Every home needs a drainage plane.
Today's homebuyers are concerned about durability. A key aspect of durability is effective water management. Attractive exterior finishes make a home look great, but you can't rely on them to provide flawless water protection. No exterior finish, be it brick, stucco, or siding, is waterproof. Brick and stucco are porous. Siding expands and contracts, creating gaps between the boards. Some water from rain and snow will penetrate the exterior finish. That's why every home needs a drainage plane.
A drainage plane is a layer of water-resistant material that completely covers the home's exterior. When water penetrates the exterior finish, it moves down the drainage plane and back to the exterior. The drainage plane protects sheathing and framing from getting wet and prevents moisture buildup in exterior wall cavities.
Housewrap
In production homebuilding, housewrap has become a material of choice for creating the drainage plane on exterior walls. Housewrap keeps water out while allowing water vapor to pass through, so any water inside the wall cavity can dry to the exterior.
A home's exterior presents a challenge to effective housewrap installation. If home exteriors were designed to look like barns or sheds, with no windows, doors, or utility penetrations, installing housewrap would be a fairly straightforward process. But homes aren't simple structures; they not only have windows, doors, and penetrations, but also intersecting walls, roofs, and decks. So how does water drain around all of these features? The housewrap is integrated with flashing to guide water around them.
Windows
To guide water around window openings, housewrap is integrated, shingle style, with window flashing at the head, jambs, and sill. Head flashing is layered over jamb flashing, which is layered over a sill pan, which in turn is layered over the housewrap at the sill, to deflect water around windows, guiding it downward, so it doesn't enter the home's structure through the rough opening.
Doors
To guide water around door openings, housewrap is integrated, shingle style, with the door flashing at the head, jambs, and sill. Roll flashing is used to seal the door's brick mold to the housewrap. Head flashing is layered over jamb flashing, which is layered over a sill pan, which in turn is layered over the housewrap at the sill. A drip cap drains water away, so it doesn't sit on top of the door frame.
Utility penetrations
Around utility penetrations, preformed flashing panels, integrated with the housewrap, guide water downward and seal the penetrations against water entry.
Intersecting roofs
Where a wall terminates at a roof, water that has penetrated the exterior finish drains, via flashing, from the wall's drainage plane to the roofing. This flashing strategy keeps water from continuing downward into the home to wet the sheathing and framing.
Foundations and block walls
Where the framed wall meets the foundation or a block first-floor wall, the housewrap is extended at least 1" below the frame-to-foundation or frame-to-block intersection to bridge the gap and guide water downward as it continues down the drainage plane.
The deck
If the home has a deck, the deck ledger board, which is a piece of pressure-treated wood, is attached to an exterior wall to support the deck joists. As water moves down the wall, it may run behind or rest on top of the ledger board. Flashing, integrated with the housewrap, helps ensure that water will drain away from the wall assembly and the ledger board.
