Resources for Historic Homeowners
#32 Chimney Flue Lining
“I own a 1772 center chimney cape. The chimney is original with four fireplaces, one bake oven, and it serves the furnace and a water heater. I don't want to use any of the fireplaces until I have it repaired. The masonry is in excellent condition, as assessed by two different masons who inspected it. I have one quote for $38K to have 7 clay flues added to the chimney I have another quote for about $25K for cast in place concrete flue liners. Since the masonry is in very good condition, I'm wondering if I can repoint the fireboxes, parge the throats and possibly get the interior of the main stack repointed or parged. I've heard of a process where masons can actually lower themselves down into these massive chimneys and repoint or parge them from the inside. Is it possible to make the chimney safe while leaving the historic masonry in place?” – Pete Hicks
First a few basics. “Repointing” is replacing the mortar between the bricks. “Parging” is a coating of mortar over the face of the brick masonry. Be very cautious if the contractor is using the words “cast in place concrete liners.” “Concrete,” in the specific use of the term, is a very hard dense material, that may not be suitable for casting into a traditional chimney. There is an inflatable tube system, called “SupaFlu” that uses a light-weight cementious insulating material that is cast in place. The light weight material is very different than concrete. I have had a few of these SupaFlu systems installed on my projects. There may be other cast-in-place systems that use other materials. These cast-in-place systems seem to work best on more modern simple chimneys.
I think your traditional methods approach would be better on complex older chimneys. The problem with these interior monolithic castings inside old chimneys is that the old chimneys are constantly settling and moving slightly and are designed and built of materials that allow for that movement. When you cast a ridged monolithic structure inside a moving chimney, the chimney can literally tear itself apart, "working" against the immovable cast internal structure. This may not be evident at first but I've seen two cases where, over just a couple of decades, cracks are developing and bricks are falling out of chimneys.
Yes, there are traditional masonry crews with a small skinny worker who can get down the flue for parging and repairs. One time he got stuck in there and we had to tear out the side of the chimney to recover him! But it is more common to simply open up the side of the chimney in the first place and do the interior work then replace the bricks.
There is a significant difference between modern masonry and traditional masonry.
With traditional masonry the bricks are much weaker than most modern brick and are laid in place with lime or clay mortar that is somewhat weaker than the bricks and much weaker that modern cement mortar. It takes weeks and months for the mortar to set and develop its strength. This traditional lime or clay mortar has the ability to "heal" itself as the mass of the masonry shifts slightly from time to time. In the traditional masonry systems the mortar holds the bricks apart, spreading the vertical loads out over the entire surface of the brick. So, what holds the a traditional masonry mass together? Gravity!
With modern masonry the high-strength bricks are literally glued together with high-strength cement mortar that sets up within hours and develops its strength within days. Modern masons use designs and methods that depend on this adhesion and fast mortar setup.
Modern masons do not often understand how gravity can be depended on to hold the bricks together, they insist on gluing the brick together with their high-strength mortar. When modern masons use their fast methods and high-strength mortars on old bricks in old chimneys the mortar is stronger than the brick and as the mass of the chimney shifts slightly over the long-term the strong mortars damage the bricks, cracking them in half and chipping off the edges. This is just the opposite of what should happen. If anything fails in a traditional masonry system it should be the mortar, leaving the bricks sound. This works well because it is far less costly to re-point damaged mortar and to reset existing bricks than it is to replace damaged bricks.
Parging mortar recipes can be somewhat different than brick laying mortar, but we've found that sometimes it's the same mortar between the bricks and in the parging. So, when you talk with your masons, talk mortar. Ask if they will be buying pre-mix mortar. If so, will it be Type S, N, O or P? These are modern specifications for the strength and hardness of the mortar, with Type S made with cement for use with modern high-strength bricks. Types O and P are softer lime-rich mortars more suitable for older chimney. Or will he mix his own? Ask what materials he uses in his mortar. What kind of lime & sand, does he use cement in it? Has he ever used clay mortar? With traditional masonry repairs you want to use mortar that matches the strength characteristics of the surrounding original mortar. This might be lime-rich mortar, straight lime mortar or even plain clay mortar, make of just clay and sand.
In the end your mason may use a combination of traditional masonry repairs, and a modern flue lining system.
Richard Irons - Irons Masonry
North American SupaFlu Systems, Inc.
Call to find your local SupaFlu contractor and make sure they know how to deal with traditional masonry chimneys.
John Leeke is a preservation consultant who helps homeowners, contractors and architects understand and maintain their historic buildings. You can contact him at 26 Higgins St., Portland, Maine, 04103; or by E-mail: johnleeke@HistoricHomeWorks.com; or log onto his website at: www.HistoricHomeWorks.com
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