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Construction glossary · Envelope and finishes

What is flashingin construction?

Flashing is the sheet metal or membrane material installed at joints, transitions, and penetrations in a building envelope to direct water away from the assembly and shed it to the exterior. On a commercial job it shows up as base flashing, counter flashing, step flashing, through-wall flashing, and cap flashing, each detailed at a specific intersection. The estimator takes it off in linear feet by type, since the stock, gauge, and labor differ for each.

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Updated August 2026 · Reviewed by the Ruh construction team

Step flashing 1 per shingle courseOverlap 2 in min lapMaterial 26 ga galv / alum / copperTakeoff Linear ft + count

Flashing locations on a building

Step, valley, drip, k…Flashing runsEvery roof-to-wall + penetration
The definition

Flashing, explained.

In US commercial construction, flashing is the formed metal or flexible membrane that bridges a discontinuity in the envelope and keeps water out of the wall or roof assembly. The trade distinguishes it by location and function: base flashing turns up at a roof-to-wall intersection, counter flashing laps over the base and is let into a reglet or under the cladding, step flashing weaves into a sloped roof against a wall, through-wall flashing collects water inside a masonry cavity and drains it to weeps, and cap flashing tops a parapet. Material matters for pricing. Galvanized steel, prefinished aluminum, copper, and stainless each carry different unit costs, and gauge or weight (for example 24 ga vs 26 ga) drives the number. As an estimator you read the wall sections and roof details, identify each flashing condition, and quantify it in linear feet against the price book for that profile. Miss a counter flashing run and the assembly leaks; the detail, not the plan, governs the count.

How it is measured

Flashing is quantified in linear feet (lf) by type and profile, with separate line items for base, counter, step, through-wall, cap, and edge metal. You pull it from the wall sections, roof plans, parapet details, and exterior elevations, scaling each run along the intersection it protects. Penetrations (curbs, pipes, scuppers) are counted as each, then converted to lf of girth where the detail calls for field-formed pans. Material, gauge or thickness, and finish are recorded per run because they change the unit price. Waste and lap allowance (commonly a few percent for overlaps and corners) is added on top of the measured length.

Types of flashing in construction

The trade distinguishes flashing by the intersection it protects, and each type is a separate takeoff line because the stock, gauge, and labor differ. The four an estimator meets on almost every commercial set are base, counter, step, and through-wall flashing, with cap flashing topping the parapets.

Base flashing

Base flashing turns up at a roof-to-wall intersection, carrying the waterproofing line from the roof membrane up the vertical surface. It is measured in linear feet along the intersection it protects, with the material and gauge recorded per run because they change the unit price.

Counter flashing

Counter flashing laps over the base flashing and is let into a reglet or tucked under the cladding above. It shares the run with the base flashing but is carried as its own linear-foot line, since it is a separate piece of metal installed in a separate operation.

Step flashing

Step flashing weaves into a sloped roof against a wall, one piece per course, shingled so each unit sheds onto the one below. The takeoff carries it in linear feet along the roof line, with the lap and corner allowance added on top of the measured run.

Through-wall flashing

Through-wall flashing collects water inside a masonry cavity wall and drains it back to the exterior at the weeps. It is measured in linear feet by profile along the cavity it drains, kept separate from base and counter flashing in the takeoff because the material and the labor differ.

Worked example

A worked flashing example.

Worked example

Take a 30 ft wall-to-roof intersection on a single-story commercial building, detailed with both step flashing at the roof line and counter flashing let into the wall above. Step flashing runs the full 30 ft, so that is 30 lf. Counter flashing laps over it for the same run, another 30 lf, giving 60 lf of formed metal before allowances. Add 5 percent for laps and corner overlaps: 60 lf x 1.05 = 63 lf. Price 24 ga prefinished metal at an illustrative US installed rate of 12 dollars per lf (material plus labor): 63 lf x 12 dollars = 756 dollars for that intersection. If the spec bumps to a heavier gauge at 15 dollars per lf, the same 63 lf reads 945 dollars. Two numbers, one detail, and the gauge call drives a 189 dollar swing on a single 30 ft run.

How Ruh handles it

How Ruh handles flashing.

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Flashing: frequently asked questions

Why split flashing into separate line items instead of one lump linear-foot number?+

Because the material, gauge, profile, and labor differ. Step flashing woven into shingles, through-wall flashing with weeps, and a parapet cap are different installs at different unit costs, so a single blended number hides risk and misprices the work.

Do I count counter flashing and base flashing as one run or two?+

Two. They occupy the same intersection but are separate pieces of metal installed in separate operations, so you carry the linear footage twice, once for each profile, even when they share a length.

How much waste should I add to flashing lengths?+

A small lap and corner allowance, commonly a few percent, on top of the measured run. Tight inside and outside corners and the lap at each joint consume stock beyond the net dimension scaled off the detail.

What does flashing mean in construction?+

In construction, flashing means the sheet metal or membrane installed at joints, transitions, and penetrations in the building envelope to direct water away from the assembly and shed it to the exterior. The word covers a family of profiles, base, counter, step, through-wall, and cap, each detailed at a specific intersection and taken off in linear feet by type.

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Related terms

Window sillHouse wrap (WRB)EIFS (exterior insulation finish system)Curtain wallVeneerMembrane roofingTPO roofingThreshold

Keep going: read the full guide or explore roofing estimating software.

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Figures on this page are illustrative. Construction estimates depend on project-specific conditions, source documents, market pricing, and professional judgment. Ruh's AI assists the estimator and does not replace professional review: your team reviews, validates, and approves every estimate, bid, and pricing decision.

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