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Roof Flashing Coverage Calculator — IRC R905.2.8 / SMACNA

Estimate step-flashing piece count and material length for roof-wall intersections, plus apron, base, and cricket/saddle flashing for chimneys and dormers per IRC R905.2.8 and SMACNA minimum lap rules.

By TradeCalc, Roofer Calculators — Code-Referenced — IRC R905.2.8 (flashing at roof-wall intersections), IRC R905.2.8.1 (cricket/saddle for chimneys ≥ 30"), IRC R905.2.8.3 (counterflashing on masonry), SMACNA Architectural Sheet Metal Manual Table 2-2 (base flashing laps)

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How to Calculate Step & Apron Flashing

Why Roof-Wall Intersections Need Flashing

Anywhere a roof meets a vertical wall (a dormer sidewall, a chimney, a second-story wall) the joint is a leak waiting to happen. Water running down the wall and water running down the roof both collect at the corner. Step flashing solves this by lacing a piece of sheet metal into every shingle course, so each course sheds water over the metal, never behind it. IRC R905.2.8 requires step flashing at every shingle-roof-to-vertical-wall intersection on roofs 4:12 and steeper.

Step Flashing Piece Count

Pieces = ceil(Wall length × 12 / Shingle exposure)

Metal length per piece = Exposure + 2" lap

  • Wall length = the horizontal run of the roof-wall intersection (ft)
  • Shingle exposure = the exposed face of one course (5" for standard 3-tab)
  • 2" lap = SMACNA minimum horizontal overlap between consecutive pieces
  • Metal width = 5" typical (4" up the wall + 4" on the roof, 90° bend) — but cut 10" wide and bend

Each step-flashing piece is bent 90° at the wall: half (typically 4") turns up the wall behind the siding, half (4") lies flat on the roof under the shingle. The piece is installed with the shingle course it serves — nail the shingle over the step flashing, then the next piece laps 2" over the previous.

Worked Example

Scenario: A two-story house has a 12-ft first-story wall meeting a 6:12 shed roof, with 5" 3-tab shingle exposure. How many step-flashing pieces and how much coil stock?

  1. Wall length = 12 ft = 144 in
  2. Shingle exposure = 5 in
  3. Pieces = ceil(144 / 5) = 29 pieces
  4. Metal per piece = 5 + 2 = 7 in (5" course + 2" lap)
  5. Total metal length = 29 × 7 in = 203 in = 16.9 lf of 10"-wide coil stock

If the same wall was a 30"-wide chimney, IRC R905.2.8 would also require an apron flashing at the downslope face (length = 30 + 6 + 6 = 42") and, because the width is ≥ 30", a cricket (saddle) behind the chimney to divert water around it.

Practical Tips

  • Don't rely on caulk or roof cement. Step flashing is the waterproof layer; caulk is a backup that fails in 3–5 years of UV. Every roof-wall intersection must have mechanical metal flashing laced into the shingles, not just a bead of mastic.
  • Counterflashing covers the up-leg. Step flashing turns 4" up the wall but the top edge is exposed. A counterflashing (reglet cut into the mortar or siding) covers the up-leg and sheds water down the face of the step flashing. IRC R905.2.8.3 requires counterflashing on chimneys.
  • 2" horizontal lap is the SMACNA minimum. Less than 2" and capillary action pulls water uphill between the pieces. In ice-dam climates, step the lap up to 3" to resist ice-lift.
  • Chimneys ≥ 30" wide need a cricket. IRC R905.2.8 requires a cricket (saddle) behind any chimney wider than 30" to divert water around it. The cricket is a small gable framed behind the chimney, flashed with a one-piece saddle. Below 30" a single apron flashing is permitted.
  • Pre-bent step flashing saves time. Buying 7" × 10" pre-bent step flashing (W.R. Grace, Gibraltar) costs ~50% more than coil stock but eliminates the brake work. For a 12-ft wall the time saved is 1 hour; for a whole house with multiple dormers it is a full day.

Code References

IRC R905.2.8 (flashing at roof-wall intersections), IRC R905.2.8.1 (cricket/saddle for chimneys ≥ 30"), IRC R905.2.8.3 (counterflashing on masonry), SMACNA Architectural Sheet Metal Manual Table 2-2 (base flashing laps)

Frequently Asked Questions

How many step flashing pieces do I need per foot?
Pieces per foot = 12 / shingle exposure. For 5" 3-tab exposure that is 12/5 = 2.4 pieces per linear foot, so a 12-ft wall needs 29 pieces. For 6" architectural exposure it drops to 2 per foot (24 pieces for 12 ft). Each piece is bent 90°, 4" up the wall and 4" on the roof, with a 2" lap over the previous piece.
What is the minimum lap for step flashing?
SMACNA requires a 2" minimum horizontal lap between consecutive step-flashing pieces and a 4" minimum up-leg on the wall. The 2" lap prevents capillary action from pulling water uphill between pieces; in ice-dam climates, increase to 3". Vertical base flashing against a wall should extend at least 4" up and 4" onto the roof deck.
When does a chimney need a cricket (saddle)?
IRC R905.2.8.1 requires a cricket behind any chimney, trellis, or obstruction wider than 30" measured parallel to the roof ridge. The cricket is a small framed gable behind the chimney that diverts water around it instead of damming against the back wall. Smaller chimneys (< 30") can use a single apron flashing without a cricket.
What is the difference between step flashing and continuous flashing?
Step flashing is a series of short (7") pieces laced into each shingle course — it handles roof-wall intersections on sloped roofs because each piece moves independently as the roof expands. Continuous (apron) flashing is a single long piece used at the base of a vertical face on a <em>flat</em> or low-slope roof, where there is no course to step into. Continuous flashing on a sloped roof buckles as the metal expands and pulls fasteners out — always step-flash slopes 4:12+.