Let’s Keep Water out of the Chimney Assembly

We will address two types of chimneys here. Brick chimneys and framed-in chimney chases. We will address keeping water out of each separately, but we can cover some issues in common.

Lets describe each of these types of chimneys.

There are 3 critical interfaces to chimneys that keep water out of your home.

  • The interface between the roof deck, the roofing and the vertical surfaces of the chimney
  • The exterior surfaces  of the chimney which are much like sidewalls of a roof on a building.
  • The crown and flue interface which keeps water shedding off the horizontal portions of the chimney.

We will look at each of these separately.

Brick Chimney

This is not your average brick chimney. Good looking corbelling there.

The brick chimney has a masonry flue sticking up out of matrix of  brick and mortar. Issues here are signs of porosity in the mortar-  gaps in the mortar, cracks in the crown that are allowing moisture to enter into the brick and mortar.

Brick, well constructed, will last for decades. But brick work, and repairs to brick work and chimney flashing assemblies- up on the roof and out of sight- can get pretty sketchy. I am not going to go into great detail here. Brick Doctor Inc., working out of the DFW area, has a great page on integrating brick chimneys and roofing materials. I would be hard pressed to improve on this page so we will use it as a reference.

Let’s talk about chimney caps for a minute. While a chimney cover is essential to a framed-in chimney, brick Chimneys can get by with just a concrete crown, BUT, your brick chimney really needs a cap.  A chimney cap keeps water out of your flue, and prevents damage to your damper. A chimney cap can keep critters from trying to homestead your chimney and prevent water stains inside your fireplace. Again Brick Doctor Inc in the DFW area does a great job talking about brick chimneys and flue caps and chimney covers. You can find that information here.


framed in chimney

Good looking framed chimney matches the exterior of the building. The siding has been left too close to the roof deck. This crack will collect debris; this image must have been taken the day the framing was finished, and water will wick up and destroy wood siding materials. A 2″ gap would be much better.

The  framed-in chase chimney has the standard 3 layer stainless chimney coming up through the chase but that portion coming up through the roof, or up along the edge of the building, is dressed up like the rest of the building with a framed structure that is built like a stud wall. It is then painted or covered with commercial panels or basically any standard siding material. It can even take on the appearance of brick.

Framed-in Chimneys are more delicate than brick; wood and engineered woods rot when exposed to moisture, and the structural layers make it easier to hide construction flaws. A brick chimney is available for inspection. Poor masonry workmanship shows through to the trained eye, but the frame chimney is just like the cladding on your home. It can look great, but the ability to keep water out of your walls, depends on the quality of the drainage plane behind the cladding. The cladding is not water shedding, instead, it is the weathering layer. You can find more information on cladding and water shedding on wall structures, here.

Brick Chimneys are covered over with a crown of masonry or concrete that is tapered from the flue to the edge of the chimney for water drainage. Cracks in crowns occur, often because of poor material choice. If you think you might have an issue with your brick chimney crown, check out the reference material, again at Brick Doctor Inc. They are very good with details.

On a framed-in chimney the covering is sheetmetal with the cap integrated right in.

First, there is a sheet metal base cap. This must be carefully matched to the framed opening and sized properly. The cap must fit but fit securely so it can be fastened properly.

sheetmetal framed chimney cover

This sheetmetal  framed-in chimney cover has been carefully measured to fit over a triple walled stainless metal chimney. Welds need to be corrosion resistant, not a trivial matter; and the metal heavy gauge to remain stable in wind. I like this fabricated cover. The metal has been “broken” to give it positive drainage and a stiffer quality.

The vertical portion of the base cover creates a barrier for water blowing along the surface of the cover. From here a flashing collar on the surface of the triple wall stainless chimney will shed water over this vertical edge of the base cover. The round chimney cap tops of the triple wall and mostly protects the flashing collar from having to shed much water. This is the proper setup of a framed- in chimney flashing. Things can go wrong from here.

stainless steel chimney flashing assembly. Not all components are stainless.

