Drywall ceiling water stains are traced to a leak in the chimney roof flashing during heavy rain. This project explains how to repair a leaky chimney by repairing the roof and chimney flashing. Total material and labor cost for repairing the chimney was about $300.
Leaky Chimney Repair Steps
The chimney leak repair steps are shown in the following series:
- Locating the chimney leak
- Chimney box inspection and install roof brackets to get on the steep roof.
- Chimney and roof inspection. Remove the HardiPlank cement board chimney siding expose the chimney step flashing to find that heavy rain was splashing over the step flashing at the roof valley constriction causing the chimney to leak.
- The roofing crew pulls off the shingles and step flashing.
- The roofers wrap the chimney in a self-stick ice and water shield rubber mat followed by new step flashing and shingles.
- Install new chimney siding boards, caulk and paint.
A couple of years after fixing the leaky chimney the entire was roof replaced and I was careful to apply the lessons I learned here to the new roof and chimney.
Roof Leaks During Heavy Rain
Several water stains appeared on the drywall ceiling above the fireplace after recent torrential rains and high winds. The attic is directly above the ceiling and there aren’t any pipes in this area, so the roof must be leaking.
The water stains were small and somewhat faint, but this could be real trouble if the leak isn’t fixed. It’s important to inspect the attic and roof for hidden water damage, rotted wood or mold.
How to Repair a Leaky Chimney
Finding the Chimney Leak
I spoke to a roofer and he said chimney leaks are very common. The leak could be caused by a number of problems, including:
- Improperly installed or rusted step flashing.
- A problem with the sheet metal chimney cap.
- A hole in the lap siding.
- Rotted chimney corner boards.
- Missing, cracked or separated caulk.
- Other problems??
I began my inspection in the attic in hopes of locating the leak and following it up to the roof. The drywall ceiling stains correspond to the right corner of the chimney box as it rises out of the attic through the roof, so I focused my search here.
I brushed away the blown fiberglass insulation in several spots and quickly found evidence of a roof leak by the corner of the chimney box. The back of the ceiling drywall was slightly warped/puckered with a faint outline of a water stain. The fiberglass insulation is matted at the edge of the chimney box – clear evidence of having been wet. Everything was dry now.
I didn’t find water stains or trails on the roof rafters or oriented strand board (OSB) sheathing outside the chimney box, so I turned my attention to inside the chimney box. The chimney box is constructed of 2×4 framing, OSB “Exposure 1” rated sheathing and HardiPlank® siding.
The chimney box can be accessed through a small doorway in the side. The doorway was cut to install the flue pipe and perhaps for ventilation to prevent moisture buildup.
The leak appears to originate from the far right corner of the inside the chimney box behind the double 2×24 wood beams.
I couldn’t squeeze around the flue pipe to get a direct look at the far side of the chimney box, but was able to use the view screen of the digital camera to reach out and get some good photos for later inspection on the computer.
Water stains are clearly visible on the 2×4 sole plate of the chimney box, especially the dark stains on the right which is very close to the drywall ceiling water damage.
Find a Roof Leak with Toilet Paper Tell-Tales
Although I knew the general vicinity where the leak is happening, I didn’t know the precise source of the leak or if the roof leaks everytime it rains. To better identify the location and frequency of the roof leak, I placed sections of toilet paper where I thought the leak might be happening. Toilet paper is an excellent tell-tale (i.e. indicator) for finding leaks because it’s sensitive to small amounts of water and stiffens as it dries.
I checked the tell-tales a few days later after a night of heavy thunderstorms. Success! The tell-tales have been wet indicating a significant amount of rain water is leaking in by the chimney.
Both sections toilet paper tell-tale were dry and stiff as cardboard when I picked them up the next afternoon, having dried during the heat of the day.
I replaced the tell-tales with new sections of toilet paper and checked after every rain over the course of a week.
The toilet paper tell-tales told me a lot about the nature of the roof leak:
- The roof leaked during heavy downpours, but did not leak during light steady rains.
- The leak is coming out at the bottom corner of the chimney box, on both sides of the box wall.
- The toilet paper placed on top of the 2×4 double wood beam is always dry, no leak here.
- The leak is fairly light because the toilet paper absorbed the rain water before it could wet the attic insulation. However, the leak is proportional the intensity and length of heavy downpours and would be worse during prolonged periods of severe weather.
Based on the evidence, my belief is I’ve got a leak at the corner of the chimney, perhaps due to a problem with the step flashing. Here’s a view of the chimney and corner where the leak is found inside the attic.
To figure out what’s happening, I’ll have to go on the roof and inspect the chimney.
