Why Flashing and Sealing Matter

Roof and wall panels cover most of a steel building, but many water leaks occur around the connections between those panels.

Flashing, trim and sealing materials form the final waterproofing line at:

• Ridges

• Eaves

• Gables

• Corners

• Doors

• Windows

• Panel overlaps

• Rooflights

Correct detailing can significantly improve long-term envelope performance.

Common Flashing Components

Ridge Cap

Installed at the roof ridge to close the gap between opposing roof slopes.

Gable Flashing

Protects the connection between the gable wall and roof.

Eave Trim

Finishes and protects the roof edge.

External Corner Trim

Covers and seals external wall corners.

Internal Corner Trim

Used at internal wall intersections.

Window Sill Flashing

Directs water away from the bottom of window openings.

Window Head Flashing

Helps prevent rainwater entering above windows.

Door Head Flashing

Protects the top of door openings.

Base Flashing

Protects the lower wall connection.

Basic Flashing Design Principles

A good flashing design should follow the natural direction of water.

As a general concept:

Upper components overlap lower components.

This allows rainwater to flow outward instead of entering joints.

Other key principles include:

• Adequate overlap

• Continuous sealing where required

• Compatible materials

• Appropriate fastener spacing

• Correct drainage direction

Butyl Tape

Butyl tape is commonly used in metal building envelope joints.

Applications may include:

• Panel overlaps

• Rooflight overlaps

• Flashing joints

• Metal-to-metal sealing

The tape should be compressed properly and installed on a clean surface.

Neutral Silicone Sealant

Neutral silicone sealants are widely used around:

• Flashing edges

• Window connections

• Door connections

• Metal panel joints

The sealant should be compatible with the panel coating.

Neutral-cure products are generally preferred over acidic sealants where corrosion of coated metal could be a concern.

EPDM Sealing Washers

Self-drilling screws used in exposed-fastener roofing often include EPDM sealing washers.

The washer creates a seal between the fastener head and metal panel.

Fasteners should not be over-tightened because excessive compression can damage the washer.

Polyurethane Foam

PU foam can be used to fill irregular gaps.

However, it should normally be considered an auxiliary material rather than the sole waterproofing solution.

Surface Preparation

Sealants should be installed on surfaces that are:

• Dry

• Clean

• Free from oil

• Free from loose dust

• Compatible with the sealing material

Environmental temperature and curing conditions should also follow the sealant manufacturer's requirements.

Common Waterproofing Mistakes

Typical causes of leakage include:

• Insufficient overlap

• Missing butyl tape

• Incorrect screw installation

• Damaged EPDM washers

• Discontinuous sealant

• Poor flashing geometry

• Water flowing against the overlap direction

Good quality control during installation is therefore essential.

Conclusion

Flashing, trim and sealing materials may represent a relatively small portion of a steel building, but they strongly influence waterproofing performance.

The long-term success of a steel building envelope depends not only on the main panels, but also on careful detailing of every joint and interface.