An example of a framed -in chimney pipe assembly. Not a great example. This is a cheap hardware box store base cover in a standard dimension. The framed-in chimney frame was sized to fit the cap. Which makes sense, but the construction of such a cover is standard and cheap. It lacked slope so water has sat on the metal and corroded the galvanizing. The joint is an inadequate light bead. Your local sheet metal company can build this for you to match the dimensions of your framed-in chimney. If you are hiring a contractor, make sure you/they specify the materials and the manufacture of this base flashing cap. The other components are standard and work fine. You can choose the material. Stainless steel lasts better, but is more expensive. Galvanized works fine in a thicker gauge. Light flimsy galvanized is not a good choice. Make sure they break the metal for slope and stability.

We need to talk about the framed chimney – roofing interface. We will briefly describe this using asphalt shingles as the roofing material.

head wall flashing schematic

Headwall flashing diagram showing the
5. underlayment
4. primary flashing secured to the roof deck and
3. counterflashing which is oddly mottled, I swiped the most appropriate image I could find- more on this below, and
2. the house wrap, building wrap which covers over the top edge of the counterflashing and completes the drainage plane for this wall section.

Proper overlaps and proper layering of building materials keep water out of structures. Super duper materials, self sealing membranes, urethane caulks, space age sealants will not make up for proper application.

A couple points need to be made about the above image. The Counterflashing is shown as more of a tape that an actual metal counterflashing. A metal counterflashing is better, because it makes a physical break with the primary flashings on the lower edge and the step flashing moving up the edge of the side wall rake edge. More on roof flashings and rakes below. This metal counterflashing also allows the roof flashing to be changed out with  tearing the house apart. Time flies, asphalt roofs go south, and a hail storm can speed up such issues. Allowing for easy but sound roofing work in the future makes sense rather than someone figuring out the easiest way to cobber a transition.

I would recommend an appropriate sealing tape over the top edge of the counterflashing not shown. The top edge of the metal is sharp and the tape protects the housewrap here, and also back stops the housewrap.

There is a siding issue also in this image

I also do not like the siding, housewrap outline here. The computer design technician here was attempting to show some critical feature and added the siding and housewrap as a toss-in maybe. There should always be an airgap between the house warp and the siding- we want double wall construction here, cladding over sheathing protected by a drainage plane-  so there is no chance to trap water, and that water that does get through the siding or soaks into the siding has a chance to evaporate.

Now I do not have the time, inclination or talent for creating images of all such proper transitions, you contractor will have to do these for you. I hope I am able to give you a sense of how these should go together. Water shedding materials work by making sure water runs over material transitions and not under or along these transitions. You either make gravity your friend or it becomes your nemesis.

Flashing metal transition between step flashing and head wall flashing

A step flashing overlaps a head wall flashing. The vulnerable point here is the cut corner of the head wall flashing at the corner. This vulnerability is protected from water penetration by the overlapping step flashing. You will see diagrams where the step and head flashing are cut and folded together, maybe even including soldered joints. These kinds of corners require a journeyman sheet metal wrangler. Go simple, yet sound- unless you have the talent available, and want to pay extra for the appropriate metals.

So now we will look at the field transition of roofing materials to the step flashings against the side wall of your framed-in chimney. This next diagram gives a pretty clear rendering of the layers of a asphalt shingle roof assembly. Steel panels, or tile roofing will have their own appropriate spec that stops water from outflanking your roofing materials. Proper performing flashings will divert water from the side wall onto the onto the roofing assembly. Of course, the drainage plane and cladding is missing here. Again, more details about keeping water out of walls is here.

asphalt roofing shingles terminating with metal step flashing

Shingles are layered with step flashing as a stop to keep water from outflanking and running off the edge of the end shingle. You can see more such diagrams under roofing pages and other links provided on this page.

And then the top edge is terminated with a pan flashing, head flashing or valley flashing which directs water around the top side of the framed-in chimney when the roof continues up slope and the framed chimney must have water diverted around it. This next image is a roofing cricket or saddle which is superior technology to divert water around an object on a roof.

A roofing cricket or saddle for the framed-in chimney.

A roofing saddle or cricket diverts water from the roofing assembly upslope around the framed chimney. The schematic details here are analogues of the other diagrams on this page. Proper layering keeps water running over overlaps not against or off the end laps.