The leak in the attic is coming from the bottom corner of the chimney box, but before going up on the roof I decided to take a closer look inside the chimney for evidence of the leak since water flows downhill and the leak has to be originating somewhere higher up. The chimney cap looks great and I saw no evidence of a leak on the chimney box walls.
Chimney Box Leak Inspection
One particular 2×4 stud inside the chimney box had what might be severe water stains – or it could be nothing more than a pine sap pattern in the wood grain – it was hard to tell. It felt dry but it may be wet under the surface of the wood. I was suspicious because this stud is near the corner where the leak is occurring and water could be wicking under the 2×4 sole plate making it’s way to the corner.
The dark pattern runs up the stud towards the chimney cap, but ends before reaching the top.
The dark pattern is only on this side of the stud, but water can trickle down and follow a strange path.
Roof Leak Testing with a Moisture Meter
Short of waiting for the next thunderstorm, the only way to really know is to test the wood with a moisture meter, so I bought the General Tools MMD4E Digital Moisture MeterHand Tools) for $32.98 (battery included!) from Amazon:
The MMD4E Moisture Meter is easy to use and has two different settings: 1) wood and 2) construction materials (concrete, brick, drywall, carpet, etc.). It has a “hold” button to freeze a reading on the display in case you’re making a measurement in a tight spot where you can’t see the display. The meter beeps as it’s takes a reading to let you know when the reading is stable. A percent moisture table is on back of the meter for quick reference – a nice touch so you don’t need to carry around the manual.
I set the moisture meter to the “wood” setting and pressed the two needle probes into the 2×4 stud in the darkened area. The wood stud was very dry at 5.3% moisture content. The dark area on the stud is therefore not caused by a chimney leak.
I checked several spots on the stud, the OSB sheathing, joists and rafters inside the attic; everything was consistently reading 5% to 6% moisture content. For comparison, my wood deck read a 7.1% moisture content.
Steep Roof: Brackets and Ladders
Having finished the attic inspection, I needed to get on the roof to inspect the chimney to find the leak. I don’t like working on steep roofs and especially the high and steep 12×12 pitch roof on my house. A 12×12 roof slopes at a 45 degree angle because it rises 1 foot for each foot of horizontal run.
I’m very safety conscious (meaning worried) when getting on the roof, so I took time to prepare by:
- Screwing a 2×4 stop to the wood deck to brace the feet of the 24 ft extension ladder.
- Install two roof brackets with a 2×6 step board. Each bracket is slipped under the shingle and fastened with three (3) pan head wood screws. (The screw holes under the shingles will be filled with roofing caulk when the brackets are removed.)
- A second 16 ft extension ladder is laid on the 12×12 pitch roof against the 2×6 step board.
- The 16 ft ladder is tied off to another roof bracket to prevent it from sliding sideways. The extra roof bracket also serves as a handhold for getting on/off the hip roof.
You might want tie the 24 ft extension ladder to the lower roof bracket above the gutter so there’s no chance of it falling sideways and leaving you stranded on the roof. I didn’t tie off the lower ladder because the deck is level and I always had my son with me on the deck.
View of the 16 foot extension ladder laying on the 12×12 roof and tied off to the roof bracket. It’s very easy to climb up the roof with a ladder this way. Note that both sections of the ladder are tied together because the extension ladder will easily separate in two pieces if pulled. I used an Anchor Bend knot to tie the rope with a finishing half hitch because this knot is easy to tie and secure.
The satellite dish prevented placing the ladder closer to the chimney. It looks like a long ways down because it is!
The leak is near the corner of the chimney box, which is located at the narrow end of a roof valley. I can see how this could be a problem with water splashing and piling up against the corner of the chimney during a heavy rain. The valley tends to catch twigs, leaves and catkins that get stuck and form a dam.
It’s plain the architect could’ve done a better job on the roof design. As I drive around the neighborhood I see many complex roof lines that make for similar impractical drainage situations.
Closer inspection of the chimney siding reveals high water marks, the HardiPlank® siding is damaged due to delamination/spalling and the caulk is separated at the chimney corner board.
Another issue is the HardiPlank lap siding was incorrectly installed against the shingles; HardiPlank requires a 1 to 2 inch gap between the siding and the roof.
Chimney Box Siding Removal
Given what I see here, I suspect the chimney step flashing is leaking, and leaks around the chimney flashing are common. The only way to tell for certain is to remove the bottom rows of HardiPlank lap siding to inspect the flashing.
I began by cutting the caulk line between the siding and the corner board with a utility knife.
