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Chimney Weather Cap or Chimney Crown Construction
Details for building a masonry chimney cap

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Chimney weather cap or chimney crown construction suggestions.

This article describes the construction of the concrete crown or weather cap found at the top of masonry chimneys and flues.

This article series illustrates common chimney cap & crown types, choices, & defects, and we cite pertinent chimney top cap / crown building codes & standards for fire and other safety concerns.

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Chimney Weather Cap - Chimney Crown Construction Details

Photograph of a damaged masonry chimney.In our photo the red arrow points to the concrete chimney weather cap or chimney crown. This chimney crown was not properly constructed: too thin, wrong mix (mortar mix that deteriorated), no expansion gap or control joint at the clay flue tile, no drip overhang.

Watch out: the risks of a poorly-constructed weather cap on a masonry chimney are more than cosmetic damage: rain or frost damage to the chimney and flue tiles can make the chimney unsafe, risking a building fire or even fatal carbon monoxide poisoning.

[Click to enlarge any image]

On a masonry chimney the chimney cap is a pre-cast concrete or poured in place concrete seal around the clay flue tile (on a modern masonry chimney).

In our photo the chimney cap, also called a mortar cap, is the gray concrete visible around the projecting flue tiles at the top of the chimney.

The Masonry Institute of America, model codes, and other chimney experts call this chimney top surface seal around the flues the chimney cap or weather cap or chimney crown.

We're following their terminology, though actually I prefer to call this concrete top seal area the "chimney crown" in an effort to avoid confusion between the chimney cap (red arrow) and the chimney rain cap (blue arrow).

7 Masonry Chimney Weather Cap Construction Recommendations

Chimney top cap construction details (C) Carson Dunlop Associates

Carson Dunlop Associate' sketch shows some of the details of good chimney weather cap (crown) construction. The object of these details is to avoid water and frost damage to the flue or to the chimney itself.

Carson Dunlop Associates is a Toronto home inspection, education, & report writing company.

  1. 4-inches or more of concrete at abutment to the clay flue tile liner.

    Masons use rock wool insulation or equivalent, topped with loose stone or brick chip infill, stuffed down between the chimney top and the clay flue tile liner combined with backer rod or foam wrap both to keep the concrete from falling down into the air space between flue tiles and masonry chimney (that'll cause future chimney flue breakage) and to provide the necessary expansion gap.

    You'll see those details in our page top photo and again in a larger image below on this page.

    Watch out: be sure to use a suitable flexible backer rod or foam wrap or use adjustable steel forms to prevent direct contact between the concrete crown or weather cap and the surface of the clay flue tile liner. Else changing temperatures may crack both the concrete and the flue tile.

    Watch out: for pouring the chimney weather cap or crown, use construction-grade concrete mix, not mortar mix.

    The latter is too soft and will not weather well at the chimney top. Some masons include a reinforcing expanded wire lath mesh mid-level inside the crown for added strength.
  2. Weather cap slopes away from flue tile liner to its outer edge
  3. Weather cap extends past chimney face by 2" or more to provide a drip protection
  4. Flashing under weather cap and up the flue tile sides, including an outer bent-over drip edge to prevent capillary roll-back of water under the cap and into the masonry chimney
  5. Expansion gap: 1/4" to 3/8" gap between flue tile outer surface and face of concrete weather cap (expansion joint) to be sealed by a foam backer rod or foam wrap of appropriate width -1/4" to 3/8" (9.5mm)

    Watch out: do not use a fiberglass or other wrap around a flue tile line where it might be exposed to weather at its upper edge.

    Doing so will invite water and in cold climates frost damage to the chimney.
  6. Flexible exterior-rated sealant at open joint between concrete weather cap and flue tile liner.

    Where the foam wrap or backer rod control joint material is used between the concrete crown and the flue tile its upper edge should be weatherproof or should be sealed. I use a high-temperature high grade silicone sealant or "caulk".
  7. Rain cap (not shown in sketch) to protect the flue from water damage & protect from down-drafts

    See those details at CHIMNEY RAIN CAP / RAIN COVER INSPECTION

Question: what do I use to seal the chimney crown around the flue liner?

Bruce said:

What material that I can buy from my local Home Depot, can be used for the expansion joint around the chimney flue?

Thank you - 2019/04/26

This Q&A were posted originally at CHIMNEY CAP & CROWN DEFINITIONS

Reply:

Rutland refractory cement, furnace cement, high temperature stove & gasket cement sealants that can work around a clay flue liner - (C) InspectApedia.com

I'm unclear what we're fixing, Bruce - are you asking about a sealant or about actual chimney material?

For just sealing a metal flue into a thimble or sealing between insulated flue sections one might use refractory cement - at masonry - or a product like Rutland HI Temp Stove Gasket Cement, between metal insulated flue sections.

Reader follow-up: how to seal the expansion gap around the flue tile

Bruce said:

Hello, sorry that I was unclear. I will try again.

I need to create an expansion gap between a chimney crown and chimney flue.

