Roof flashing is thin, corrosion-resistant sheet metal or membrane that the 2021 International Residential Code (IRC) requires at wall and roof intersections, slope changes, and roof openings, with metal not less than 0.019 inch thick (Sections R903.2 and R903.2.1). The main types are step, base and cap, valley, kickout and chimney flashing, plus crickets that divert water around wide obstructions.
Flashing is any material that keeps water from getting through a joint or intersection in the roof assembly by directing it onto the roof covering and off the roof. It is most needed where the roof meets something vertical, because that is where shingles cannot lap in the normal way.
Key facts
- Flashing must be installed at wall and roof intersections, wherever there is a change in roof slope or direction, and around roof openings (2021 IRC Section R903.2.1).
- Where flashing is metal, it must be corrosion resistant and not less than 0.019 inch thick, which is No. 26 galvanized sheet (2021 IRC Section R903.2.1).
- A cricket or saddle is required on the ridge side of any chimney or penetration more than 30 inches wide, measured perpendicular to the slope (2021 IRC Section R903.2.2).
- Base flashing against a vertical sidewall must be continuous or step flashing, at least 4 inches high and 4 inches wide (2021 IRC Section R905.2.8.3).
- Mineral-surfaced roll roofing used as base flashing must weigh not less than 77 pounds per 100 square feet (2021 IRC Section R905.2.8.1).
- Open valley linings of metal must be not less than 24 inches wide (2021 IRC Section R905.2.8.2).
Where is roof flashing required?
Flashing is required where water could enter at a joint, intersection or penetration. Section R903.2 states the goal: flashings must prevent moisture from entering the wall and roof through joints in copings, through moisture-permeable materials, and at intersections with parapet walls and other penetrations through the roof plane.
Section R903.2.1 then lists the locations:
- Wall and roof intersections, such as a sidewall, a front wall, or a dormer cheek.
- Changes in roof slope or direction, which includes valleys.
- Around roof openings, such as chimneys, skylights, plumbing vents and roof hatches.
- Where the eave of a sloped roof intersects a vertical sidewall, where a flashing must divert water away.
What are the types of roof flashing?
The types are named by where they sit and how they are formed. The code has rules for some and defers others to the shingle manufacturer.
Types of roof flashing, where each goes, and the 2021 IRC section that covers it for asphalt shingle roofs.
| Type | Where it goes | 2021 IRC |
|---|---|---|
| Step flashing | Sidewall, in pieces lapped with shingle courses | R905.2.8.3 |
| Continuous (apron) flashing | Sidewall or front wall, one long piece | R905.2.8.3, R905.2.8.4 |
| Base and cap flashing | Base under the covering, cap over it at the wall | R905.2.8.1 |
| Valley flashing or lining | Inside corner where two planes meet | R905.2.8.2 |
| Kickout (diverter) | Bottom of a sidewall at the eave | R903.2.1 |
| Chimney and vent pipe flashing | Around chimney, soil stack, vent pipe | R905.2.8.4 |
| Cricket or saddle | Ridge side of wide chimney or penetration | R903.2.2 |
| Drip edge | Eaves and rake edges | R905.2.8.5 |
The drip edge guide covers the last row in full.
How does step flashing work?
Step flashing is a run of separate bent metal pieces, one lapped under each course of shingles, with one leg on the roof and the other turned up the vertical wall. It is the usual way to meet the base flashing rule where a sloped roof meets a sidewall.
Section R905.2.8.3 says base flashing against a vertical sidewall shall be continuous or step flashing, not less than 4 inches in height and 4 inches in width, and shall direct water away from the sidewall onto the roof or into the gutter. The same section covers wall coverings:
- Siding on the sidewall. The vertical leg of the flashing must be continuous under the siding.
- Anchored masonry veneer. Base flashing goes in per the section, and counterflashing per Section R703.8.2.2.
- Exterior plaster or adhered masonry veneer. Base flashing goes in per the section and Section R703.6.3.
The code does not set the length of each step piece, the nail position or the head-lap between pieces. Those details come from the shingle manufacturer's installation instructions, which Section R905.2.8 makes part of the requirement.
What is the difference between step and continuous flashing?
Step flashing is made of many small pieces laid with each shingle course, so every piece sheds onto the shingle below it. Continuous flashing is one long piece, which the code also permits against a sidewall. Section R905.2.8.4 sends flashing against a vertical front wall to the manufacturer's printed instructions.
What is kickout flashing and is it required?
Kickout flashing is an angled piece at the bottom end of a step flashing run that throws roof runoff into the gutter. The word kickout is trade language, not code language, but the 2021 IRC covers the condition: Section R903.2.1 requires a flashing to be installed to divert the water away from where the eave of a sloped roof intersects a vertical sidewall.
