What Is a Chimney Flashing Area Calculator?
Chimney flashing is one of the most leak-prone details on any roof, because it's built from several separate pieces that all have to overlap correctly: a base (apron) flashing on the low side, step flashing woven up both sides, a back piece (or, better, a cricket) on the high side, and — on any chimney wider than about 30 inches — a cricket to split water flow around it. This calculator breaks material area into those components so your estimate isn't just a single rough number.
How to Read Your Results
Total Flashing Material Needed
The combined area of base, back, step and cricket flashing, with your waste allowance applied — a starting point for how much flashing stock to buy.
Base + Back Flashing
The apron flashing on the downslope face and the equivalent piece on the upslope face, each sized by the chimney's width times the flashing leg height.
Step Flashing (Both Sides)
The individual overlapping pieces that run up both sides of the chimney, sized by the chimney's depth times the leg height, doubled for both sides.
Cricket Flashing
A simplified plan-area estimate for the saddle-shaped cricket that diverts water around the chimney's upslope side. Actual sloped cricket surface area will run somewhat higher than this flat estimate.
Real-World Example
A 32in × 24in chimney with a 6-inch flashing leg, a 16-inch cricket extension, and 10% waste:
| Component | Area |
|---|---|
| Base + back flashing | ≈2.7 sq ft |
| Step flashing (both sides) | 2.0 sq ft |
| Cricket flashing | ≈3.6 sq ft |
| Total (with 10% waste) | ≈9.0 sq ft |
The cricket is the single largest component here — a reminder that wider chimneys need proportionally more cricket material, not just more base and step flashing.
Tips for Chimney Flashing
- Any chimney over about 30 inches wide (measured across the slope) should have a cricket per most residential codes, not just step and base flashing.
- Use compatible metals — mixing dissimilar metals (like copper against galvanized steel) can cause galvanic corrosion.
- Step flashing pieces are installed one per shingle course, not as one long strip, so budget nail and sealant quantities accordingly.
- Counter-flashing set into mortar joints is not included in this material estimate — add it separately if applicable.
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Last updated: August 2026 · Reviewed by: Simple Calculator Tools Editorial Team