I need to wrap something around the flue to create the expansion gap, but do not know the best or recommended material to use. I assume that I can then use a silicone product to seal the top of the expansion gap.
Thank you

Moderator Reply: foam backer rod or foam wrap to create expansion joint between clay flue tile and concrete chimney weather cap or crown

CRL-USALUM foam baker rod 3/8" suitable for chimney top expansion joint at concrete crown & clay flue tile - cited & discussed at InspectApedia.comOk so you want an expansion joint or gap whose top is sealed using a high temperature flexible sealant - at that height heat-resistant silicone caulk ought to be fine.

If your gap is wider than the sealant can bridge then something's off in the construction but you can perhaps fabricate or have a metal worker fabricate a collar that's sealed to the flue and to the crown.

Typically the mason either uses a 3/8" foam backer rod or she wraps a thin 1/4" - 3/8" flexible foam wrap around the flue tile liner before pouring the concrete. This is a waterproof, flexible material (not rated for high temperature) that gives a reasonable expansion gap whose gap doesn't exceed the bridging capacity of high quality exterior-use silicone sealant (caulk) or equivalent.

If you use a foam wrap instead of a backer rod between the concrete crown top and the flue tile, you'll want the foam to extend above the intended top of the concrete crown during the pour.

Once the crown has set and cured sufficiently hard and dry that the silicone sealant will bond properly, trim off the exposed foam and then apply the sealant around the flue tile.

Shown here, CRL-USALUM ALUM-EF38C-VCP-1, 3/8" Closed Cell Backer Rod produced by C.R. Laurence Co., Inc., Tel: 1-800-421-6144 Website: http://www.crlaurence.com and sold at hardware stores & building suppliers. This product is sold in thicknesses from 1/4" to 2" diameter.

Product description:

CRL Closed Cell Backer Rod will not absorb water or wick water to joint walls, which could cause adhesive failure. It is compatible with cold-applied sealants, and its round shape promotes the preferred "hourglass" shape for joint sealants. It withstands hot and cold temperature extremes from -45° to 160°F (-43° to 71°C).

Reader follow-up: using sill gasket foam or foil-faced tape around the flue tile to create an expansion joint

What about the material that is used to wrap around the flue? See the attached photo. This is not my photo, but just an example. In the photo it looks like some type of insulation. I found some sill gasket foam, but not sure if that can be used.

Wood frame to pour concrete crown or weather cap around a masonry flue - InspectApedia.com reader Bruce

Moderator Reply: details of sealing the chimney crown or weather cap to the clay flue tile

Bruce

Your photo shows Nashua 324A Cold Weather Premium Foil Tape, or foil faced insulation wrap in the same product line. In your photo this tape was probably being used in the hope that there will not be frost cracking caused by moisture between the concrete chimney crown and the top most chimney flue liner, by providing an "expansion joint" filler between the concrete weather cap and the clay flue tile.. According to Nashua's cited here, that is not an intended use.

"Nashua - 324A cold weather foil tape features a high performance acrylic adhesive and a bleached Kraft liner that resists mold and mildew growth. Ideal for permanent flexible seals. Meets UL 181 A-P standards for use as a closure material on fiberglass duct systems and UL 181 B-FX standards for use on flexible air ducts and closures Can be installed at temperatures as low as -10ºF"

Nashua 324A is intended for use as a joint sealing tape on metal HVAC system ducts and can be used on dryer ducts - it is not intended for exterior use.

In addition the product has a total thickness of 4.8 mils of which 2.1 mils is the foil facing. That means that the compressible thickness is a total of 2.8 mils.

That is a compression thickness of 0.0028 inches in an application where we normally use an expansion joint of 0.25 to 0.375 inches (1/4" to 3/8"). That is a compression thickness of only about 1/100th of what is required at a chimney top weather cap.

Watch out: Therefore in my OPINION your photo shows an incorrect use of that product. And it violates typical chimney codes:

R1003.9.1 Chimney Caps:

Masonry chimney shall have a concrete, metal or stone cap, a drip edge and a caulked bond break around any flue liners in accordance with ASTM C1283. The concrete, metal or stone cap shall be sloped to shed water.

Chimney weather cap or crown construction details (C) InspectApedia.comI'd have preferred a foam wrap or a foam backer rod as discussed below. The latter may be a bit trickier to use when pouring concrete around the flue tile.

You can read the intended applications of this foil-faced tape at

If you fake a look at the concrete chimney crown photos on this page you'll see that most of the Masons simply pour the concrete right against the clay chimney tile.

Really? Well yeah, but that, too, is not the best practice when constructing a chimney weather cap.

Often, over time you see a fine crack there and then in some climates that can lead to frost damage that could easily be avoided by use of a high-grade sealant in that location once the concrete has cured.

Chimney construction experts such as MIA, the Masonry Institute of America, recommend a simple bead of sealant at the chimney cap/crown, sealing any gap between the concrete crown and the clay flue tile liner.

In proper design it is the slope of the chimney cap away from the flue tile liner and other construction details that avoid water and frost damage to the masonry chimney top.

In the MIA reference I cite below you'll also find these additional chimney weather cap details. If you compare the chimney weather cap construction details we cite at the start of this article you'll see that this older specification is a bit incomplete by contemporary best-practices.

 

Section 5.17: chimney weather cap construction specifications

Chimney Weather Cap Construction References


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