A mortar cap, or chimney crown, that has been improperly constructed, or one that has become deteriorated over time, may be the cause of water leaking into your attic through cracks or broken mortar in the crown. When this happens, it is sometimes better to replace the crown than attempting to repair it. If your plans include the replacement of a chimney crown, and if you have never replaced one, you will likely need information on materials and tools needed, and best techniques for you to use. You will find that information below.
Step 1: Clean Old Mortar off Bricks
Chip off old mortar from the existing chimney crown, but using a hammer, chisel, and wire brush. To avoid bonding problems between old brick and new concrete, be sure to clean off all dust and debris from the old brick.
Step 2: Build Your Cap Frame
Use 2x4 lumber to make your new concrete form. With your bench saw, rip the board, leaving a 15-degree bevel, then attach the board to a 1x6 inch board, using nails and a hammer. Finally, saw this piece into four equal pieces, and nail the pieces together at the corners. This will create a form you can place on top of the chimney.
Step 3: Place the Form on the Chimney
When you have the form resting on the chimney, tighten and reinforce it with a band clamp. Be sure, when it is finished, that there are no cracks or holes the wet concrete that will be added will not seep through. If there are cracks, seal them by applying tape to the cracks. Then, when the flue expands when it is heated, the hard concrete made by the form will not crack.
Step 4: Make an Expansion Joint
Around each flue, place a flat piece of cardboard that will help keep the form from sticking to the concrete when the concrete is set and the form removed. Apply oil to the form to facilitate its release from the concrete. Without this oil, the form may not release from the concrete without breaking or chipping the cured concrete.
Step 5: Mix and Add Your Concrete
Climb down off the roof and begin mixing the concrete you will pour into the form you just created. Combine dry concrete mix, water, and light aggregate such as gravel or crushed rock. Mix the wet concrete in a wheelbarrow or other large container until it has the consistency of thick pancake dough. Then carry the wet concrete up to the roof in buckets small enough that you can carry them. Fill the form half full with concrete. Insert rebar into the concrete, with a 2-inch space left between the end of the rebar and the form. Finally pour more concrete into the form until it is filled, then use a trowel to finish the concrete and taper its edges.
Allow the concrete to set for 24 hours, then remove the form, seal flue joints with caulk, and use a waterproof coating to seal the crown.