OK, so we should have some kind of idea how things should work, how materials should be put together, but something is likely wrong or why would you be here and this far down into this long winded story.

How is your Chimney doing to deep water out?

Are you looking for a leak around your chimney or are you just looking things over for potential problems?

If water is coming in, and nothing looks particularly porous, you can always look inside and outside the attic chase area in the attic. That is discussed here. Or continue below, and follow through the discussion about potential problems with the framed in chase structure. Remember the chase boundaries. Look where water is showing up. Water upgrade from the roof deck means water is not entering through the roof assembly area. Water damage to sheathing of the framed in chimney means water is penetrating the framed-in walls or the crown cap area. Water on the outside of the framed in sheathing looks different than water on the inside of the sheathing. Water cannot span gaps. Water does not travel along beams and rafters. Water pretty much shows up where is is leaking. Good luck.

We are evaluating the framed chase chimney you have, and so lets look at some issues that might concern you. Let’s start at the top with the sheet metal cap. Is it badly corroded? Is it covered with patching materials? Does it have joints that are not soldered, but sealed together with some kind of sealant and fastener combination holding the parts together? These are all signs of trouble and the solutions here will depend on your enthusiasm for climbing ladders. Rust at soldered joints means you could have an issue with porosity. Patching materials on metal just hide real problems. When you have a problem make sure you are buying a fundamental problem or you must might be purchasing a puzzling problem obscured by defective patching. Metal has a high temperature coefficient which means it grows when it gets hot. Overlapping joint thus saw and dislodge patching materials.

You can poke at the rust and see if it is a light surface dusting where you can see the entire perimeter of the oxidation or does the oxidation disappear under the collar of the chimney? Or at a soldered joint. These leave the possibility of porosity. If you are not having trouble now, you have not spotted the damage, or the leaks are coming soon.  If you have access to the rust and can get to a perimeter you can treat rust with proper surface preparation and a quality rust inhibitor.

Does the cap hold water? Is the corrosion in a pattern that looks like the edges of standing water? Do you have an issue here with structural deformation of the original cap? If the cap was a little tight yet forced to fit the framed in  chimney frame the outer edges will pop up and create standing water. Your original cap could have lacked certain virtues- a formed crown with proper drainage designed in. Do you remember the fuss about breaking the sheet metal cap at the corners to provide drainage and stiffness?

This means when it does fail your home interior will catch the continuing leakage from a baby bathtub. it will fail as draining a puddle on this cap versus a small pin hole leak. This I leave to your judgment. How big is the puddle and how many hours can it rain- and you can decide whether it is time to schedule a new and proper cap to be fabricated.

Does the cover fit right? Often covers like this are too small and get distorted as they are fit over the outer edge of the vertical siding. Note defects and add these to your specifications for a replacement cap. This metal cap needs to be crowned and fit properly for proper drainage. This is a important issue not an emergency.

So lets turn our attention to the wall portion of the framed chase chimney. The framed chase needs more maintenance than the brick. Brick done right can last for many decades. Wood cladding built right and maintained properly will last for decades.

Your framed chase chimney needs to be checked for water penetration into siding. Since we do not know the condition of the drainage plane- for more info on drainage planes look here–  we have to make sure we are doing good work keeping water out of the siding. Are your chase walls made of wood? Or are they covered in fiber cement siding? Your maintenance procedures with proceed from here.

Is the structure sound and strong? The structure inside is most likely studs and plywood or OSB sheathing. If this gets wet, your structure will fail and things will begin looking askew. We will also put the chimney chase on our tour of the attic looking for water  issues. If the interior  of your chase structure is being damaged you have water inside your home. If you find water stains around the chase, you know you have issues with your cap or your framed chase walls or the flashing areas. The roof deck leaking will put water on the outside of the chase predominantly.

Flashing issues around a framed chase are quite similar to the issues of a brick chimney so here is another link to the Brick Doctor page discussing brick chimney flashings. Their discussion deals with the fundamental issues with roofing and chimney protrusions through the roof deck.


Do you have any questions or comments. Feel free to add to the discussion.

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