Water-Resistive Barriers and Flashing

Why the wall needs a second line

Siding isn’t watertight. Wind drives rain through laps and joints, and water runs behind trim, around windows, and down from decks. The defense is a second drainage plane behind the siding: the water-resistive barrier (WRB), with flashing at every opening and joint, arranged so water that gets behind the siding runs down and back out instead of into the wall.

The barrier

Wisconsin requires a WRB on every wood- or metal-frame exterior wall, from the highest point to the bottom of the siding ¹. Acceptable materials include polymeric housewraps, spray-applied barriers, #15 asphalt felt, and extruded foam sheathing with permanently taped joints ¹. Housewraps must have a vapor permeability of at least 5 perms, so moisture inside the wall can still dry outward ¹. Spray-applied WRBs may not be compatible with foam insulation, so check before mixing them ¹.

Sequence matters: cavity insulation may not go in until the wall has a water-resistant covering and the windows, doors, and at least roof underlayment are installed ¹.

Shingle lapping

Every layer of the drainage plane follows the rule roof shingles follow: upper over lower. For sheet WRBs ¹:

• Horizontal seams: the upper layer laps over the lower at least 2 inches. Start at the bottom.
• Vertical seams: lapped at least 6 inches.
• Rips and tears: patched with the same laps.
• Penetrations: fastener holes need no sealing; small penetrations (5 square inches or less) get caulked; larger ones get flashed.

A reverse lap, where a lower piece sits over an upper one, catches water and funnels it into the wall. It’s the most common mistake, often made when someone tucks housewrap behind a window flange at the head instead of over it at the sill.

Where to flash

Corrosion-resistant flashing has to extend to the face of the siding and keep water from re-entering. Joints in flashing are lapped 6 inches if vertical and 2 inches if horizontal, and sealed with compatible exterior-grade sealant ¹. Wisconsin requires flashing at ¹:

1. The top of every exterior door and window, unless the window is self-flashing with at least an inch of flange all around.
2. Where chimneys and masonry meet frame walls.
3. Under and at the ends of copings and sills.
4. Continuously above projecting wood trim.
5. Where porches, decks, or stairs attach to the wall or floor framing.
6. At wall and roof intersections.
7. At built-in gutters.
8. Along the bottom of door openings above grade, which also prepares for a future deck.

Two roof-wall details get specific rules: at a dormer, the rake-wall flashing runs at least half an inch down past the head-wall flashing, and where an eave meets a sidewall, the flashing’s end must kick water away from the wall and onto the roof or into the gutter, the “kickout” that prevents the rotted wall below so many roof-wall corners ¹.

The deck ledger is the classic failure: a deck bolted to the rim joist without flashing over it rots the rim from the outside in.

What to take from this

Behind every siding is a drainage plane: a WRB (housewrap of 5 perms or more, felt, or taped foam) from the top of the wall to the bottom, lapped upper over lower, 2 inches horizontal and 6 vertical, with flashing at windows and doors, masonry, sills, trim, decks, roof-wall intersections, gutters, and elevated doors. Kick out water where roofs meet walls, flash deck ledgers, and don’t insulate until the wall, openings, and roof are covered.