How Thick Should a Chimney Crown Be?

What the crown does

The crown is the slab across the top of a masonry chimney, surrounding the flue tile or liner. Its job is simple and constant: shed water away from the flue and off the face of the brick, so rain and melting snow never sit on top of the chimney or soak down into it. Every other part of the chimney below it relies on that.

A crown is not the same as a cap. The cap is the metal cover with a screen that sits above the flue opening to keep out rain, animals and sparks. A chimney needs both, and the cap does not make up for a crown that lets water in.

Thickness and slope

Thin concrete cracks. A crown that is only a skim of mortar a little over an inch thick has no strength to resist freeze-thaw, the thermal movement of a hot flue or the weight of a person working on it. Common trade practice is a slab at least a couple of inches thick at its thinnest point, the outer edge, rising toward the flue so water runs outward. Wider chimneys with several flues usually call for more thickness and often reinforcement.

The slope matters as much as the thickness. A flat crown holds puddles that freeze and soak in. A crown that falls away from the flue on every side sends water off quickly, which is the whole point.

Overhang and drip edge

A good crown overhangs the brick on every side, commonly by about two inches, so water falls clear of the chimney face instead of running down it. On the underside of that overhang, a small groove called a drip edge or kerf runs parallel to the edge. Water clinging to the underside reaches the groove and drops off rather than creeping back to the brick.

Without the overhang and drip, every rain sheets water down the top courses of brick, which is why so many older chimneys show their worst spalling and open mortar joints in the top few feet.

Crown types compared

Look at the top of almost any older brick chimney and you will find one of these.

• Mortar wash: mortar troweled over the top course, flush with the brick. No overhang, little thickness, and it cracks early
• Poured-in-place crown: concrete cast in a form on the chimney, with an overhang and drip. Durable when thick, sloped and isolated from the flue
• Precast crown: a slab made off site to the chimney's size and set in place. Consistent quality, but it needs accurate measuring and bedding
• Stone or slab cap: a stone or concrete slab on piers above the flues, found on some older homes. Protects well if its joints are kept sound

The gap around the flue

The flue tile or liner gets hot and expands every time an appliance runs; the crown around it stays near outdoor temperature. If the crown is cast tight against the flue, that movement cracks the crown, the tile or both. A properly built crown has a bond break: a flexible gap, often filled with a sealant made for the purpose, between the crown and the flue, so each can move on its own.

The flue should also stand a few inches above the crown's surface. A flue cut flush with the crown lets water and debris run straight into it, and makes a proper cap fit harder.

Why so many crowns fail

In older homes, the most common problems, roughly in order, are a mortar wash that was never a real crown, crowns cast tight to the flue that cracked from expansion, flat or back-sloped crowns that hold water, and crowns with no overhang. Freeze-thaw winters and nor'easters do the rest. Once a crack opens, water gets under the crown, freezes, lifts it and runs down inside the chimney, where it rusts dampers, stains ceilings and breaks down flue tile.

Previous patching adds its own layer: a crown smeared with roofing tar or caulk usually hides cracks rather than fixing them.

What you can see from the ground

Stand back where you can see the top of the chimney, and use a phone's zoom. A crown flush with the brick, with no visible lip, is probably a wash. Dark streaks running down from the top corners, missing mortar in the highest courses, white staining on the brick, or vegetation growing at the top all point to water getting in at the crown. Inside, water in the firebox or rust on the damper can point the same way. Leave close inspection to someone with proper roof access.

Precast or poured in place

Both approaches produce a durable crown when the basics are right. A poured-in-place crown is formed on the chimney, which makes it easy to fit irregular old chimneys, several flues or a chimney that is out of square, and to shape the slope exactly. It needs forms, time to cure, and protection from rain and frost while it sets.

A precast crown is cast off site to the chimney's measurements, often with the overhang, drip edge and flue openings built in. It goes on quickly and its concrete is made under controlled conditions, but it depends on accurate measuring and a solid, level mortar bed on the brick. On a chimney whose top courses are loose or uneven, those courses are rebuilt first either way. The choice usually comes down to the chimney's shape, the number of flues and access to the roof.

Repair or replace

Fine hairline cracks in an otherwise sound, well-built crown can be sealed with a crown sealant made for that use, which flexes with the masonry. A crown that is a thin wash, is cracked through, has pieces missing, sits tight to the flue or holds water needs to be replaced. Replacement means removing the old crown down to sound brick, repairing the top courses if needed, forming or setting a new crown with proper thickness, slope, overhang and drip, and fitting a bond break around each flue.

A new crown is usually paired with a new cap and a look at the flue tops, since all three work together. Concrete needs time to cure, so crown work goes on the calendar for mild weather rather than a hard freeze.