cricket, also sometimes called a saddle, is basically a small roof built to divert runoff water away from the upper side of a chimney. Without a cricket or diverter, water can build up behind the chimney, which acts as a dam as water runs down from a peak. Water that collects or pools behind a chimney will eventually cause the shingles, metal flashing and roof decking to deteriorate and leak.
Remove any old roofing and flashing from an existing chimney; strip it down to the basic masonry and roof decking, just like you would for a new roof. Install waterproof membrane over the roof and up the side of the chimney. Cover that with metal flashing, bent at 90 degrees, with one side over the roof and the other up the chimney.
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Determine the pitch or slope of the roof behind the chimney. Put an end of a level against the chimney, lay the other end on the roof, set it level and use a tape measure to get the height from roof to level and the distance from chimney to roof, to figure a triangle. Calculate the pitch; if the length is 3 feet and the height is 3 inches, the roof is a 3/12, sloping 3 inches per foot.
Cut a piece of 2-by-4-inch framing lumber with a circular saw to make a vertical support for a ridge beam from the top of that board to the roof. Adapt the height to the pitch of the roof; the beam board should be horizontal at the same height as the level when you figured the pitch. Screw that brace vertically to the roof framing with galvanized screws and a screw gun. Place a 2-inch face against the chimney. Angle screws from both sides. Set another length of two-by-four on top of that support, level it with a level and mark the angle of slope on the roof to make it flush against the roof decking and cut it for a ridge beam.
Fasten that ridge beam to the vertical support and the upper roof decking with galvanized screws. Measure the triangle along the sides of the cricket beam, the length up the roof, the width against the chimney and the angle between the outside of that line and the top of the cricket beam. Cut 3/4-inch plywood to fit that triangle space, for both sides. Make a template of cardboard and test it if you are not experienced at figuring and cutting roof angles.
Secure the plywood triangles to the roof decking and cricket ridge with galvanized screws. Install a waterproof membrane over the cricket sides and edges, fastened to the cricket with a construction stapler. Put metal flashing along the sides of the cricket, nailed in place to overlap seams with the roof. Put more flashing up the angled sides along the chimney. Add counterflashing on top of that, another 90-degree metal strip secured to the masonry and extending down over the top of the base flashing. Use a masonry saw to cut sloping notches in the chimney to secure the top of the counterflashing.
Cover the roof and cricket with shingles, secured with shingle nails and a hammer. Bend shingles over the peak of the cricket and across the seams on both sides. Make sure the tops of the shingles are covered with other shingles up the roof; install the crickets first before shingling up a new roof; lift the tabs on existing shingles to nail down cricket shingles on an existing roof.