Module 3: Waterproofing and Flashing Systems

Principles of Water Management

Effective water management is critical to preventing moisture intrusion, which can lead to structural damage, mold growth, and energy loss. The key principle is "drainage over reliance on sealants"—allowing water to flow away from vulnerable areas rather than depending solely on barriers.

Core Concepts:

1. Overlap & Shingle Principle

• Flashing layers should overlap like shingles, directing water down and out.

• Bottom flashing must always be installed first, followed by sides, then top.

2. Capillary Breaks

• Small gaps (1/8"–1/4") between materials prevent water from wicking upward.

3. Drainage Plane

• A continuous path (e.g., WRB + flashing) ensures water exits the wall system.

4. Redundancy

• Multiple layers (e.g., flashing + sealant) provide backup protection.

Types of Flashing

Flashing materials vary in durability, flexibility, and application methods.

1. Self-Adhesive (Peel-and-Stick) Flashing

• Material: Rubberized asphalt or butyl with a sticky backing.

• Pros:

• Easy to install (no tools required).

• Conforms well to irregular surfaces.

• Cons:

• Can fail in extreme heat or UV exposure.

• Requires a clean, dry surface for adhesion.

• Best For:

• Window/door rough openings (sill pan flashing).

• Integration with WRB (e.g., Vycor, FlexWrap).

2. Metal Flashing

• Materials: Aluminum, copper, galvanized steel.

• Pros:

• Extremely durable (50+ year lifespan for copper).

• Resists UV and temperature extremes.

• Cons:

• Requires cutting/bending tools.

• Can corrode if dissimilar metals contact (e.g., aluminum + steel).

• Best For:

• Roof-to-wall intersections.

• Masonry applications (e.g., through-wall flashing).

3. Flexible Flashing

• Materials: PVC, EPDM, or composite membranes.

• Pros:

• Stretches to fit curves (e.g., arched windows).

• Resistant to tears.

• Cons:

• May degrade under prolonged UV exposure.

• Best For:

• Stucco or stone veneer details.

• Complex geometries.

Integration with Weather-Resistant Barriers (WRB)

A WRB (e.g., Tyvek, Typar) is the primary moisture barrier behind cladding. Flashing must integrate seamlessly to maintain continuity.

Installation Sequence:

1. Install WRB over sheathing (lapped shingle-style).

2. Apply sill flashing (self-adhesive or metal) over WRB at the bottom.

3. Side flashing overlaps sill flashing and WRB.

4. Top flashing overlaps side flashing and WRB.

5. Terminate WRB at the flange or use fluid-applied flashing for a sealed transition.

Critical Details:

• Window/door flanges must lap over flashing (not behind).

• End dams (folded metal edges) prevent water from spilling off sides.

Sealant Selection and Application

Sealants fill gaps and provide secondary waterproofing but should not replace proper flashing.

1. Silicone Sealant

• Pros: Flexible, UV-resistant, long lifespan (20+ years).

• Cons: Not paintable, can attract dirt.

• Best For: Exterior joints (window perimeters, metal flashing).

2. Polyurethane Sealant

• Pros: Paintable, strong adhesion, good movement tolerance.

• Cons: Shorter lifespan than silicone (10–15 years).

• Best For: Gaps between dissimilar materials (e.g., wood-to-masonry).

3. Butyl Tape

• Pros: Stays pliable, excellent for irregular gaps.

• Cons: Messy to apply, not for high-movement areas.

• Best For: Setting window/door flanges before fastening.

Application Tips:

• Clean surfaces (dust/oil reduces adhesion).

• Backer rod fills deep gaps before sealant.

• Tool the bead for a water-shedding shape (concave for vertical joints).

Key Takeaways

✔ Water must flow outward—flashing layers should overlap correctly.
✔ Match flashing type to the application (metal for durability, self-adhesive for ease).
✔ Sealants are secondary—never rely on them alone for waterproofing.