Poured-in-place chimney cap
We needed a chimney cap to keep the rain from pouring down our fireplace flue, but the estimate we got to install a 2-1/2-ft. by.3-1/2-ft., 2-in. thick piece of stone atop our chimney was outrageously high. I decided to pour a concrete cap for the chimney, knowing that the primary challenge lay in getting the thing to the top of our 2-1/2-story house. My solution was to pour the cap in place, and then remove the formwork with the help of my car jack.
As shown in the drawing, I made a rectangular form with a two-piece, 1/2-in. plywood bottom and 1×3 pine sides. A crosstie held the longer opposing sides together. Near the center of the bottom I cut two U-shaped notches. They would eventually allow me to raise the cured cap enough to remove the formwork.
I supported the form with a stack of mortared bricks under each corner. Then I placed a 2×6 over the flue, and jockeyed the car jack around on it until it was under the holes in the bottom of the form. To make it easier to remove the formwork, I lined the form with polyethylene. The poly also kept the concrete from escaping through the hole in the bottom of the form.
When the form was ready, I mixed the concrete at ground level and hauled it up a series of ladders to the form. I reinforced the concrete with wire mesh, and placed anchor bolts in the wet concrete to secure the base of a lightning rod. I let the cap cure a few days under wet burlap bags, then used the jack to lift the cap enough to slip out the plywood. At the same time, I slipped a pingpong-ball sized blob of epoxy under each corner to bond the cap to the bricks. Eleven years have gone by, the cap hasn’t moved and the hearth remains dry.
John D. Hallahan M.D., Media, PA