Masonry Chimney Height

[Raymond Wand]

Note the height of the masonry chimney on the north west side of the house.

I pointed out to my client that I thought the chimney required bracing due to height and possibility of toppling due to high winds.

House is 23 years old, and chimney was plumb. Two flues. I guessed its height at between 12 feet to 15 feet.

I haven't a clue as to how to go about having it braced but thought it wise to reduce my risk if not called out.

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[John Kogel]

It looks like a design flaw to me. That upper storey could have gone all the way over and the chimney would then have had some moral support.

You are right to mention it at least. I say moral support because even up against a wall, there is a 2" gap between the wood and the masonry.
But if something happens, the structure would be there to limit the movement, and that is what's missing in this place. A big ugly clamp and two steel rods back to the house would be one way.
My own choice is to install a direct vent gas fireplace and a high efficiency furnace and tear the brick phallus down.

[Raymond Wand]

John

About a decade ago down the road from me a lady was sitting in her first floor den watching the boob tube. The weather was frightful, high winds and rain.

The house had a wide tall chimney which was unsupported.

The chimney toppled, it came through the roof into the den carrying the lady her recliner down into the basement. She is now in permanent repose.

[CHARLIE VAN FLEET]

RAYMOND

needs bracing if over 12 feet

cvf

[Rod Corwin]

Possibly internally braced with say re-bar? Was it built so high to help prevent smoke from drifting as easily to ground level? Surely would point it out.

[Raymond Wand]

Oh yes I pointed it out alright.

The house had some other issues and I understand my clients passed on the house.

[Ken Amelin]

Residential chimneys are free standing structures - they support themselves.

I am not aware of any requirement for bracing, only a solid footing on stable earth, below frost line.

They are certainly not supported by the house frame. In fact, two inches of clearance is required between house and chimney. If a chimney deflected two inches, enough to touch the house frame, it would tumble to the ground.

[Raymond Wand]

They have to have lateral support. Many chimneys have metal brick ties attached to the wood framing.

[Ken Amelin]

They have to have lateral support. Many chimneys have metal brick ties attached to the wood framing.

Not in this country. (as far as I know) - All free standing.

ANY MOVEMENT in a masonry chimney will cause it to fail.

[Raymond Wand]

So there is no lateral support even on a 30' chimney? What happens in high winds?

[Raymond Wand]

Ken,

Take a look at this.

http://www.google.ca/url?sa=t&rct=j&...jSfQHw&cad=rja

Tying a chimney to a house - Fine Homebuilding Question & Answer

[Ken Amelin]

That's pretty interesting.

It is for areas that are in earthquake zones and require that type of protection. (I don't live in an area that mandates earthquake protection construction)

For those that do live in an area where earthquake protection construction is mandated - you should read Raymond's link.

Although bracing is recommended, its purpose is to prevent collapse and personal injury from earthquake damage, NOT structural bracing or protection against failure.

Thanks Raymond.

[Bob Harper]

The codes do not address tall chimneys. When you build a smokestack, you must engineer it for many forces including wind loads as well as gravity. It is legal to construct a high aspect/ratio masonry chimney even though we know the wind can have a significant effect on the broad side.

The article and drawings for chimney bracing are only for seismic zones. The intent of this bracing is not to protect the chimney from damage but simply to allow the occupants to escape BEFORE the chimney falls during an earthquake. Now, bracing a chimney to the exterior roof is asking for trouble. Note the drawings showing bracing deep into the attic tying into the joists. The joists have a LOT more inherent stability. The roof framing, on the other hand, moves a lot! It flexes. BTW, so does a large masonry chimney. That's one reason to use step and counter flashing instead of one step flashing cut or caulked directly to the chimney. The two part flashing acts like an expansion joint.

As soon as a masonry chimney gets out of plumb, gravity begins to work against it. As the mortar joints erode, you are on a collision course with eventual collapse.

Note that the uBC methods of rebar and fully grouting around flue tiles is in conflict with the IRC and NFPA 211, which both require a 1/2-4" air space around the flue tiles to allow for thermal expansion.

Also note the lack of substantial seismic reinforcement for factory built fireplaces. In general, they tend to flex and wiggle with the house during a quake and actually perform quite well.

There are tens of thousands of damaged fireplaces in and around Richmond, Va. from this summer's quake.

[Jerry McCarthy]

Two observations;
1. Just how much support do you think four #4 steel rebar grouted into a fireplace masonry chimney will provide?
2. My money is on the premise that the second story was an add-on and the chimney was extended due to height clearance requirements. Now don't you think that's an even worse scenario then just an original high chimney?

[Raymond Wand]

Jerry

The chimney and 2nd floor are all original. Circa 1988.