Without it, the water running down along the wall turns the corner at the eave and goes down behind the siding. That is a common source of hidden wall rot, which is why a kickout is the usual way to satisfy the diverter rule. The code gives no dimensions for it, so use the product made for the job and the manufacturer's installation instructions.
When does a chimney need a cricket?
A chimney needs a cricket when it is more than 30 inches wide, measured perpendicular to the slope. A cricket, also called a saddle, is a small ridge built behind the chimney to split the water flowing down the slope and send it around both sides.
Section R903.2.2 sets the rule:
- A cricket or saddle shall be installed on the ridge side of any chimney or penetration more than 30 inches wide as measured perpendicular to the slope.
- Cricket or saddle coverings shall be sheet metal or the same material as the roof covering.
- Exception: unit skylights installed in accordance with Section R308.6 and flashed in accordance with the manufacturer's instructions may be installed without a cricket or saddle.
A cricket is a framing element as well as a flashing one: it needs small framing members and sheathing on the roof deck before the metal goes on. On a roof framing plan it shows as a small peaked shape on the ridge side of the chimney. See roof framing plan. For chimney flashing itself, Section R905.2.8.4 says chimney, soil stack and vent pipe flashing shall be applied in accordance with the asphalt shingle manufacturer's printed instructions.
What are base flashing and cap flashing?
Base flashing is the lower layer that sheds water onto the roof covering, and cap flashing, or counterflashing, is the upper layer that covers the top edge of the base flashing at a wall. Section R905.2.8.1 says both go in per the manufacturer's instructions. Base flashing must be corrosion-resistant metal of minimum nominal 0.019 inch thickness or mineral-surfaced roll roofing weighing not less than 77 pounds per 100 square feet. Cap flashing must be corrosion-resistant metal of minimum nominal 0.019 inch thickness.
What are the valley flashing options?
Valley linings go under the shingles before they are applied, and the code allows several materials. Section R905.2.8.2 permits:
- Open valley, metal: not less than 24 inches wide, in a corrosion-resistant metal from Table R905.2.8.2 (for example aluminum at 0.024 inch minimum thickness or galvanized steel at 0.0179 inch, 26 gage).
- Open valley, roll roofing: two plies of mineral-surfaced roll roofing, the bottom layer 18 inches and the top layer not less than 36 inches wide.
- Closed valley: one ply of smooth roll roofing not less than 36 inches wide, or either lining above, or self-adhering polymer-modified bitumen underlayment in lieu of the lining.
Widths for tile, slate, wood and metal-shingle roofs differ, and the roof valley guide tabulates them rather than repeating them here.
How does flashing work with underlayment and ice barrier?
Flashing sits in the water-shedding sequence: underlayment and any ice barrier lie below the shingles and valley linings, and the flashing laps over or under them so each layer sheds to the next. Underlayment follows Section R905.1.1, and the ice barrier follows Section R905.1.2, which extends 24 inches inside the exterior wall line at the eave. The ice and water shield guide covers it. The order of flashing and shingle components is set by the manufacturer's installation instructions.
What flashing mistakes cause leaks?
- Skipping the diverter at an eave and sidewall. Section R903.2.1 requires it.
- Flashing narrower than 4 inches by 4 inches at a sidewall. That is the minimum in Section R905.2.8.3.
- Metal thinner than 0.019 inch. That is the code minimum for metal flashing.
- No cricket behind a chimney wider than 30 inches. Water ponds behind the masonry.
- Siding laid over the vertical leg. The leg must be continuous under siding, not trimmed behind it.
- Treating chimney flashing as a code detail. The code defers it to the shingle manufacturer's printed instructions.
Putting flashing on a plan
Flashing belongs in the roof details and sections. The framing plan shows what the flashing needs: wall intersections, openings, valleys, and crickets. Framing Plan is a preliminary drawing aid that draws joist layouts, beams and openings and exports PNG, PDF and DXF. It does not check code or select flashing, and it works in millimeters. The adopted code edition and local amendments govern, and manufacturer instructions govern each product.
Summary
The 2021 IRC requires flashing at wall and roof intersections, slope changes and openings, in corrosion-resistant metal at least 0.019 inch thick. Sidewall base flashing is continuous or step, at least 4 by 4 inches, and a diverter is required where an eave meets a sidewall. A cricket or saddle is required behind any chimney or penetration wider than 30 inches.
Open the framing studio to draw the walls, openings and chimney framing in a preliminary roof plan before it goes to your designer or plan reviewer.