The top row of siding was removed first with a hammer and blue pry bar because it easiest to start here and work down. I was fortunate these bottom rows were face nailed so I could get at the nail heads. I worked the flat end of the blue pry bar under the bottom of the siding and gently pulled it up to raise the nails, taking care to disturb the upper plank as little as possible.
The next rows were easier to remove.
Box Chimney Weather Resistant Barrier
Notice anything odd? The builder didn’t install a weather resistant barrier (WRB) such as building paper, tar paper/roofing felt or house wrap over the OSB sheathing! HardiPlank should be installed over a weather resistant barrier (WRB) per the HardiPlank installation instructions and 2006 residential building code.
I called my local Building Dept. and asked the building inspector if the weather resistant barrier was required when the home was built in 2002. He confirmed that back then the code did not require weather resistant barrier for lap siding installed over OSB sheathing; however it is required with the 2006 residential code.
The OSB sheathing is “Exposure 1” rated by the Engineered Wood Association (APA) – meaning has a fully waterproof bond and is designed for applications where long construction delays may be expected prior to providing protection (e.g. lap siding), or where high moisture conditions may be encountered in service. So there’s no cause for alarm but it wouldn’t have cost much for the builder to have installed tar paper over the OSB sheathing.
The Building Inspector went on to say that in spite of the 2006 residential code, a large percentage of homes continue to be built without a weather resistant barrier and that no barrier is better than incorrectly installed house wrap because the house wrap could trap water.
I know from experience the chimney is only the only place on the house without a weather resistant barrier because the main walls have blue foam board behind the siding and the stucco walls have tar paper over the OSB.
Back to the main story…
The last row of siding was soaking wet and just about ready to fall apart:
Did you notice the whitish horizontal water lines matching the siding on the OSB sheathing in the above photo? A good indicator water is being picked up at the nose of the HardiPlank where it lays against the roof and traveling or wicking along the length of the siding, wetting the OSB sheathing above the flashing.
The dark areas are saturated with water on this last row of siding:
It’s been several days since it last rained with temperatures in the 70’s and the HardiPlank is still soaked.
Cause of the Box Chimney Leak
The cause of the chimney leak is now clear: During heavy downpours rain water is being forced up and behind the siding, which runs down the back of the step flashing at the corner of the chimney box.
- There’s strong evidence water is being pickup up by the sharp nose of the HardiPlank and carried sideways above the step flashing as noted in the next photo. This happens because water is piling up at the end of the narrow roof valley by the corner of the chimney AND the HardiPlank was incorrectly installed touching the roof.
- Another factor is the chimney corner board sticks out beyond the step flashing into the roof valley partially obstructing the drainage path and the caulk was separated here. Any tree debris caught at the corner would dam the valley and make things worse.
There’s a chance the step flashing may be rusted and I do see some rust stains along the edge of the shingles. I can’t tell for certain without pulling up the shingles.
I checked the other side of the chimney and found the HardiPlank here is badly cracked and spalled.
Closeup of the HardiPlank along the roof line. (Click on the image for a full size view.) The bottom edge of the HardiPlank is looking pretty bad. That bottom row is badly cracked and pieces are missing.
Again, the problem is the HardiPlank was incorrectly installed touching the roof when there should be a 1 or 2 inch gap between the siding and roof. When the HardiPlank sits on the shingles, it stays wet, the water freezes and expands causing the cement fiberboard layers to separate. The result is cracking, spalling and delamination as shown above.
During my conversation with my local building inspector, he said sees HardiPlank installed all the time without the proper gap between the planks and roof. Or the siding is touching the ground, which invites termites and you can’t see the mud tunnels.
I decided to pull off and replace the damaged HardiPlank where it met the roofline on the west face of the chimney. Recall that the siding on the north side of the chimney was previously removed in Part 3.
HardiPlank Chimney Siding Removal
The last four rows of HardiPlank were incorrectly installed against the shingles. As I pulled off the first plank, I was surprised by dark water stains on the OSB sheathing! Uh oh, this could be bad – meaning “expensive”.
After removing the bottom four rows of siding, I inspected the OSB sheathing. The sheathing has a curious wedge-shaped water stain, but was dry and solid. Oddly, the water stains stopped at the step flashing, this told me it wasn’t causing a roof leak, but something else was going on.
Notice the water lines on the step flashing where the HardiPlank used to be:
I discussed this photo with the local building inspector. He and his supervisor had two theories for what caused the water stains on the OSB siding:
Chimney Condensation Theory:
The chimney firestop (or floor) in the attic is even with the top of the water stain. The places the lower section of the chimney on the air conditioned side of the house, which is cooled and causes water to condense on the OSB sheathing in the humid summers. Basically, the lower section of the chimney sweats like a cold can of soda.
