Problematic chimney flashing issues can allow roof leaks in the transition area between the chimney and the roofing materials. Diagnosing leaks around the chimney is often a problem to diagnose, due to accessibility.
Is the leak a roofing assembly problem? Is the leak caused by deterioration in the brick chimney or crown? Or, perhaps a faulty counter flashing assembly is allowing water to bypass it?
Both the roofer and the brick mason play important roles in preventing leaks. Roofing materials and chimney components depend on each other to prevent leaks. A good and sound chimney crown diverts water over the sides of the exterior brick and mortar. The counter flashing is installed into the brick and allows water to shed from the exterior of the brick onto the water shedding portions of the primary flashing installed by the roofer as part of the roofing assembly.
The following discussion and schematic depicts asphalt roofing materials only. The pictures illustrate primary flashing installed with roofing shingle materials. Other types of roofing materials have similar flashing components, though the appearance and specifications may vary somewhat.
The diagram above illustrates a proper intersection of roofing and counterflashing on a brick chimney. This is the rake edge running upslope along the sides of the brick chimney. The individual step flashing is layered with each row of shingle as part of the primary flashing assembly. (Feel free to call for help understanding roof transition schematics. Photo- courtesy NRCA.)
The image above shows the relationship between the counter flashing and step flashing, which are typically installed by the mason…and the sheet metal primary flashing assembly, which is installed by the roofer.
The continuous flashing is either installed in lengths or saw cut into the brick face. The step flashing method is embedded into the mortar between courses of brick. In either case, the counter flashing extends from the layer of bricks and then bends down, overlaying the top edges of the primary (step) flashing.
When all components are sound, the water sheds off and down over the brick chimney surfaces, down over the counter flashing and over the outside edge of the step flashing, onto the roofing assembly, and then off the roof.
Finding and Segregating Leak Sources…
In some instances, framed chimneys consist of stud framing sitting on the existing ceiling joist. These are built with stud walls to the appropriate height, and sheathed in, then covered with a choice of cladding (siding). A brick chimney is self supporting from the foundation up while a framed chimney rests upon the existing ceiling joists, in which case in your attic, you may have a stainless steel pipe enclosed in the chimney chase as shown in the diagram below.
What is the significance of the type of chase configuration in diagnosing leaks?
This basic structural difference between framed chimneys and full brick chimneys totally changes the configuration of the chimney chase. The full brick chimney puts a physical gap by way of the chimney chase- between the roof sheathing and the self supporting brick chimney. Water cannot cross this gap, so leaks on the roof side of the gap are related to a roof assembly problem. Water coming through the components of the brick chimney are either showing up inside the fireplace, or running down the outside of the inside brick. If sheetrock abuts the chimney brick, this edge could show water stains.
This image is a sub-section of the flashing schematic above, which highlights the gap between roof decking and the self supporting brick chimney. This gap provides a great tool to help determine which part of the housing structure is allowing water leakage. Infiltrating water cannot cross this gap. If water stains show up on the roof deck, the roof assembly has a leaking issue. If the infiltrating water appears in the firebox or on the outside of the brick fireplace you have a brick and mortar problem in the chimney structure. A caveat here would be a sheetrock edge being wetted by water running down the outside of the chimney.
A framed chimney does not provide the same demarcation line for water infiltration. The frame chimney is build right into the ceiling framing and the chimney chase is inside the frame. This makes a framed chimney more susceptible to water damage because infiltrating water has more space and co-dependent components to propagate problems like rot and mold, and manufactured wood products like plywood and OSB are much more susceptible to water damage than brick and mortar…
A framed chimney chase configuration totally changes this clear physical water boundary.
A framed chimney is supported by the ceiling joints. The chimney chase in this configuration is inside the framed opening and only surrounds the stainless steel chimney insert. There are many points of possible water entry with a framed chimney
— the water shedding component which covers the large top cover and,
— the large area of cladding around the vertical sides.
There is no clear separation in the framed chimney opening between water coming in through the roofing assembly and the water shedding materials. Leaks coming through the chimney framing components are trickier to diagnose, with fewer rational conclusions drawn. The chimney chase is inside the framed chimney and just surrounds the stainless steel chimney insert.
Brick Doctor often deals with issues like this when the cladding on a framed chimney looks like brick or is created with brick or thin brick, and some framed chimney component has failed and is allowing water in. We are called upon to repair such leaking issues and we accept the job when allowed to rebuild with real brick and mortar to our standards for proper water shedding of rain of the framed structure.
When flashing materials are working properly and the bricks and mortar are sound, your chimney is watertight. BUT even a minor breach in the chimney brick, mortar, or the counter flashing can cause a significant leak, as water which should be shedding over the counter flashing is instead outflanking and getting behind the step flashing and pouring into the interior of your house. Here it can destroy wood, create mold, and/or damage drywall or flooring.
More Flashing Illustrations
Here is a specific side view of the primary flashing- head flashing detail- with a shingle roof covering the base of the head flashing. This section of the primary flashing extends across the face of the chimney to divert runoff around the chimney. The counter flashing placed by or repaired by the mason keeps water running off the chimney from getting behind the primary flashings.
This is an end view of the head flashing which provides the pan to catch roof runoff and divert the runoff around the protruding edges of the masonry chimney.
The soundest of masonry chimneys will not stop a leak caused by defects or faults in the roofing assembly, including the primary flashing assembly which is layered into the roofing materials by the roofer.
The most critical part of the primary flashing is the head flashing up slope from the chimney, which must divert water from the slope around the chimney and insure that water then sheds properly around the sloped sides of the chimney. Water running down the roof slope accumulates volume and velocity until it reaches the drip edge and leaves the roof. The head flashing must catch this runoff and divert it around the chimney. The two diagrams below lay out this component visually.
The quality of the primary flashing and the roofing materials and the quality of the application of these materials are in the hands of the roofer. This is why no responsible bricklayer will guarantee your roof to be leak-free… even when he knows his brick work (and any counter flashing installed by him) properly sheds water. A brick chimney is usually quite visible from the roof, and can be visually inspected for possible breaches. The chimney crown, and every brick, every mortar joint, and every flashing component can be checked, and reasonably ruled out as a potential source of any chimney leak.
Roofing materials, however are layered together. Fasteners are hidden, overlaps in the underlayment are not visible, and properly judging the integrity of primary flashing is not an obvious skill. The bottom line… roofing materials can often look sound, and yet be defective.
The end solution to a confounding leak around a chimney is to make sure the masonry chimney is not porous. When the chimney is sound, you know the issue is with the roofing materials and the primary flashings.