During my 31 years as a home inspector, I’ve seen a lot of houses that suffered costly water damage because of bad chimney caps. It seems that many homeowners seldom give their chimney caps a thought, and the ones who do go about the repairs all wrong. The mortar cap (or crown) on your chimney may not appear important from the ground, but it actually is critical in keeping your chimney sound and your house dry. When a cap crumbles or cracks, water can run down behind the masonry facing, bypassing the flashing where the chimney meets the roof. In fact, people often blame the chimney flashing for leaks when the water actually enters through the cap and travels between the flue tiles and the masonry facing. It usually appears as a wet spot in the ceiling or as a drip at the front of the fireplace.
Heat and water cause cracks in chimney caps. To build a cap that will last, leave an expansion gap at the flue by placing corrugated cardboard around the outside of each flue. Direct water away from the chimney with a Duplex nail beveled drip edge.
Beat the enemy
Most chimney crowns are nothing more than a thin layer of mortar spread over the top of a masonry chimney that tapers down to the outside edges. Because they are so thin and lack either reinforcement or expansion joints, they typically start to break up after only a few years.
Once water gets between the flue and the chimney surface, problems follow quickly. If the water freezes, it can pop the hard-fired surfaces off the bricks. Spalled brick not only looks bad, it also allows even more water to penetrate the porous surface. A chimney crown should be made with reinforced concrete that is at least 2 in. thick at its thinnest point. It should be arched slightly to shed water. And it should overhang the chimney so water runoff free-falls rather than runs down the surface. For added insurance, the overhang should be beveled at the bottom to create a drip edge.
Build a better crown
To replace the crown, begin by chipping off the old mortar. Eye protection and work gloves are musts; and a brick chisel with a plastic hand guard is easier on your knuckles. Clean the bare bricks with a wire brush, removing loose mortar that could cause the new cap to bond poorly.
Prepare a wooden form by ripping a 2x4 with a 15degree bevel. Nail it to a 1x6 so the square end is flush with the bottom and the short face is against the board. Use as few nails as possible until you lay out the cuts for the mitered ends. Then cut it into four sections so the 2x4s fit tightly around the top course of bricks. This will create a 2in.thick cap with a 11/2in. overhang.
Screw the sections together at the four corners and drive a couple of duplex nails through each section from the outside so they just graze the top of the bevel. Set the form over the chimney so it rests on the nails. Then reinforce it with a loadbinder, band clamp or even a tightly twisted rope.
If you spot any gaps between the form and the chimney, seal them with duct tape before you pour. Next, wrap the flue tiles with corrugated cardboard and secure it with duct tape. This will create an expansion joint so the concrete doesn’t crack when the flue gets hot and expands.
Once everything is in place, oil the outside of the cardboard and the inside of the form so they don’t stick when you strip them. Be careful not to get oil on the brick tops, or the concrete won’t bond properly.
Make the crown with small aggregate concrete mix — not mortar mix. Mix it on the ground and hoist it to the roof in half-full 5gallon buckets. The mix should be fairly stiff.
Wet the top of the chimney and pour enough concrete in the form so it fills the beveled edge and is about 1 in. above the top of the bricks. Install No. 3 rebar about 2 in. from the edges and between flues as shown. Then add the remaining concrete, slightly overfilling the form, and trowel the concrete smooth so it tapers down along the edge. In hot or windy weather, lay a sheet of plastic over the concrete and staple it to the form so the concrete will dry slower and cure harder.
Strip and preserve
The next day, strip the form by removing the clamp and the corner screws. Lift the cardboard off the flue and seal the joint with a high temperature caulk such as Polyseam seal’s HighTemp Red Silicone Adhesive Sealant. For added protection against water infiltration, seal the crown with a waterproofing coating such as Quikrete’s Acrylic Concrete Cure And Seal.
A note of caution: Depending on the slope and height of your roof, you may need to erect a level platform and wear a safety harness (fall arresting gear). The platform may be as simple as nailing a couple of roof-jack brackets under the shingles and laying a short plank across them. But it could require erecting pipe scaffolds from the ground up.
Step 1: Chip away the old cap with a brick chisel. Scour the brick with a wire brush so the new cap bonds well. Wear eye protection and work gloves.
Step 2: Assemble the form with screws and clamps. It hangs from nails driven in from the sides and rests on the top course of bricks.
Step 3: Install reinforcing rods (No. 3 rebar) midway through the pour to strengthen the concrete and prevent cracks. Place the rods along all four sides and between each flue.
Step 4: Slightly overfill the form and slope the concrete away from the flue during troweling so the cap sheds water. Cover the concrete with plastic if the weather is hot or windy.
Step 5: After removing the cardboard spacers, seal the flue joints with high-temperature caulk. Seal the concrete with a waterproofing coating.