By Eric Moshier – Certified Heater Mason, Third-Generation Mason, MHA Technical Committee Member, ASTM E1602 Masonry Heater Group Member
Solid Rock Masonry LLC of Duluth, Minnesota restores stone and masonry chimneys and fireplaces throughout northern Minnesota and northwestern Wisconsin, within about 90 minutes of Duluth. Owner Eric Moshier is a Certified Heater Mason, a member of the ASTM E1602 masonry heater standards committee, and a third-generation mason with more than 25 years and 800 completed projects. We do not patch chimneys. We find out why the chimney failed, we fix that, and then we repair the damage it caused. That order matters, and it is the difference between a repair that lasts thirty years and one that fails again in five. This page explains how masonry chimneys and fireplaces actually fail in a freeze-thaw climate, what a real repair looks like, and how to tell whether the contractor standing in your driveway is about to fix your chimney or just hide it.
Every masonry chimney failure is a water failure
There is essentially one cause of masonry chimney failure in this climate, and it is water.
Stone and brick and mortar are porous. They absorb water. In Duluth we go through dozens of freeze-thaw cycles every winter, and each time water inside the masonry freezes it expands by roughly nine percent. Nine percent does not sound like much until you remember it is happening inside a solid material that cannot move.
So the water finds the weakest thing and breaks it. It pops the face off a stone. It crumbles a mortar joint. It cracks a crown. It rusts a lintel. Then the crack it just made lets in more water next year, and the failure accelerates.
Every genuine chimney repair therefore has to answer one question before anything else: where is the water getting in?
A contractor who does not ask that question is not repairing your chimney. He is buying himself five years.
The four places water gets into a chimney
In our experience, on the chimneys we open up, water is coming in through one of four places, and usually more than one.
1. The crown
This is the single most common failure we see, and it is almost always the result of the crown never having been built properly in the first place.
The crown is the slab on top of the chimney that sheds water off the masonry. On far too many chimneys, what is up there is not a crown at all. It is a wash: mortar troweled over the top of the brick, sloped a little, feathered out to the edge.
Mortar is not a roofing material. A mortar wash cracks. It cracks because mortar shrinks as it cures, because it has no reinforcement, because it is bonded rigidly to a flue liner that expands and contracts every time you light a fire, and because it is thin at the edges where it needs to be strongest.
Once the crown cracks, the chimney becomes a funnel. Water pours down the inside of the masonry, saturates it from within, and starts breaking the chimney apart from the top down. You will see the damage on the outside, near the top, and you will be tempted to think you have a mortar problem. You have a crown problem.
Here is how we build a crown, and it is a layered answer.
The best crown is concrete with a metal crown over it
A cast concrete crown gives you what a mortar wash never can: real mass, a real slope, a real overhang past the face of the chimney, and a drip edge so water falls clear of the masonry instead of running back down it. It is reinforced, and it has a bond break where it meets the flue liner, because the liner moves every time you light a fire and the crown does not. Cement those together and it cracks. Every time.
But concrete is concrete, and concrete in a freeze-thaw climate is on a clock. Sooner or later, weather wins.
So we put a formed metal crown over the top of it.
The metal is fabricated to fit the chimney, sloped, with a generous formed overhang and a turned drip edge all the way around. It cannot crack, because there is nothing in it to crack. It does not shrink as it cures, because it does not cure. It has no reinforcement to rust. Where the flue passes through, the penetration is sealed with a flexible sealant that moves with the liner instead of fighting it.
Now look at what that combination actually does. The concrete does the structural work. The metal takes all the weather. The concrete crown never sees rain, snow, ice, or a freeze-thaw cycle again, so the thing that destroys concrete crowns never gets to touch it.
That is not a belt-and-suspenders flourish. That is the whole philosophy of this company in one detail. Do not build a crown and hope. Build a crown, then make sure the crown is never exposed to the thing that kills crowns.
If we are not pouring new concrete, a metal crown goes on anyway
Not every chimney needs, or every budget allows, a new cast crown. Sometimes the existing crown is sound enough, or the scope is smaller.
In that case we still install a new formed metal crown over what is there. Always. Because a metal crown over a tired concrete crown stops the water, stops the freeze-thaw, and stops the deterioration where it stands. It buys that chimney decades.
What we will not do is trowel a fresh skim of mortar over a cracked crown, tell you it is fixed, and cash the check. That crack is still moving under there, and it will come straight back through your new mortar inside of two winters.
