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How to Repair a Leaky Chimney – Part 1

Bob Jackson
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by Bob Jackson

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 (this article).
  • Part 2 – Chimney box inspection and install roof brackets to get on the steep roof.
  • Part 3 – 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.
  • Part 4 – The roofing crew pulls off the shingles and step flashing.
  • Part 5 – The roofers wrap the chimney in a self-stick ice and water shield rubber mat followed by new step flashing and shingles.
  • Part 6 – 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.

Water Leak on Drywall Ceiling above Fireplace

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.

Roof and Chimney Box inside the Attic
Roof and Chimney Box inside the Attic

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.

Roof Leak: Water Stains on Drywall Ceiling by the Chimney
Roof Leak: Water Stains on Drywall Ceiling by the Chimney

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.

Chimney Box inside the Attic
Chimney Box inside the Attic

The leak appears to originate from the far right corner of the inside the chimney box behind the double 2×24 wood beams.

Roof Leak: Chimney Box Leak Inspection
Roof Leak: Chimney Box Leak Inspection

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.

Rain Water Leak Stains inside the Chimney Box
Rain Water Leak Stains inside the Chimney Box

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.

Toilet Paper Tell-Tales to find the Roof Leak
Toilet Paper Tell-Tales to find the Roof Leak

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.

Finding a Roof Leak with Toilet Paper Tell-Tales
Finding a Roof Leak with Toilet Paper Tell-Tales

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.

Leaky Roof: Toilet Paper Tell-Tale
Leaky Roof: Toilet Paper Tell-Tale

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.

Roof Leak: Chimney Box with Hardiplank Siding
Roof Leak: Chimney Box with Hardiplank Siding

To figure out what’s happening, I’ll have to go on the roof and inspect the chimney.

This project is continued in How to Repair a Leaky Chimney- Part 2.

Thanks for reading,

Bob Jackson

attic inspectionchimney boxdrywall ceilingFinding a Roof LeakHardiPlank
Bob Jackson
Bob Jackson
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