The old shingles and step flashing are removed to install the self-stick ice and water shield rubber mat. This project is continued from How to Repair a Leaky Chimney – Part 3.

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 removed in Part 3.

Deteriorated HardiPlank Siding at West Roofline

Deteriorated HardiPlank Siding at West Roofline

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”.

Leaky Chimney: HardiPlank Chimney Siding Removal

Leaky Chimney: HardiPlank Chimney Siding Removal

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.

Box Chimney: Water Stains on OSB Sheathing Behind Lap Siding

Box Chimney: Water Stains on OSB Sheathing Behind Lap Siding

Notice the water lines on the step flashing where the HardiPlank used to be:

Box Chimney: Water Stains on OSB Sheathing

Box Chimney: Water Stains on OSB Sheathing

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:
Chimney Box Firestop and Roofline

Chimney Box Firestop and 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.

How to Repair a 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 rubber membrane 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.

Chimney Leak Repair: Remove the Shingles and Step Flashing

Chimney Leak Repair: Remove the Shingles and Step Flashing

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:

Box Chimney Corner Board Replacement (2 years ago)

Box Chimney Corner Board Replacement (2 years ago)

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:

Box Chimney Leak Repair: Remove the Corner Boards

Box Chimney Leak Repair: Remove the Corner Boards

The Owens Corning WeatherLock® Mat ice and water shield is unrolled and cut to length.

Chimney Leak Repair: Owens Corning WeatherLock Ice & Water Barrier

Chimney Leak Repair: Owens Corning WeatherLock Ice & Water Barrier

The plastic film backing is peeled off before installing the self-stick ice and water shield.

Chimney Leak Repair: Peel the backing off the Self-stick Ice and Water Shield

Chimney Leak Repair: Peel the backing off 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.

Box Chimney Leak Repair: Ice and Water Shield on Roof Deck and Chimney

Box Chimney Leak Repair: Ice and Water Shield on Roof Deck and Chimney

This project is continued in How to Repair a Leaky Chimney – Part 5.

Take care,

Bob Jackson

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