That is a crown. Anything else is a countdown to needing restoration work in the near future.
2. The flashing
Flashing is where the chimney meets the roof. It is also where a great many contractors reach for a tube of caulk.
Correct chimney flashing is two separate layers of metal that work together. Step flashing is woven into the roof shingles, one piece per course, running up the side of the chimney. Counterflashing comes down over the top of the step flashing and is let into a joint in the masonry, then sealed. The two overlap. Water that gets behind the counterflashing lands on the step flashing and runs back out onto the roof.
The bandaid version is a bead of caulk or roofing tar where the chimney meets the shingles. It works for a season or two. Then it dries, shrinks, cracks, and every drop of water running down the roof goes into your house.
On the up-slope side of a wide chimney you also need a cricket, a small peaked structure that splits the water and sends it around the chimney rather than letting it pile up against the back of it. Chimneys wider than about 30 inches need one. Most do not have one, and the back of the chimney is rotting because of it.
3. The mortar joints
Mortar is the sacrificial part of a masonry wall. It is supposed to be softer than the stone or brick around it, and it is supposed to wear out first. That is by design.
So when joints erode, that is not necessarily a failure. It is the system working. It is a maintenance item. The failure is leaving it, because once the joints are cracked or recessed and open, water sits in them and goes straight into the wall.
The failure is also, very often, in how the joints were repaired last time. We will come back to that, because it is the single most damaging thing done to masonry in this country.
4. The top of a stone chimney, and the coping
Stone chimneys have a particular vulnerability that brick chimneys do not. The stones on top are often large, flat, and irregular, with wide mortar joints between them. Water sits on those joints. It gets in. It freezes. Over a few decades the top of the chimney is loose, the mortar is gone, and the stones can be lifted off by hand.
We see stone chimneys where the top three or four courses are effectively just stacked. There is nothing holding them but gravity and mortar dust.
The thing that ruins more historic masonry than water: the wrong mortar
This is the most important section on this page, and it is the one that separates a restoration mason from a contractor with a mixer.
Mortar must be softer than the masonry it holds together.
Old stone and old brick are relatively soft and relatively porous. They were laid in lime mortar, which is also soft and porous. That system works because the mortar is the weak link on purpose. Water moves through the mortar and evaporates out of the mortar. When the wall moves, as every wall does, the mortar cracks and can be replaced. The stone survives. The mortar is a consumable.
Modern Portland cement mortar is enormously strong and nearly impermeable. Point an old soft wall with hard Portland mortar and you have inverted the entire system. The mortar is now the strongest thing in the wall. The stone is now the weak link.
So the water that used to leave through the joints cannot. It goes into the stone instead. It freezes in the stone instead. And instead of a crumbling mortar joint, which you can rake out and replace in an afternoon, the face of the stone pops off.
That is spalling. And a spalled stone cannot be repaired. It is gone. You either live with it, cut it out and replace it, or turn it around if the back is sound.
Here is how you can spot it yourself. Walk around an old building that has been repointed with hard mortar. The mortar joints will be standing proud of the masonry, sharp and intact, while the brick or stone between them is eroded, pitted, and hollow. The mortar is winning. That building is being destroyed by its own repair.
We match mortar to the masonry. On historic work that usually means a lime-rich mortar, Type O or Type N, sometimes a straight lime putty mortar. We match it for strength, for permeability, for color, for aggregate, and for joint profile. On newer chimneys built with hard modern brick, a harder mortar is correct. The point is that it is a decision, made deliberately, based on what the wall is made of.
Most contractors use whatever is in the bag on the truck. That is how a 100-year-old chimney gets destroyed by the people hired to save it.
The bandaids, and why they fail
Here is what gets sold as chimney repair around here, and what actually happens.
- Caulking the cracks. Caulk is not structural and it is not permanent. It hides the crack from you. It does not stop water, because the water is coming from somewhere else anyway.
- Elastomeric paint on patches that always fail over time.
- Sealing or painting the face of the masonry. This is the worst thing you can do to a wall. A film-forming sealer or a coat of paint stops water going in through the face, but it also stops water coming out. Water still enters from the top, from the joints, from behind. Now it cannot leave. It freezes inside the wall and blows the face off from the inside. Painted and sealed masonry fails faster, not slower.
