Through-Wall Flashing: How Masonry Walls Manage Water
Masonry walls are rain screens, not waterproof barriers. Wind-driven rain penetrates the outer wythe (the exterior brick or stone face) as a matter of course. The building envelope system manages this by collecting the infiltrating water at specific locations and directing it back out through weep holes.
Through-wall flashing is the metal or membrane layer that spans the full thickness of the masonry wall at these collection points, acting as a shelf that catches water and routes it to weep holes spaced at 24" on center. Key locations for through-wall flashing in masonry construction include:
- At the base of the wall just above grade or the foundation ledge
- Above all window and door lintels (opening heads)
- At floor-line shelf angles and relieving angles that support the masonry above
- At roof-to-wall junctions and parapets (as base flashing)
Cap Flashing at Parapets
At parapet walls, cap flashing works differently: rather than being embedded through the wall, it is inserted into a reglet near the base of the parapet interior wall face (or at the wall cap) and hangs down to lap the base flashing below it. This two-piece system allows the base flashing (attached to the roof) and cap flashing (attached to the wall) to move independently — critical for long-term performance at the roof-to-wall junction.
Profile Geometry
Through-wall flashing profiles vary by application:
- Flat (horizontal) TWF — a horizontal sheet that spans the wall thickness, with a drip return at the outer face; used at shelf angles and base-of-wall locations
- Lintel TWF — similar to flat TWF but with a back-dam at the inner face to prevent water from draining inward; used above lintels
- Cap flashing (parapet) — an L-shape inserted in a reglet with an exposed leg that laps the base flashing; see the Reglet & Counterflashing page
Material Selection
| Material | Weight/Thickness | Application |
|---|---|---|
| Copper | 16 oz. | Traditional standard for TWF in masonry — durable, mouldable, bonds with mortar |
| Stainless steel | 26 ga. | Marine environments, where copper staining is unacceptable |
| Aluminum | .040" | Cap flashing at parapets; not recommended for mortar-embedded TWF (alkali attack) |
| Lead-coated copper | 3 lb./sq ft | Historical restoration, complex geometry |
Material note: Do not use aluminum in mortar-embedded through-wall flashing applications. Mortar is highly alkaline and will corrode aluminum within a few years. Copper or stainless steel are the correct choices for mortar-embedded flashing.