Chimney caps: mortar bad...concrete good

I ran into a chimney sweep who explained that the reason so many caps crack is that since the mason is up there finishing the top of the chimney and he’s got a batch of mortar and he’s all set up to mix mortar, he creates the cap out of mortar, which doesn’t last as well when exposed horizontally as it does when exposed vertically.
This sweep said that when he does a replacement, he pours all his caps out of concrete and they last a lot longer. Seems to make sense.

That is a very general statement. Of the two types of mortar (Lime and Cement) a lime mortar does not harden under water and is not as strong as cement mortar. The best type of cement mortar is a mixture of portland cement, sand, water, and a small amount of lime. And, if an air-entrainment agent is added it will have added resistance to weather deterioration. Most concrete mixes (I think) run between 3,000 & 4,000 psi but I do not know what mortar mixes are designed for. I suspect that it is not that important - the governing factor (my guess) would be its elastic properties - those gained with air-entrainment.
I hope to have my preconceived notions repaired by you experts, so please throw me a treat here.

Also, Kenton, if you head South and are nimble footed (to make it across the Freeway) you will find the US Dept. of Interior’s national “concrete research” complex - a very interesting place to visit. Maybe you can drop in on them and let us know what they have to say about this?

Hi. guys, hope I can help.

There is no way that I would approve of a Mason to give me a chimney cap made of mortar cement. It was not designed for that purpose and should be made of concrete of no less than 3000 PSI. with air-entrainment of at least 6% +/-.
Some sort of wire mesh or other should be provided and either cast in place with an overhang and drip or pre-cast and set in place.
The outline below should help in indentifying the different mortar types in use and where used.


Mortar Type
Building Segment
Exterior, above grade
Load-bearing wall
Non-load bearing wall
Parapet wall


S or M
N or S

Exterior, at or below grade
Foundation wall, retaining wall, manholes, sewers, pavements, walks, and patios
M or Nc
Load-bearing wall
S or M
Non-bearing partitions

a This table does not provide for many specialized mortar uses, such as chimney, reinforced masonry, and acid resistant mortars.

b Type O mortar is recommended for use where the masonry is unlikely to be frozen when saturated, or unlikely to be subjected to high winds or other significant lateral loads. Type N or S mortar should be used in other cases.

**c Masonry exposed to weather in a nominally horizontal surface is extremely vulnerable to weathering. Mortar for such masonry should be selected with due caution.

The table below will show the different strengths of the different type of mortars.

TABLE 4 - ASTM C270 Property SpecificationA
Mortar Type
Average Compressive Strength at 28 Days (min. psi)
Water Retention
(min. %)

Air Content
max. %

Aggregate Ratio
Not less than 2¼ and not more than 3 times the sum of the separate volumes of cementitious materials
A Laboratory Prepared Mortar Only

B When structural reinforcement is incorporated into cement-lime mortar, the maximum air content shall be 12%

Hope this helps.

Marcel:) :slight_smile: :stuck_out_tongue:

Good stuff. Thanks!

What I never see on Tile Chimneys with a cement cap is a bond break and then caulked .
Why this is required is when the cement cap is poured it sticks to the tile. When a fire is lite the Tile expands and grows .
This can break the tile or lift the cap or break the cap .
I think this a lot of the reason why we end up with cracked caps gaps in the tiles when the cool and shrink and the tiles do not always slide back down into place .
This can lead to early decay of the chimney from water penetration from above or exhaust gasses going into the gap in the tiles causing the fule acids to help decay the chimneys.

Roy Cooke

Roy, it sounds like you’re describing an expansion joint similar to what’s done around the basement floor perimeter when they pour basement slabs. Here, for that they like to use strips of Celotex.

What material should be used that will allow the concrete cap and flue tile to expand and contract at different rates but still maintain a watertight seal?

As I said in my post caulk and the Caulking I would recommend is any of the elastomeric caulking is what I would use as for a bond breaker it can be as simple as plastic or tape . I am not an expert on proper procedures just on seeing if it has been done or not almost always not.
Not a code expert so I had to look it up as I forgot from when I took my code course many years ago .Book is still almost new .

Ontario Building code 1997 is Chimney caps (3) Cast-in-Place concrete caps shall be separated from the chimney liner by a bond break and shall be sealed at that location.
Water as we all know can be our enemy .
When it freezes it can generate a pressure of 25,000 lbs per sq. inch and destroy just about any thing.

Roy Cooke

I believe Roy is trying to point out what I have offered before, and he is correct in his statements.

Hope this helps.


Great picture thanks Marcel.
What I was trying to say, it shows as a backer rod and sealant .
I do also like the flashing under the Chimney cap too bad this is not been done on any chimneys that I could see.
It is so sad to see where the Home owner has tried to water proof his Chimney Cap with Asphault . NO ,NO, No, . this breaks down in the sun and also traps water making the problem worse.
$15;00 of fixing can save a $750:00 chimney repair.
Roy Cooke

Roy; You are right about that and also noticed that it appears you may have cheap Masons in your area for that price. ($750) ha. ha.

Marcel :slight_smile: :slight_smile:

Nice, this is a good little discussion…
I’m about to start another proper, roof-up chimney rebuild next week, I’ll show you one way to form and pour it right with some pictures soon. For that area, form the flue with 1/2 plywood, (oiled if you like) then remove and seal properly (requires flue flashed to a drip edge through the brick [in galvanized, stainless or copper. -Never in aluminum] for proper water tight seal). Some customers like the concrete cap capped in sheetmetal with a drip edge as well!

For bits and kibbles, what concrete mixtures (and admixtures) do others here use for their cast in place concrete caps in North-East North America?