Flashing problems in a house: a major issue for the building’s waterproofing

Flashing is an essential component of the building envelope. Unfortunately, when poorly designed, deteriorated, or missing, it becomes a common source of water infiltration, structural deterioration, and mold growth. As a building expert, it is imperative to understand the different types of flashing, their respective functions, and the associated issues.

1. Foundation flashing: protection at the wall/foundation junction

Function

Foundation flashing is placed at the junction between the base of the exterior wall (siding) and the foundation. It acts as a waterproof barrier, preventing water from seeping between these two elements. It also redirects water to the outside of the envelope.

Common Problems

• Absence or discontinuity of flashing : In older buildings or during non-compliant renovations, the foundation flashing is sometimes omitted, leaving a direct path for water.

• Poorly positioned flashing : Too low or buried below ground level, it can no longer play its drainage role.

• Deformation or corrosion : Metallic materials (galvanized, aluminum) are susceptible to rust if not adequately protected.

• Thermal bridging and condensation : Poor flashing design can create a thermal bridge, promoting interior condensation and mold growth.

2. Head flashing: protection above openings

Function

Header flashing is installed above doors and windows. It prevents water from seeping into the wall through the junction between the opening and the exterior cladding.

Common Problems

• Common Omission : Header flashing is often missing in older homes or self-builds, exposing the structure to chronic leaks.

• Concealed Flashing : Some types of siding, such as masonry, can hide the flashing, making inspection or maintenance difficult.

• Poor overlap with the weather barrier : If the flashing is not properly integrated with the water barrier membrane, water can seep behind it rather than being redirected to the exterior.

3. Roof flashing: critical junctions to monitor

Function

Roof flashings are located at the junctions between the roof covering and other components (chimney, wall, skylights, ventilation, dormers, etc.). They prevent water from penetrating under the roofing materials.

Common Problems

• Poorly sealed or cracked flashing : Over time, caulking joints deteriorate, allowing water to seep in.

• Freeze/Thaw Deformation or Heave : Weather cycles can dislodge flashing, creating gaps invisible to the naked eye.

• Improper installation when replacing shingles : During roof renovations, some roofers reuse old flashings or install them without respecting the minimum overlaps.

• Debris Buildup : Leaves and debris can block drains around flashings, causing overflows.

Consequences of Flashing Failures

• Water infiltration inside

• Deterioration of the structure (frame, insulation, gypsum)

• Mold growth in walls or ceilings

• Efflorescence on masonry

• Degradation of thermal performance

Inspection and maintenance: an essential priority

Proper maintenance and periodic inspections can help prevent flashing problems. Here are some recommendations:

• Check the integrity of visible flashings (roof, foundation, openings).

• Ensure that the caulking joints are intact and flexible.

• Confirm the presence of compliant flashings during renovations.

• When purchasing real estate, have the flashings inspected by a recognized expert.

Conclusion

Flashings play a fundamental role in the waterproofing of a building's envelope. Too often neglected, their poor condition or poor design are the cause of numerous water infiltrations and hidden defect disputes. A rigorous assessment by an expert like GEQ – Bâtiment Conseil helps prevent damage and recommend appropriate fixes before problems worsen.