Residential Chimney Tiebacks

[Kilndry]

Is anyone familiar masonry chimney tieback requirements? I'm researching how masonry chimneys are constructed along an exterior wall of a normal 2-story residence in non-seismic/non-high-wind zones. (and without much extension above the roof line). The IRC doesn't seem to require ties unless the building is in a high-seismic zone which makes me think the ties are only there to prevent a catastrophic collapse and the main stability originates from the foundation. However, do you really design the chimney to cantilever up two stories from the foundation? Any ideas are appreciated.

 

[BAretired]

The chimney should be considered laterally braced at the roof line.

BA

 

[Kilndry]

I'm really trying to put some documentation behind this. Is that a rule of thumb you use or is it cited somewhere? The brick association doesn't seem to provide this particular guideline.

 

[BAretired]

It is not a rule of thumb and I doubt very much if it is cited anywhere. It is just common sense. Why would you cantilever a chimney from the foundation when it is so easy to tie it laterally at roof level?

BA

 

[Kilndry]

I would think the flexibility of the wood framing is greater than that of the chimney.

 

[MusicEngineer]

Can you control the deflection of the wood that the cantilevered masonry chimney is tied into such that both the wood and the chimney meet the same deflection criteria?

 

[Kilndry]

I think it's doubtful that anyone has gone to that extent. I mean, the IRC (Chapter 10) doesn't even require these tiebacks unless you're in a higher seismic zone...which is leading me to question what their true purpose is.

 

[MiketheEngineer]

All I have ever seen are just normal "ties" to the walls. Don't forget the codes do cite certain heights above the roof that are required.

 

[Kilndry]

Ties or tiebacks, the IRC doesnt appear to differentiate.

Yep, for proper venting, I see those extension above roof requirements. I'm really most concerned what the standard of care is in the design as it relates to everyday lateral stability as it seems to be an art at best.

 

[msquared48]

In addition to seismic requirements, I think the ties provide a link to decrease the chance of developing a roof leak at the chimney/roof interface - in addition to any flashing.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[MiketheEngineer]

You could always do a wind and seismic calc and figure how many ties you need. (BTW - most of them are not very good)