- Smearing new mortar over the old crown. A skim coat over a cracked crown cracks again along the same line within a year or two, because the crack underneath is still moving.
- Repointing without addressing the crown or the flashing. New joints, same water. It looks great for two winters.
- Grinding joints with an angle grinder and no care. Fast, cheap, and it cuts into the edges of the brick or stone permanently. Those cut marks are forever.
- Sandblasting or high-pressure washing. Strips the hard fired surface off brick and the weathered skin off stone, exposing the soft porous interior. It looks clean for one season, then absorbs water like a sponge and deteriorates rapidly. It is irreversible.
If a contractor proposes any of the above without first explaining where your water is getting in, get another opinion.
What a real chimney restoration looks like
Here is how we work.
We find the water first
Before we quote anything, we look at the crown, the cap, the flashing, the joints, the liner, the roofline, the gutters, and the interior. Water stains inside the house tell us as much as anything on the roof. A chimney that is wet on the north face and dry on the south is telling us something. We are trying to build a picture of where water enters and where it goes.
We fix the source, then the damage
In that order, always. There is no point in rebuilding the top of a chimney under a crown that will crack again next spring.
Rebuilding above the roofline
The portion of a chimney above the roof takes the worst of the weather, and it is often the only part that needs to come down. We take it down to sound masonry, salvage the stone or brick where we can, and rebuild it with mortar matched to the original. Then we cast a proper crown.
Repointing, done correctly
Repointing is not smearing mortar over a joint. The joint is raked or carefully cut out to a depth of at least two and a half times the joint width, so the new mortar has something to grip. The back of the joint is squared, not left rounded, because a feathered edge of mortar has no strength and will fall out. The masonry is dampened so it does not suck the water out of the fresh mortar and kill the cure. The mortar is packed in tight, in layers, not slapped in all at once. And it is tooled to match the original joint profile, because the profile is not decoration, it is what sheds water off the joint.
Done properly, a repointed joint lasts decades. Done badly, it falls out in two years and takes some of the masonry with it.
The liner
A chimney flue that is cracked, spalled, missing, or was never lined is a fire and carbon monoxide risk, and no amount of cosmetic work outside changes that. If the liner is compromised, it gets addressed. A beautiful chimney that leaks combustion gas into your house is not a successful project.
The firebox and the smoke chamber
Inside a fireplace, the firebox has to be firebrick laid in refractory mortar. Regular masonry mortar in a firebox fails. It cannot take the heat, and it deteriorates until the joints are open and the fire is in contact with the brick behind.
Above the firebox, the smoke chamber should be smooth, parged, and correctly shaped. A rough, corbelled, unparged smoke chamber creates turbulence, kills the draft, and is a common reason a fireplace smokes into the room. People blame the chimney height. Frequently it is the smoke chamber.
Why the fireplace smokes, and what it usually is
We get called about smoking fireplaces constantly. The causes, roughly in order of how often we find them:
- Smoke chamber built wrong. Rough, unparged, wrong shape. Fixable.
- Flue too large or too small for the firebox opening. There is a ratio, and if it is wrong the fireplace will never draw properly no matter what you do.
- Chimney too short. The rule is that the chimney must be at least 3 feet above the roof penetration and at least 2 feet higher than anything within 10 feet. Roofs get replaced, additions get built, trees grow. The chimney that worked in 1960 may not work now.
- The house is too tight. Modern windows and air sealing mean there is no makeup air. The fireplace is trying to pull air up the flue and the house will not give it any. This is increasingly common and it is not the chimney’s fault.
- Damper, throat, or lintel geometry.
The point again is diagnosis. Adding two feet of chimney is a common and expensive guess. Sometimes it is right. Often the problem is in the smoke chamber, and adding height accomplishes nothing.
Stone chimneys in particular
Stone is not brick, and stone chimneys have their own problems.
Stone units are irregular, which means joints are wide and variable, which means there is more mortar exposed to weather than on a brick chimney. Wide joints hold water. Stone is also frequently harder and denser than brick, which means it is even more important that the mortar be soft, because a hard mortar in a wide joint between two hard stones has nowhere to go but into the stone.
Stone chimneys built with fieldstone and rubble often have a soft, weak core behind the visible face – a rubble fill of small stone and mortar that was never intended to be exposed to water. When the face fails and water gets into that core, the chimney can lose structural integrity quickly and without much warning from the outside.
