Flashing a chimney is probably one of the most complicated part of roofing a home. Old flashing pieces can be used as templates, but keep in mind that by reroofing or tearing off the old roof, the dimensions of the flashing have changed. Chimney flashing typically consists of base flashing (this wraps around the front of a chimney), step flashing (places up the sides of the chimney), cricket flashing (diverts water from the up-slope side of the chimney) and pieces of counter flashing. This can also be called cap flashing in some instances which hangs over all other flashing pieces.
If the chimney is 24 inches or wider, build a cricket along the upper side of the chimney. This helps keep water and snow from building up in this critical area. Before flashing, complete all final roofing up to the base of the chimney and no further.
Step 1.) Adding Base Flashing
Base flashing is applied at the base of the chimney. Make your own by bending a piece of aluminum with a straightedge. After you bend the aluminum, mark where it intersects the open mortar line and bend a lip along the upper edge. Put on some gloves and cut the wings that wrap around the sides of the chimney with a tin snips. Seal the lip of the flashing in the mortar line with mortar or caulk. Overlap the roofing with the lower edge of the flashing.
Step 2.) Applying Step Flashing
Install step flashing as you shingle your roof, cement each step to the roof but not to the chimney and your roof may settle a little independently from the chimney.
Step 3.) Fitting Counter Flashing
Counter flashing is a layer that sheds water onto step flashing. With a large sheet of paper, make a template or have one made by a sheet metal shop that stair-steps down according to the mortar line. The lips of the flashing are set 1/4 inch into the mortar line. Trace the outline onto the sheet metal and cut it with your tin snips. Use the edge of a screwdriver to score the lines along which the lips bend. Use two scraps of wood to clamp and bend the lips. Set the finished flashing in place, mortaring or caulking the lops into the chimney. Do not connect the counter flashing to the other flashing pieces because the roof and the chimney are on difference foundations and thus settle independently. Do not smear the bottom pieces with caulk or roofing cement because this will attach both the roof and chimney together.
Step 4.) Adding a Cricket
A cricket is used to keep any precipitation or moisture and debris from piling up behind a chimney. The height of the cricket is one-half the width of the chiney. Crickets are made of wood that is roofed over or sheet metal exposed as flashing. Given a sketch of what is needed for your cricket, a sheet-metal shop will cut and break a cricket for you in less than a day. If you are going to make one yourself, the simple approach is to add a 2×6 frame behind your chimney. Cut the edges in 45 degree angles so they fit together like a picture frame. Once fitted around your chimney and nailed or screwed into place, add felt to the flashing and roll roofing.