Thread: Siding over New Masonry Chimney

I just installed a brand new masonry chimney on my house, using blocks and liners. I followed all proper procedure and code, but i had planned on siding over the blocks when all said and done. I was curious if anyone knows if it is safe, assuming I keep a clearance over all sides of the masonry blocks. I did not think that there would be an issue with it, but i am about to do it this weekend, I want to make sure everything is SAFE.

When you figure out how to make anything in this world "safe" let me know because it is an impossible state.

I'm curious how you plan on attaching combustible siding to a masonry chimney at stated clearance to combustibles and how you plan on keeping the rain out at the top. You never want to place a moisture resistant barrier over masonry as it needs to breathe. Sorry but this is a bad idea in so many ways. BTW, what size CMU block did you use and are the cells fully grouted or hollow?

You need to check with your AHJ to see what is allowed.

There are too many "what ifs" to give an answer.

Well my plan was to add on metal studs them dura rock board then nail my siding, vinyl, to that. I was gonna wrap the sides in tyvek. I used the 8x13 blocks (inside dim) and the matching clay flues. Thick this will work?

Originally Posted by Andrew Gallant

Well my plan was to add on metal studs them dura rock board then nail my siding, vinyl, to that. I was gonna wrap the sides in tyvek. I used the 8x13 blocks (inside dim) and the matching clay flues. Thick this will work?

Well, you would have to get approval from your building dept. not that they know what they are doing. Metal studs conduct heat. They would have to create the air space required. Note I said "air space". Encapsulating the chimney like this is untested and unknown but could result in significantly elevated temps. that could at the very least cause failure of the siding. You are also trapping a lot of moisture creating an interstitial space where water could condense.

You are spending a ton of money on an unproven system with a lot of significant drawbacks at the very least.


If your blocks are 6" wide or more they must be fully grouted unless there is another wythe of bricks inside this course. I would have strongly recommended against terra cotta flue tile flue when you have modern listed factory chimney available. Bad choice. So, what kind of mortar did the mason use laying your flue tiles?

Medium duty refractory laying flue tiles. The blocks are solid, not hollow. What can I do to safely dress it up?

Originally Posted by Andrew Gallant

Medium duty refractory laying flue tiles. The blocks are solid, not hollow. What can I do to safely dress it up?

Put lick and stick stone (faux stone) on the chimney and make it look like stone. Have a proper crown placed on top along with a proper spark arrestor/cap. Stay away from vinyl! Another option would be to cover it in hardcoat stucco.

Will the faux stone still give me the proper breathability that is required?

Originally Posted by Andrew Gallant

Will the faux stone still give me the proper breathability that is required?

I have no idea without doing some research on it. You could contact a faux stone manufacturer, I'm sure they already have guidelines already to go for this type of application.

Originally Posted by Andrew Gallant

Will the faux stone still give me the proper breathability that is required?

Download the Masonry Veneer Manufacturers Association Installation
Guide. It should give you installation guidelines for manufactured stone on block. Modern block is usually too smooth for masonry to adhere properly. Wire lath is recommended.

What is this chimney venting?

It's a woodstove in the basement, chimney is approx. 35 ft tall, made of solid blocks 8x13 (inside dim) blocks, with matching clay flue. All joints on flue were made in medium duty refractory mortar. My originals thought was to box around it, attaching to masonry units and house, leaving an airspace from sheathing, and siding the sheathing. I have seen it done, but not sure i want to do it if it is not safe. I didn't think that blocks would heat up enough to pose a threat to my siding or frame on them? Any suggestions will help. Thank you so far for all the help!

Originally Posted by Andrew Gallant

It's a woodstove in the basement, chimney is approx. 35 ft tall, made of solid blocks 8x13 (inside dim) blocks, with matching clay flue. All joints on flue were made in medium duty refractory mortar. My originals thought was to box around it, attaching to masonry units and house, leaving an airspace from sheathing, and siding the sheathing. I have seen it done, but not sure i want to do it if it is not safe. I didn't think that blocks would heat up enough to pose a threat to my siding or frame on them? Any suggestions will help. Thank you so far for all the help!

I am assuming that this chimney is outside of the building (i.e. against an exterior wall). If so, then 1-inch clearance to combustible materials is still required.