We have taken apart stone chimneys where the exterior looked acceptable and the inside was effectively a bag of gravel.
When it should come down
Sometimes the honest answer is that the chimney is beyond repointing and needs to be rebuilt, in whole or in part. We will tell you that.
We will also tell you when it does not. We have looked at plenty of chimneys that a previous contractor said needed full replacement, where a proper crown, correct flashing, and repointing the top few courses was the right answer at a fraction of the cost.
The diagnosis drives the scope. Not the other way around.
The work we are proudest of
Restoration is not only chimneys and house walls. Some of the best work we have done is on structures that belong to everybody.
We have restored a historic stone arch bridge: falsework and centering built under the arch, the arch ring taken down and rebuilt stone by stone, and the whole thing put back the way the original masons left it. An arch is held up by nothing but compression and the accuracy of the stones. There is no glue, no steel, and no second chance. You either cut them right or the arch does not stand.
We have restored historic brick and stone buildings in Duluth, including grand Victorian houses where the brick and the brownstone had been weathering for over a century. We restored the stone pillars and patio at Chik-Wauk. We have rebuilt stone retaining walls on the Lake Superior shore, where the water and the ice do things to masonry that no inland wall ever has to survive.
That work is the reason we talk the way we do about mortar and water. When you have taken a two-hundred-year-old arch apart and put it back together, you understand exactly how much the last repair matters, and exactly what a careless one costs.
What it costs
Chimney and fireplace restoration ranges enormously, because the scope ranges enormously. A crown replacement and flashing repair is a very different project from taking a stone chimney down to the roofline and rebuilding it.
We give you a written estimate that spells out what we are doing and why. Our pricing is competitive because we do a lot of this work and we are fast at it, and our experience shows in the number. If something on our estimate does not make sense to you, ask, and we will explain what it is for.
Frequently asked questions
How do I know if my chimney needs work?
Look for: cracks in the crown, mortar joints that are recessed or crumbling, pieces of masonry or mortar on the roof or in the yard, white staining (efflorescence) on the chimney, water stains on the ceiling near the chimney, a damp or musty smell in the firebox, or rust on the damper. Any of those means water is getting in.
What is spalling?
Spalling is when the face of a stone or brick pops off, leaving a fresh, crumbly interior exposed. It is caused by water freezing inside the masonry. It is very often accelerated by hard Portland cement mortar or by sealing or painting the face of the wall, both of which trap water inside. Spalled units cannot be repaired. They have to be replaced or turned.
Should I seal my chimney?
Not with a film-forming sealer or paint. Those trap water inside the wall and make things worse. There are breathable, vapor-permeable water repellents that reduce absorption while still letting the wall dry, and there are situations where those are appropriate. But a sealer is never a substitute for fixing the crown, the flashing, and the joints. Sealing a wall that is taking on water from the top is like putting a raincoat on a man standing in a bathtub.
What is the difference between tuckpointing and repointing?
In common usage they are used interchangeably to mean replacing deteriorated mortar joints. Strictly, tuckpointing is a historic decorative technique using two mortar colors. What matters is not the word but whether the joints are cut out to proper depth, squared, and filled with a mortar matched to the masonry.
Why does my fireplace smoke?
Most often the smoke chamber, the flue-to-opening ratio, chimney height, or a house that is too tight to supply makeup air. It is a diagnosis, not a guess. We look before we quote.
Can you match my existing stone and mortar?
Yes. Matching the mortar for strength, permeability, color, and aggregate is a core part of the work, not an afterthought. A repair that is the wrong color is a cosmetic problem. A repair that is the wrong hardness is a structural one.
Do you work on historic buildings?
Yes. Historic masonry is where the mortar-hardness question matters most, and where the wrong repair does permanent damage. See our restoration work.
How far do you travel?
Restoration and stonework within roughly 90 minutes of Duluth. Masonry heater work is nationwide.
Talk to a third-generation mason
If you have a chimney that is losing mortar, dropping pieces, staining, or smoking, we would like to look at it before someone caulks it.
We will tell you where the water is coming in. We will tell you what actually has to be done and what does not. And if the last contractor pointed it with the wrong mortar, we will tell you that too, because you deserve to know what happened to your building.
Solid Rock Masonry LLC
5347 Howard Gnesen Road, Duluth, MN 55803
218-343-2978
eric@solidrockmasonry.com