Siding Water Wick Theory:
Water is wicking it way along the HardiPlanks because it’s in contact with the roof. The water evaporates and condenses on the OSB sheathing.
Both are very plausible theories, but the water wick theory is correct because:
- Water lines are seen along the step flashing following the HardiPlank (see above photo).
- The water stains are only behind the HardiPlanks which were in contact with the roof.
- I know the roofline is above the firestop because I observed daylight shining through the nail holes after I pulled off the siding. It’s also plain from the attic photos in Part 1 the chimney firestop/floor is well below the west roofline. Therefore, the lower section of the chimney box is not on the air conditioned side of the house. This next photo illustrates the firestop and west roofline:
The chimney condensation problem should be remedied by installing new HardiPlank with the required 1 to 2 inch gap above the roof such that water isn’t wicked along the planks.
Fixing the Leaky Chimney
At this point in the project the bottom rows of siding in contact with the roof have been removed from the north and west sides of the chimney box. The chimney leak will be repaired by:
- Remove the shingles and step flashing by the chimney.
- Install an ice and water shield between the roof deck and chimney.
- Install new step flashing and shingles.
- Install new HardiPlank lap siding.
- Caulk all joints and screw heads.
Carpentry work I can do myself, but I wanted professional roofer to do the shingle and flashing work. I hired a roofing subcontractor who’s worked on my home before and I’ve been happy with the results. The roofer supplied the Owens Corning WeatherLock® Self-Sealing Ice & Water Barrier and step flashing since it was on his truck and I didn’t need a lot of material. I bought the shingles, roofing caulk and HardiPlank Cedarmill lap siding.
The roofers began by cutting the shingles 1-1/2 tabs back from the chimney, then removed the shingles and flashing. The original roofing felt was left in place.
Aside: The chimney corner boards were replaced about about 2 years ago because the bottom of the old boards had rotted. New cedar corner boards were installed with stainless steel screws – a good decision because the screws can be loosened to slip the ice and water barrier behind the boards. I just happened to have a photo of the work from 2 years ago on my iPhone:
The stainless steel screws were simple to back out to loosen the corner boards for this repair. Had the corner boards been nailed, it would’ve been difficult to remove the nails without damaging the boards:
The Owens Corning WeatherLock® Mat ice and water shield is unrolled and cut to length.
The plastic film backing is peeled off before installing the self-stick ice and water shield.
The ice and water shield is applied between the chimney and roof deck and overlapped like roof felt to shed water. The ice and water shield seals around nails to maintain the waterproof seal. Notice the shield is slipped behind the corner boards and wrapped around upper corner of the chimney.
The Owens Corning WeatherLock® Mat Waterproofing Underlayment (ice and water shield) is installed against the chimney and the roof deck. Notice how the roofers slipped water shield behind the first piece of step flashing and under the shingle to shed water.
Chimney Leak Repair with Ice and Water Shield
The Owens Corning WeatherLock® ice and water shield is applied in the three sections against the chimney to prevent leaks:
- Section 1 – first against the west side (lowest side) and wrapped of the chimney corner.
- Section 2 – from the roof across the valley to about 12 inches up the chimney. This high piece will keep out splashing rain water.
- Section 3 – against the north side of the chimney to seal the roof joint.
Here’s a closeup view of the ice and water shield installation chimney corner:
The original construction had a slot sawn in the chimney corner board for the step flashing to pass through. This left an lip or edge of the corner board sticking out into the roof valley which obstructed flow of rain water and could catch tree debris. A notch is sawn in the corner board to remove this obstruction:
Chimney Step Flashing and Shingle Installation
New shingles and step flashing are installed from the bottom working up. I admired how quickly the roofers installed the flashing and shingles, like watching a piano player. Notice how the ice and water shield rises about twice the height of the step flashing against the chimney.
In retrospect, I should’ve asked the roofer to apply a section of roofing felt over the exposed section of OSB sheathing and lap it over step flashing. It probably won’t matter since the new HardiPlank will be correctly installed with a minimum 1 inch gap above the roof so it doesn’t wick water and cause condensation against the OSB sheathing.
The new shingles are interleaved with the old shingles:
The original roof shingles on my home are Atlas Chalet Designer in the Weathered Shake color, which have been discontinued. The replacement shingles are GAF ELK Timberline® Natural Shadow™ shingles purchased from Home Depot. The roofer said the GAF Timberline is a very good quality shingle and pointed out it’s a better continuous design (i.e. no tabs) versus a 3-tab style. The GAF Natural Shadow turned out to be a very close match with the Atlas Chalet Weathered Shake shingles.
