Expert: Bart Ogden - 10/14/2008

Question
QUESTION: I have a concrete block chimney where it exits the roof which contains 2 tile flues, and a third prefab chimney flue next to the concrete chimney which is made up of an inner and outer round galvanized tubing separated by about 2 inches. Tile flue on left is for a first floor fireplace used occasionally. Tile on right is for wood stove in basement used all winter. Metal flue is for prefab fireplace used occasionally, manufacturers cap removed. The chimney was originally enclosed by constructing a 2x4 wall around the concrete blocks and metal flue, sheathing with plywood and vinyl siding. The chimney crown was poured in place concrete over OSB. The crown drip edge was wood and the whole mess covered in roofing tar to effect some degree of waterproofing. Needless to say, it's been leaking for years and I discovered rot in the OSB, studs, sheathing and even in the roof sheathing when I demolished the enclosure/crown.
Once I repair the roof sheathing, can you outline how to correctly enclose the the concrete chimney and metal flue (is a stud wall and plywood sheathing structure OK?), especially how to construct a chimney cap, if it is on top of a wood stud wall? I do remodeling but have not come up against this kind of repair before.


ANSWER: Hi Dan,

You've got some real issue here that will need to be solved.

You can't do any type of combusible on top of the masonry portion, period.  You also should maintain at least 2" to the block exterior from any combustibles.

It appears that the clearance from the framing to the metal flue is insufficient.  It's typically 2", but you will need to see what it says on the piping itself.  There should be a sticker or a stamp on the piping telling you what is required.

BOTH woodburning chimney flues, at their shortest point, must stick out of the deck at least 36" and must extend 24" above anything within 10'.

Masonry should never sit on top of wood, only on other masonry - that's the way it's designed.

I suggest you separate the two chimneys (the masonry and the metal) so they can be flashed and terminated independently.  This way you can do a full-masonry termination on the masonry as is required and a wood chase with a metal chase cap on it as should be done on the metal flue.

I recommend you contact a local CSIA Certified Chimney Sweep to do some on-site diagnosis of your problem.

You can find a CSIA Certified Chimney Sweep at www.csia.org.

Thanks for using AllExperts.Com and good luck with your project.

Sincerely,
A . Bart Ogden, President
Home Safe Hearth & Chimney, Inc.
Wichita, KS
Voice  316-265-9828
Fax  316-265-9890
www.HomeSafeKS.com

CSIA Certified Chimney Sweep #2653
CSIA Certified Dryer Exhaust Technician #10
NFI Certified Wood Specialist
NFI Certified Gas Specialist
NFI Certified Pellet Specialist
NFI Master Hearth Professional
NCSG Technical Advisory Council Member




---------- FOLLOW-UP ----------

QUESTION: Does Code prohibit capping all three flues with a metal chase cap, nothing combustible? Would be much easier to rebuild the chase around all three, with proper 2 inch clearances.

10 foot clearance is there but I will have to extend flues to meet the 36 inch rule.

Probably a good idea to get a pro out to examine this one!

Answer
Hi Dan,

It's not prohibited, but it's very hard to seal up.

I would also recommend the chase be ventilated.

Thanks for using AllExperts.Com and good luck with your project.

Sincerely,
A . Bart Ogden, President
Home Safe Hearth & Chimney, Inc.
Wichita, KS
Voice  316-265-9828
Fax  316-265-9890
www.HomeSafeKS.com

CSIA Certified Chimney Sweep #2653
CSIA Certified Dryer Exhaust Technician #10
NFI Certified Wood Specialist
NFI Certified Gas Specialist
NFI Certified Pellet Specialist
NFI Master Hearth Professional
NCSG Technical Advisory Council Member