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Enter the chimney width, depth and roof pitch to calculate step flashing pieces for each side, apron flashing across the downslope face, counter flashing let into the mortar joint, ice-and-water membrane around the opening and cricket material where the chimney exceeds 30 inches across the slope. Chimneys are the most common leak point on an otherwise healthy roof, and the failure is almost always the metal rather than the masonry.
Flashing a chimney takes four separate details, and the quantities come from the chimney dimensions rather than the roof area. Step flashing is counted per shingle course, so it follows the length of the chimney sides.
| Chimney side length | At 5 in exposure | At 5⅝ in exposure |
|---|---|---|
| 2 ft | 5 | 5 |
| 3 ft | 8 | 7 |
| 4 ft | 10 | 9 |
| 5 ft | 12 | 11 |
| 6 ft | 15 | 13 |
| 8 ft | 20 | 18 |
Both sides need it, so double the figure. Add a few spare — the first and last pieces on each side are usually cut.
| Chimney width | 4/12 | 6/12 | 8/12 | 12/12 |
|---|---|---|---|---|
| 30 in | 5 in | 7.5 in | 10 in | 15 in |
| 36 in | 6 in | 9 in | 12 in | 18 in |
| 48 in | 8 in | 12 in | 16 in | 24 in |
| 60 in | 10 in | 15 in | 20 in | 30 in |
| 72 in | 12 in | 18 in | 24 in | 36 in |
A cricket is required on any chimney more than 30 inches wide measured across the slope. Below that, an apron flashing across the up-slope face is normally enough.
Every one of the four has to be present and lapped in the right order, over the piece below and under the piece above. Miss one and the others cannot compensate — which is why a chimney is the most common leak point on a roof and why roof cement smeared round the base is the most common failed repair.
| Pitch | Angle | Slope % | Multiplier | 2,000 sq ft footprint becomes |
|---|---|---|---|---|
| 1/12 | 4.8° | 8.3% | 1.0035 | 2,007 sq ft |
| 2/12 | 9.5° | 16.7% | 1.0138 | 2,028 sq ft |
| 3/12 | 14.0° | 25.0% | 1.0308 | 2,062 sq ft |
| 4/12 | 18.4° | 33.3% | 1.0541 | 2,108 sq ft |
| 5/12 | 22.6° | 41.7% | 1.0833 | 2,167 sq ft |
| 6/12 | 26.6° | 50.0% | 1.1180 | 2,236 sq ft |
| 7/12 | 30.3° | 58.3% | 1.1577 | 2,315 sq ft |
| 8/12 | 33.7° | 66.7% | 1.2019 | 2,404 sq ft |
| 9/12 | 36.9° | 75.0% | 1.2500 | 2,500 sq ft |
| 10/12 | 39.8° | 83.3% | 1.3017 | 2,603 sq ft |
| 12/12 | 45.0° | 100.0% | 1.4142 | 2,828 sq ft |
| 14/12 | 49.4° | 116.7% | 1.5366 | 3,073 sq ft |
| 16/12 | 53.1° | 133.3% | 1.6667 | 3,333 sq ft |
| 18/12 | 56.3° | 150.0% | 1.8028 | 3,606 sq ft |
| 20/12 | 59.0° | 166.7% | 1.9437 | 3,887 sq ft |
| 24/12 | 63.4° | 200.0% | 2.2361 | 4,472 sq ft |
One piece per shingle course up each side. At a 5 inch exposure that is about 2.4 pieces per foot of chimney side, doubled for both sides, plus a few spare for the cut end pieces.
Four details: apron on the down-slope face, step flashing up both sides, a cricket on the up-slope side if the chimney is over 30 inches wide, and counter flashing let into the masonry over all of it.
When it is more than 30 inches wide measured across the slope. That is a code requirement, not a preference — without one, water ponds against the up-slope masonry.
No. Cement round a chimney base is the most common attempted repair and the least durable — it cracks as it shrinks and as the roof moves, and it hides the missing detail rather than replacing it.
Fixing a leaking chimney? The chimney flashing repair guide shows step and counter flashing.
Chimney wider than 30 inches? Size the saddle with the chimney cricket calculator.
Hiring it out? The chimney repair cost calculator prices flashing, crown, cap and brick repairs.
Using copper flashing on a slate or tile roof? The copper roof cost calculator shows what copper costs installed.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.