Weather-Resistive Barriers, Flashings, and Weep Systems

Why This Layer Matters

Brick and stone veneer are not waterproof. Rain, wind-driven moisture, and condensation all find their way behind the face of the wall. The cavity behind a veneer wythe is designed to collect that water—but only if three elements work together correctly: the weather-resistive barrier (WRB), the through-wall flashing, and the weep system. When any one of these fails or is installed out of sequence, moisture is trapped against wood framing, causing rot, mold, and structural damage. Getting this sequence right is one of the most important skills a mason develops.

The Weather-Resistive Barrier (WRB)

The WRB—commonly called house wrap—is a sheet material fastened to the sheathing (usually OSB) before the veneer is laid. Its job is to allow water vapor to pass outward from the framing while blocking liquid water from moving inward. House wrap is not a vapor retarder; it is a drainage plane.

Installation rules:

• Start at the bottom and work upward, just like roofing shingles—each upper course laps over the course below by at least 6 inches so water drains down, not under.
• Vertical seams must lap a minimum of 6 inches.
• Fasten with cap staples or cap nails in the manufacturer's recommended pattern; loose, baggy wrap tears and admits water.
• At window and door openings, the WRB integrates with the window flashing pan and sill tape. The sill pan flashing goes in first, then the window unit, then the face of the WRB laps over the nailing flanges, and finally tape seals every penetration. Tape must be compatible with the WRB material—check the manufacturer's data sheet.
• All penetrations (pipes, conduit, anchor bolts) must be sealed with the manufacturer-approved tape or fluid-applied flashing material.

Through-Wall Flashing

Through-wall flashing sits at the base course of the veneer—and at any horizontal interruption like shelf angles or window heads—and directs water that has collected in the cavity back to the exterior. It must span the full width of the wythe and turn up behind the WRB on the back side.

Key requirements:

• Flashing material must be compatible with the masonry and mortar. Flexible self-adhering modified-bitumen membranes are widely used because they seal around fasteners and conform to irregular surfaces.
• The flashing runs continuously across the wall. Pieces must be lapped at least 6 inches and sealed so there are no gaps.
• End dams are upturned returns at each end of a flashing run (at corners, door jambs, or wall ends). Without end dams, water simply runs off the ends of the flashing and back into the wall cavity. End dams are bent from sheet metal or formed from the membrane material itself.
• The front edge of the flashing laps over the drip edge (a stainless or hot-dipped galvanized metal edge piece). The drip edge projects slightly beyond the face of the veneer, directing water out and away from the wall surface rather than letting it run back under.
• Flashing must not be punctured by ties or fasteners within the field of the wall. Masonry ties must be placed above the flashing plane.

Weep Systems

Weeps allow water that collects on top of the flashing to drain out of the cavity. They are placed in the first head joint above the flashing at regular intervals—typically every 24 inches on center, though local codes and manufacturer guidance may vary.

Two common types:

• Open head joints: A head joint is simply left open (no mortar) to create a hole. These drain quickly but can allow insects to enter the cavity. Mesh inserts or open-cell foam weep vents address that concern.
• Rope-wick weeps: A cotton or synthetic rope is placed in the head joint, with one end inside the cavity on the flashing and the other end hanging out the face. Capillary action draws water out. Rope wicks are less vulnerable to mortar plugging but must not be completely encased in mortar.

Mortar droppings are the enemy of weeps. During laying, keep the cavity and the surface of the flashing as clean as possible. Mortar that falls onto the flashing can harden and block weep openings. Some contractors place a mortar-collection device—a mesh strip or gravel bed—on the flashing to keep weeps open.

The Colored-Water Test

A simple field test verifies that the flashing and weep system actually work: pour or squeeze colored water into the top of the cavity above the flashing and watch where it exits. Water should appear promptly from every weep opening with no pooling visible. Any location where water does not exit—or where it appears outside a weep opening—points to a flashing gap, a blocked weep, or a missing end dam.

Safety Points

• Cuts: Utility knives used to cut house wrap and membrane flashing are a leading cause of hand lacerations. Always cut away from your body and retract the blade when not in use. Wear cut-resistant gloves when scoring metal drip edge.
• Falls: Work on panels that are properly supported or braced. Never lean a panel against a wall without securing the base.
• Adhesive membranes: Self-adhering flashings contain solvents and adhesives. Work in ventilated areas and wear nitrile gloves to protect skin.
• Eye protection: Flying staple tacks and mortar splatter both require safety glasses throughout this activity.
• Housekeeping: Keep the floor clear of tape scraps and cut material to prevent slip-and-fall hazards.