How to Repair a Leaky Chimney – remove the cement plank chimney siding to reveal the chimney is leaking at the narrow roof valley. This project is continued from How to Repair a Leaky Chimney – Part 2.

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.

Leaky Chimney Repair: Leak at Roof Valley Pinch Point

Leaky Chimney Repair: Leak at Roof Valley Pinch Point

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.

Leaky Chimney: Damanged HardiPlank Siding

Leaky Chimney: Damanged HardiPlank Siding

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.

Leaky Chimney Repair: Cut the Siding Board Caulk Lines

Leaky Chimney Repair: Cut the Siding Board Caulk Lines

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.

Leaky Box Chimney Repair: HardiPlank Siding Removal

Leaky Box Chimney Repair: HardiPlank Siding Removal

The next rows were easier to remove.

Leaky Box Chimney: Remove the Second row of Siding

Leaky Box Chimney: Remove the Second row of Siding

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 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:

Chimney Flashing Leak Repair: Bottom Row of Lap Siding

Chimney Flashing Leak Repair: Bottom Row of Lap Siding

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:

HardiPlank Chimney Siding: Wet and Damaged

HardiPlank Chimney Siding: Wet and Damaged

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.
Box Chimney Leak: Water behind Step Flashing

Box Chimney Leak: Water behind Step Flashing

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.

Deteriorated HardiPlank Lap Siding at Bottom of Chimney

Deteriorated HardiPlank Lap Siding at Bottom of Chimney

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.

Cracked and Delaminated HardiPlank Siding at the Roof Line

Cracked and Delaminated HardiPlank Siding at the Roof Line

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.

This project is continued in How to Repair a Leaky Chimney – Part 4.

Regards,

Bob Jackson

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