I recently worked on a project where a contractor built a fireplace on a porch that was attached to a pool house. The masonry at the back of the firebox was in contact with the wall sheathing. The owners used the fireplace for a long time during a party in winter weather and the pool house caught fire.

Originally Posted by Andrew Gallant

It's a woodstove in the basement, chimney is approx. 35 ft tall, made of solid blocks 8x13 (inside dim) blocks, with matching clay flue. All joints on flue were made in medium duty refractory mortar. My originals thought was to box around it, attaching to masonry units and house, leaving an airspace from sheathing, and siding the sheathing. I have seen it done, but not sure i want to do it if it is not safe. I didn't think that blocks would heat up enough to pose a threat to my siding or frame on them? Any suggestions will help. Thank you so far for all the help!

From NFPA 211-2013: 13.4.4 Flue Cross-Sectional Area. for residential-type, natural draft solid fuel-burning appliances, the flue shall meet the following conditions:
(3) The cross-sectional area of the flue of a chimney with one or more walls exposed to the outside below the roofline shall not be more than two times the cross-sectional area of the appliance flue collar.

If your stove has an 8" collar then it is allowed. If it has a 6" collar then it would need to be relined. I hate to say it but you have been given some poor advice on this chimney from the start. You could have installed a UL listed factory chimney rated to withstand a 2,100F chimney fire then built a chase around it with your matching siding and a stainless steel chase top for the same or probably much less money than you are now facing. Whomever was guiding you on this did not do you any favors. You keep mentioning you want it to be "safe" whatever that means but it doesn't appear you sought the advice of someone certified within the hearth industry before starting. Also, it must comply with your local building codes and possibly/ probably require a permit and inspection. If not done properly, your insurance carrier may have grounds to cancel your policy or refuse payment in the event of a claim.

Can you provide the make, model and flue collar size of the stove and if it is listed to UL 1482 or certified to EPA Phase II? Any pics? What about your floor and wall protection inside and how it attaches to the flue?

Sorry.

A permit was pulled, I'm having my inspection tomorrow, and it is an 8" collar. That much I did research into. I just did not check into siding, I assumed that because it was block it would heat up and that a frame of some sort around it would be ok. I'm looking into California stucco as an option now, and speaking with a skilled mason to do it.

- - - Updated - - -

The stove is not plumbed in yet, it's a dutchwest catalytic, and it's in the basement, so next to a concrete wall. The ceiling above it has gypsum wallboard, but is it still 5 above top of stove. I have insulted Selkirk pipe inside the house, just to prevent any burns or heat coming off stove pipe

Originally Posted by Bob Harper

From NFPA 211-2013: 13.4.4 Flue Cross-Sectional Area. for residential-type, natural draft solid fuel-burning appliances, the flue shall meet the following conditions:
(3) The cross-sectional area of the flue of a chimney with one or more walls exposed to the outside below the roofline shall not be more than two times the cross-sectional area of the appliance flue collar.

Bob, The IRC (M1805.3.1) does not seem to mention inside versus outside chimney when it comes to flue size, and it allow the flue to be up to 3 times the flue collar. Unless I missed somthing, it seems to be a little more liberal. I know that NFPA 211 has some other requirements more strict than the IRC, but in many areas the IRC is the governing code (not to say you cannot exceed code).

Originally Posted by Mark Reinmiller

Bob, The IRC (M1805.3.1) does not seem to mention inside versus outside chimney when it comes to flue size, and it allow the flue to be up to 3 times the flue collar. Unless I missed somthing, it seems to be a little more liberal. I know that NFPA 211 has some other requirements more strict than the IRC, but in many areas the IRC is the governing code (not to say you cannot exceed code).

First of all, NFPA 211 is THE national std. for installing woodstoves, venting, fireplaces, etc. It has been upheld in most state courts regardless of the local building codes, which rarely address such issues themselves but specifically often refer you back to 211. The IRC does not reference woodstove specifically anywhere. They goofed.

In your case, you list yourself as from New Hampshire, which has adopted 211 in its entirety so that is your code. Also, you should check the listed instructions for their requirements.

Your flue size may be ok then but still, you just installed an obsolete material that has a very poor track record, is unlisted, is not expected to survive a chimney fire much less contain the heat, no warranty and subject to the skill of the installer completely. BTW, 211 requires a non-water soluble calcium aluminate refractory cement mortar and not just refractory mortar. A round flue performs better than rectilinear shapes and is easier to maintain.

