Repairing the sloped slab on top of your chimney

The crown is the sloped concrete or masonry slab on top of the chimney, running from the outer face of the brick to the flue liner. Its job is to shed rain away from the wall below and keep water out of the space between the liner and the brickwork. It is not the metal cover over the flue opening — that is the cap, and the two words get mixed up constantly.

Crown

The sloped slab covering the whole top of the chimney, pierced by the flue liner. Also called the wash.

Cap

The metal cover over the flue opening, usually with mesh sides, that keeps rain and debris out of the flue itself.

Mortar wash

The cheapest form of crown: mortar troweled over the top course of brick, sloped by hand, and stuck straight to the flue tile.

Overhang and drip edge

The projection of the slab past the brick face, and the groove under it that makes water fall clear instead of curling back onto the wall.

Bond break

A deliberate separation between the crown and the flue liner, and between the crown and the brickwork, so the parts move independently instead of tearing each other.

• Real slope. The surface falls from the flue outward on every side, so water runs off instead of standing around the tile.
• Overhang on all four sides. The slab projects past the brick face, so runoff leaves the chimney in the air rather than down the wall.
• A drip formed in the underside. A groove or sharp lip under that projection breaks surface tension. Without it, water tracks back along the underside to the brick.
• A square edge, not a feathered one. Material tapered to nothing where it meets the brick has no strength and breaks away first.
• Reinforced concrete rather than mortar. A cast or precast slab with reinforcement in it can carry tension. A hand-troweled wash cannot.
• Separated from the flue and the brick. The joint around the flue tile is a flexible expansion joint, and the slab sits on a bond break rather than glued to the masonry.

The most common crown on an older house is a mortar wash troweled over the top course of brick and stuck to the flue tile. It is also the least effective, because mortar has no tensile strength to spare. A thin unreinforced layer cannot resist shrinkage and temperature movement, so it cracks in a ring around the flue and in lines running to the corners. The flue expands every time you light a fire while the slab around it does not, and with no expansion joint that movement opens the joint at the tile. The feathered edge spalls off first, and with the slab flush to the brick there is no overhang, so runoff lands on the wall face and the top courses stay wet.

Two questions decide which of the three applies: is the slab still structurally sound, and does it have the geometry it needs? Sealing and coating renew a surface. Neither adds slope, overhang, or a drip edge that were never built in the first place.

1. Crack repair and joint sealing. For a sound, correctly shaped crown with narrow, isolated cracks. The cracks are opened out, cleaned, and filled with a flexible sealant, and the joint at the flue is re-formed as an expansion joint instead of being packed solid with mortar.
2. Resurfacing with a crown coating. For a crown that is structurally sound but cracked or crazed across its surface. Loose material comes off, the surface is prepared, and a brush-applied elastomeric coating is built up over the whole top. It stays flexible and bridges the existing cracks, but it does not change the geometry.
3. Full recast or replacement. For a crown that is broken through, delaminating, missing sections, flush with no overhang, or simply a mortar wash at the end of its life. The old crown comes off down to sound brick and a new slab is formed, reinforced, and cast with slope, overhang, drip edge, and bond breaks. A precast slab reaches the same result.

The crown, the cap, and the top of the flue liner are one detail, not three items. The cap keeps rain out of the flue opening; the crown keeps rain off the masonry. Neither covers for the other, and on a chimney whose crown has no overhang a small bolt-on single-flue cap can make things worse by collecting rain and discharging it onto the defective crown. SaverSystems offers that as a rule of thumb, not a code requirement.

1. 1

   Break out the failed crown

   The old slab comes off down to sound brick, with the flue opening covered so nothing drops into the flue or firebox.

2. 2

   Set the bond breaks

   Compressible material is wrapped around the flue tile and a slip layer laid over the top course of brick, so the new slab is not fixed to either. This step decides whether the crown cracks again.

3. 3

   Form and reinforce

   Forms are set to give a projection past the brick on all four sides, with a square edge and a drip formed into the underside. Reinforcement is placed within the slab.

4. 4

   Cast and slope

   The mix is struck off with a continuous fall from the flue outward on every side, leaving no low spots where water can stand.

5. 5

   Cure, then seal the flue joint

   Once cured, the joint around the flue is finished with a flexible sealant so it stays an expansion joint rather than a rigid connection.

6. 6

   Check the cap and the drip

   The cap is refitted or replaced so it sheds clear of the new crown, and the underside of the overhang is checked for a continuous drip line.

A crown gets blamed for leaks it did not cause. Order matters: repointing alone will not fix a chimney being fed water by a failed flashing, gutter, or crown, because the source has to be corrected first or the new mortar fails too.

• Flashing. A chimney has three distinct flashing zones, and the roof intersection is only one of them — the base and the area under the crown also need flashing. A stain on a ceiling near the chimney is at least as likely to be flashing as crown.
• Open mortar joints. Water entering through eroded joints below the crown produces the same interior symptoms. That is repointing, and the mortar has to be matched rather than made stronger.
• Structural cracking. Cracks wider than a couple of millimeters, cracks through multiple bricks, stepped or diagonal cracking, widespread spalled brick, and any out-of-plane lean are structural signals. The Brick Industry Association treats them as the threshold for involving a structural engineer.