Detail of the step flashing and shingle installation at the chimney corner where the roof leak was coming in. The roofer radiused the step flashing with tin snips to prevent injury on a sharp corner. You can see how the bottom of the corner board is notched so it doesn’t obstruct the flow of water. Step flashing normally isn’t nailed against the chimney – but a single nail was used given the shingle pattern and the ice and water shield seals the nail penetration.
The chimney is caulked with BASF Sonolastic® NP 1™ high performance polyurethane caulk. You won’t find NP 1 at the big box home improvement stores, but can buy it at roofing contractor supply stores or online.
The roofer is caulking behind the first section of step flashing to keep out wind driven rain:
View of the ice and water shield, new step flashing and shingles as the roofer works his way around the chimney corner.
HardiPlank Lap Siding – Chimney Installation
I bought several 12 foot x 8.25 inch x 5/16 inch HardiPlank® primed boards in the Cedarmill© style to replace those removed from the chimney. The new siding boards were cut using the old boards as a pattern, but 1 inch shorter on the bottom slope to provide the required clearance above the roof surface.
I used my Bosch circular saw with a Dewalt diamond masonry blade to saw the HardiPlanks. At only 5/16 inch thick, the 12 foot HardiPlanks are very flexible and it would take several saw horses to support the boards. So instead of using saw horses, I set the saw blade to about 3/4 inch depth and laid the siding on 2x4s for support and ground clearance. The blade cut the HardiPlanks very quickly, but made a huge amounts of dust. I held my breath while sawing then walked away for the dust to clear.
A new set of boards ready to be installed on west side of the chimney. Instead of nails, the siding was fastened with corrosion resistant bugle head wood screws. The roofers were kind enough to install the siding for me while I sawed the boards.
The corner board seams and screw heads were caulked with more of the BASF Sonolastic® NP 1™. The lapped bottoms of the HardiPlanks are NOT caulked (per the HardiPlank installation instructions) because it needs to breathe. Also, the HardiPlanks are installed with the required 1 inch clearance above the roof.
The inset image in the above photo shows the corner board and step flashing detail so there’s nothing to obstruct the flow of water or catch tree debris at the narrow roof valley exit.
The chimney looks great after the HardiPlank is painted with a coat of Sherwin Williams Duration® exterior latex paint. I took care to paint the bottom edges of the HardiPlank above the roof and the plank end at the chimney corner.
Here’s the west side of the chimney before painting:
And after painting:
I liked how the new GAF ELK Timberline® Natural Shadow™ shingles blended well with the discontinued Atlas Chalet Weathered Shake shingles.
Chimney Step Flashing Inspection
There were rust stains on the old step flashing, but the roofer said it was cosmetic and found no rust holes. I looked at each piece of the old step flashing and found it to be in generally good condition with no holes, except where it was nailed to the roof as required. The step flashing was therefore not the cause of the chimney leak.
Toilet Paper Tell-Tales
There’s been no roof leaks since the chimney repair with ice and water shield, new flashing, shingles and siding despite several more thunderstorms. I placed some toilet paper tell-tales inside the chimney box at the corner where it had been leaking and will be checking after hard rains for a while to be certain the leak is fixed.
Leaky Chimney Repair Cost
My repair cost wasn’t that much since I did a significant amount of work myself:
- Setup the ladders and roof brackets
- Removed the damaged HardiPlank siding
- Bought the GAF ELK shingles, HardiPlank siding, Sonolastic NP 1 caulk and Sherwin Williams paint
- Measured and sawed the HardiPlank siding
The cost for repairing the leaky chimney was:
- $200 for the roofer’s labor
The roofer supplied the Owens Corning ice & water shield and step flashing. He also installed the HardiPlank (20 minutes work) and caulked the chimney.
- $26.95 for one pack of GAF ELK shingles purchased at Home Depot
- $18.57 for three HardiPlank Cedarmill boards @ $6.19 each at Home Depot
- $3.80 for a tube of NP 1 caulk
- $35.99 for a gallon of Sherwin Williams Duration paint (purchased on sale)
- I already had the tools, ladders, brackets, 2×6 step board and screws, so there was no direct cost for these items.
The total out of pocket expense of $285.31 for the leaky chimney repair. Not bad! If I had hired a roofer to do the entire job soup-to-nuts I could see the job costing me $500 to $750.
Hope this saves you some money!
A year after fixing the leaky chimney, the entire roof was replaced due to hail damage. I had the roofing contractor remove the HardiPlank siding from all four sides of the chimney to wrap it in ice & water shield, followed by new step flashing and HardiPlank siding to guarantee the chimney would be leak free. You can read about that project here.
Thanks for reading,