I'm not trying to bust on you as it seems you have done a lot of work on this but someone should have asked these questions before the first block laid. You could have used CMU chimney block and dropped an insulated listed liner for instance then applied hard coat stucco or 'lick'em and stick'em' mfd stone veneer as one option.

Good luck with it.

Originally Posted by Bob Harper

First of all, NFPA 211 is THE national std. for installing woodstoves, venting, fireplaces, etc. It has been upheld in most state courts regardless of the local building codes, which rarely address such issues themselves but specifically often refer you back to 211. The IRC does not reference woodstove specifically anywhere. They goofed.

In your case, you list yourself as from New Hampshire, which has adopted 211 in its entirety so that is your code. Also, you should check the listed instructions for their requirements.

Your flue size may be ok then but still, you just installed an obsolete material that has a very poor track record, is unlisted, is not expected to survive a chimney fire much less contain the heat, no warranty and subject to the skill of the installer completely. BTW, 211 requires a non-water soluble calcium aluminate refractory cement mortar and not just refractory mortar. A round flue performs better than rectilinear shapes and is easier to maintain.

I'm not trying to bust on you as it seems you have done a lot of work on this but someone should have asked these questions before the first block laid. You could have used CMU chimney block and dropped an insulated listed liner for instance then applied hard coat stucco or 'lick'em and stick'em' mfd stone veneer as one option.

Good luck with it.

You are confusing me the the OP. I am in PA, which has adopted the IRC. I have nothing against NFPA211, but some chimney contractors seem to insist it is required to meet this code, when that is not always the case.

Originally Posted by Mark Reinmiller

I am assuming that this chimney is outside of the building (i.e. against an exterior wall). If so, then 1-inch clearance to combustible materials is still required.
.

IRC R1001.11 requires 2" clearance to combustibles on front and sides and 4" on the back of the fireplace.

Originally Posted by Andrew Gallant

A permit was pulled, I'm having my inspection tomorrow, and it is an 8" collar. That much I did research into. I just did not check into siding, I assumed that because it was block it would heat up and that a frame of some sort around it would be ok. I'm looking into California stucco as an option now, and speaking with a skilled mason to do it.

- - - Updated - - -

The stove is not plumbed in yet, it's a dutchwest catalytic, and it's in the basement, so next to a concrete wall. The ceiling above it has gypsum wallboard, but is it still 5 above top of stove. I have insulted Selkirk pipe inside the house, just to prevent any burns or heat coming off stove pipe

What type of Selkirk pipe is in the house and what is it being used for? (A wall pass through, connector pipe, etc)

Originally Posted by Thom Huggett

IRC R1001.11 requires 2" clearance to combustibles on front and sides and 4" on the back of the fireplace.

The chimney in question is for a wood stove, and I am assuming it is on an exterior wall. See R1003.18 .

For the case I mentioned involving a fire, there was no clearance between the firebox and wall sheathing.

Mark sorry if I made it sound like I was thinking you were the OP. You are 100% correct about the clearance off the rear of a firebox but don't forget a min. 12" wall thickness of solid masonry units, too for a fireplace.

The OP is referring apparently to installing a freestanding woodstove into a brand new masonry chimney. My main point here was the design process because in my opinion, terra cotta is an obsolete material unsuitable for flue tiles. It is expected to withstand 1,000F continuous flue gas temps. and resist cracking and softening up to 1,800F, neither of which is does consistently. The ASTM C-315 std. for flue tiles does not contain any sort of temperature test because they know it cannot take high temps. or thermal shock, such as from a creosote 'chimney fire'. With the availability of listed factory chimney and liners I'm just puzzled why neither were chosen. These systems are listed against very stringent stds. including three 2,100F flue fires x 10 min. each. Factory chimneys come with warranties usually starting at 20+ years while most professional stainless steel liners carry a transferrable lifetime warranty. Masonry chimneys have no warranty or testing and are known inferior to listed systems. The OP could have constructed a masonry chimney using CMU chimney block then applied hardcoat stucco or stone veneer and cap it with a either a cast concrete crown or a stainless steel chase top and been much better off. I feel bad he has a brand new inferior chimney serving an old inefficient woodstove. I hope he has sweeping equipment so he can sweep it frequently as needed.