How to Repair a Leaky Chimney

How to Repair a Leaky Chimney – the ice and water shield rubber mat is laid against the chimney box and roof deck, then new step flashing and shingles are installed. This project is continued from How to Repair a Leaky Chimney – Part 4.

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.

Box Chimney Leak Repair: Install Ice and Water Shield

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.

Chimney Leak Repair with Ice and Water Shield

Here’s a closeup view of the ice and water shield installation chimney corner:

Leaky Chimney: Ice and Water Shield Rubber Mat

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:

Roof Leak Repair: Notch in Chimney Corner Board at Roof Valley

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.

Leaky Chimney Repair: New Step Flashing and Shingles

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:

Leaky Chimney Repair: New Roof 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.

GAF ELK Timberline Natural Shadow Lifetime 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.

Chimney Leak Repair: Corner Step Flashing

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.