Calculate step flashing, counter flashing, and pipe boot quantities for roof-to-wall intersections, chimneys, and pipe penetrations.
Roof flashing prevents water intrusion wherever the roof plane meets a vertical surface (walls, chimneys, dormers) or is penetrated by pipes, vents, or skylights. Step flashing, counter flashing, and pipe boots are the three most common types.
Step flashing is interwoven with each shingle course along a roof-to-wall junction. The standard rule is one piece of step flashing per shingle course, or roughly 2 pieces per 3 linear feet of wall. Counter flashing is installed over the step flashing and embedded in the mortar joint of masonry walls or tucked behind siding.
This calculator estimates step flashing pieces, counter flashing length, and pipe boot count based on your roof's specific intersections and penetrations. Proper flashing is arguably the most important detail in roofing — the majority of roof leaks occur at flashing points rather than in the field.
Precise calculations are essential for meeting regulatory requirements, passing inspections, and ensuring the long-term structural integrity and safety of the completed project.
Flashing is the #1 source of roof leaks when inadequate. This calculator ensures you have the right quantity of step flashing, counter flashing, and pipe boots for a leak-proof installation. This quantitative approach replaces rule-of-thumb estimates with precise calculations, minimizing material waste and reducing the likelihood of costly change orders during construction.
Step Flashing Pieces = Wall Length / Shingle Exposure (typically 5.625") Counter Flashing LF = Chimney Perimeter + Masonry Walls Pipe Boots = One per pipe penetration (by size)
Result: 64 step flashing pieces, 12 LF counter flashing, 5 pipe boots
Step flashing: 30 ft × 12 in/ft / 5.625 in = 64 pieces. Counter flashing: 12 LF around chimney. Pipe boots: 5 individual boots sized to match pipe diameters.
Step flashing protects roof-to-wall joints. Counter flashing covers step flashing tops. Base (or apron) flashing seals the bottom of chimneys and walls. Valley flashing channels water in valleys. Pipe boots seal around pipes. Skylight flashing kits surround skylights. Each type is designed for a specific vulnerable point.
Aluminum is the most common and affordable flashing material. Galvanized steel is stiffer and used for heavy-duty applications. Copper is the premium choice, lasting 70+ years with an attractive patina. Lead is still used in historic restoration. Rubber is used only for pipe boots and flexible applications.
Most flashing failures result from: 1) Using caulk instead of metal, 2) Improper overlap (water must always flow over, not under), 3) Failure to embed counter flashing in masonry, 4) Using incompatible metals (galvanic corrosion), and 5) Not replacing old flashing during re-roofing.
Step flashing consists of small L-shaped pieces of metal woven into each course of shingles where the roof meets a vertical wall. Each piece overlaps the one below it, creating a continuous water-shedding barrier. The wall siding or counter flashing then covers the vertical leg.
Step flashing is woven into the shingle courses and sits against the wall. Counter flashing is a continuous piece installed over the top of the step flashing, embedded into the wall or mortar joint. Counter flashing prevents water from getting behind the step flashing.
Standard residential step flashing is 5 inches wide by 7 inches tall (bent into an L-shape). Commercial step flashing may be larger. They are installed one per shingle course, with each piece extending 2 inches above the exposure of the shingle below.
Pipe boots have a rubber collar that seals around the pipe. UV exposure causes the rubber to crack after 10–15 years, creating a leak path. Metal pipe boots with rubber collars should be inspected annually and replaced when the rubber shows cracks.
No. Caulk is a temporary sealant that degrades in UV and temperature cycles. Flashing is a permanent mechanical water barrier. While sealant may be used to supplement flashing at certain points, it should never substitute for proper metal flashing.
A cricket (or saddle) is a peaked diverter built on the uphill side of a chimney to prevent water and debris from pooling. Chimneys wider than 30 inches should have a cricket. The cricket is flashed with step and counter flashing and soldered or sealed base flashing.