Architectural Flashing Supply
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Water management and enclosure basics · Lesson 2 of 4

Water management and enclosure basics: materials, standards and design constraints

What the 2021 IRC as published by Seattle says about where flashing goes and how big it is, the 0.019 in metal minimum compared with AFS register thicknesses, outward slope and drips, sealant as a back-up, and typical practice versus code.

You will be able to: Identify what the 2021 residential code, as one jurisdiction publishes it, requires for flashing locations, sidewall flashing, drip edge, the wall barrier and metal thickness, and the design constraints and sealant rules that apply on top.

14 min · 12 slides · 11 questions

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Slide 1 of 12

Codes set the minimum

This module reads the 2021 International Residential Code as the City of Seattle publishes it, with Seattle's amendments.

The edition and amendments your own jurisdiction adopts govern, and the maker's printed instructions apply too.

Code text names minimums. Drawings, specifications and product instructions can ask for more.

Code numbers at a glance from the 2021 IRC as Seattle publishes itTable of nine code numbers: sidewall flashing at least 4 inches high and 4 wide; drip edge overlap at least 2 inches, below the sheathing at least 1/4 inch, up onto the deck at least 2 inches, fasteners at most 12 inches on center; wall barrier horizontal laps at least 2 inches and joints at least 6 inches; a cricket behind a chimney wider than 30 inches; metal flashing at least 0.019 inch.Code numbers at a glanceSidewall flashingasphalt roofsat least 4 in high and 4 in wideR905.2.8.3Drip edge overlapasphalt roofsat least 2 inR905.2.8.5Drip edge below sheathingasphalt roofsat least 1/4 inR905.2.8.5Drip edge up onto the deckasphalt roofsat least 2 inR905.2.8.5Drip edge fastenersasphalt roofsat most 12 in on centerR905.2.8.5Wall barrier, horizontal lapat least 2 inR703.2Wall barrier, jointsat least 6 inR703.2Cricket behind a chimneyexceptions listedwider than 30 inR903.2.2Metal flashing thicknessat least 0.019 in (0.5 mm)R903.2.12021 IRC as Seattle publishes it

Table of nine code numbers. The first five are from the asphalt shingle section and are marked asphalt roofs: sidewall flashing at least 4 inches high and 4 wide; drip edge overlap at least 2 inches, below the sheathing at least 1/4 inch, up onto the deck at least 2 inches, fasteners at most 12 inches on center. Then wall barrier horizontal laps at least 2 inches and joints at least 6 inches; a cricket behind a chimney wider than 30 inches, with exceptions listed; metal flashing at least 0.019 inch.

The numbers from this lesson with their section numbers, as the City of Seattle publishes the 2021 IRC. The R905.2.8 rows sit in the asphalt shingle section and apply to asphalt shingle roofs. Your jurisdiction's edition and amendments govern.

Sources

Glossary

R903.2.1
The 2021 residential code section that says where flashing is required: wall and roof intersections, changes of roof slope or direction and around roof openings, with a diverter where a sloped roof eave meets a sidewall, and corrosion-resistant metal at least 0.019 in (0.5 mm) thick.
Water-resistive barrier (WRB)
The layer applied over studs or sheathing behind the exterior wall covering. The 2021 residential code calls for at least one layer, continuous to the top of the walls, with the upper layer lapped over the lower.
Nominal thickness
The catalog thickness of a metal, as opposed to a measurement of the actual sheet. AFS lists 26 ga galvanized steel at 0.0217 in nominal in its material register.
Outward slope
A flashing that falls away from the wall so that water runs out onto the face or into a drain. Building Science Corporation says flashing must be installed continuously with an outward slope.