Materials, Fasteners, and Sealants

Materials, Fasteners, and Sealants

Chapter Introduction

This chapter covers the essential knowledge of materials, fasteners, and sealants that every journeyperson sheet metal worker must master for the Red Seal exam. You will be assessed on your ability to select the right material for a given application, choose the appropriate fastener based on mechanical and environmental stresses, and correctly apply sealants in accordance with national standards. This chapter integrates the requirements of the Canadian Electrical Code, Part I (for duct systems), CSA B149.1 (Natural Gas and Propane Installation Code), and CAN/CSA-S16 (Steel structures) standards where relevant.

2. Basic Sheet Metal Materials

2.1 Carbon Steel (Black Steel)

Carbon steel is the most widely used material in sheet metal ventilation and heating work. It contains less than 2% carbon and is divided into three categories based on its content:

Key Properties:

Common Gauges: The American gauge system (USG) is used in Canada. The following table provides nominal thicknesses:

Rule of Thumb: For rectangular ducts, the maximum unreinforced panel width depends on the gauge. A 600 mm wide duct in gauge 26 requires reinforcement if the panel length exceeds 1,200 mm.

2.2 Stainless Steel

Stainless steel contains a minimum of 10.5% chromium, which forms a protective passive layer of chromium oxide. The most common grades in sheet metal work:

Welding: Austenitic stainless steel (304, 316) is non-magnetic and welds easily. Ferritic steel (430) is magnetic and more difficult to weld.

Watch Out for Traps: Never use stainless steel in direct contact with carbon steel in the presence of moisture — this creates galvanic corrosion. Use isolators or stainless steel fasteners.

2.3 Aluminum

Aluminum offers an excellent strength-to-weight ratio and natural corrosion resistance through oxidation.

Properties:

Common Alloys:

Exam Trap: Aluminum cannot be arc-welded with the same parameters as steel. TIG welding is preferred. Brazing requires a special flux. Never use steel filler rods on aluminum.

2.4 Copper and Alloys

Copper is used for downspouts, flashings, and certain specialty ducts.

Properties:

2.5 Galvanizing (Galvanized Steel)

Galvanized steel is coated with a layer of zinc (by hot-dip immersion or electrolysis) to protect it against corrosion.

Methods:

Coating Classes (g/m²):

Important Rule: During bending or forming, the zinc layer can crack at tight bend radii. The minimum bend radius must be at least 1.5× the metal thickness to prevent zinc cracking.

2.6 Material Selection by Application

3. Mechanical Fasteners

3.1 Rivets

Rivets are permanent fasteners used to join sheet metal. The blind rivet (pop rivet) is the most common in sheet metal work.

Types of Blind Rivets:

Rivet Materials:

Rivet Length Calculation:

The rivet length must equal the total thickness of the materials being joined + 1.5× the rivet diameter to form the closing head.

Formula: L = T + 1.5D

Where:

Example: Joining two 1.2 mm sheets with 4.8 mm diameter rivets.

L = (1.2 + 1.2) + 1.5 × 4.8 = 2.4 + 7.2 = 9.6 mm → choose a 10 mm rivet.

Rivet Spacing: For rectangular ducts, maximum spacing is 150 mm on transverse joints and 100 mm on longitudinal joints. Corners must have a rivet within 25 mm of each end.

3.2 Self-Tapping Screws and Sheet Metal Screws

Sheet metal screws are classified by their point and thread type:

Screw Designation: A No. 8-18 × 1/2" screw means:

Penetration Rule: The screw must penetrate at least 3 threads into the base material. For sheet metal, minimum penetration is 5 mm.

Self-Drilling Screws: They drill, tap, and fasten in one operation. Common sizes are No. 8, No. 10, No. 12, and No. 14. Drilling capacity is indicated by the manufacturer (e.g., 1.5 mm, 3 mm, 6 mm).

Exam Trap: Do not confuse self-tapping screws (require a pilot hole) with self-drilling screws (drill their own hole). Self-tapping screws cut the threads but do not drill.

3.3 Welding

Welding is a permanent fusion fastening method. In sheet metal work, common processes are:

MIG Welding Parameters for Mild Steel:

Shielding Gases:

3.4 Mechanical Fasteners for Ducts

According to SMACNA and Canadian practices, duct assemblies use:

Fastener Spacing Rule on Flanges: Maximum 150 mm between fasteners, with a fastener at each corner.

3.5 Supports and Anchors

Duct supports must comply with the requirements of the Canadian Electrical Code, Part I (Rule 8-200 for ventilation ducts) and SMACNA practices.

Maximum Support Spacing for Rectangular Ducts:

Maximum Support Spacing for Round Ducts:

Types of Supports:

Important Rule: Supports must be made of galvanized or corrosion-protected steel. Threaded rods must have a minimum diameter of 8 mm (5/16") for ducts up to 1,200 mm wide.

4. Sealants

4.1 Types of Mastics and Sealants

Application Rule: Mastics must be applied to clean, dry, and grease-free surfaces. The minimum application temperature is generally +5 °C, unless otherwise specified by the manufacturer.

4.2 Duct Tapes

The Canadian Electrical Code, Part I and CAN/ULC-S110 (ventilation ducts) standards require that duct joints be airtight.

Types of Tapes:

Exam Trap: Ordinary duct tape is NOT compliant with code requirements for ventilation ducts. Only UL 181A-compliant aluminum tape or mastic is acceptable.

4.3 Duct Sealing per Code

The Canadian Electrical Code, Part I (Rule 8-200) requires that ventilation ducts be airtight and smoke-tight. Sealing classes per SMACNA standards:

Sealing Methods:

Rule: Longitudinal joints (seamed or welded) of round ducts must be sealed if pressure exceeds 500 Pa. Rectangular ducts must be sealed at all transverse and longitudinal joints.

4.4 Roofing and Flashing Sealants

For flashings and roof penetrations, the following products are used:

Rule: Flashings must be installed with a minimum 100 mm overlap at joints and a minimum slope of 60° from horizontal to ensure water runoff.

4.5 Material Compatibility

Important: Acidic silicone mastics (which emit a vinegar odor) can corrode galvanized steel and aluminum. Use only neutral (alkoxy) silicones for these metals.

5. Calculations and Conversions

5.1 Common Unit Conversions

5.2 Calculating Sheet Metal Mass

Formula: m = L × W × t × ρ

Where:

Example: Calculate the mass of a galvanized steel sheet measuring 1,200 mm × 2,400 mm, gauge 24 (0.61 mm).

m = 1.2 × 2.4 × 0.00061 × 7,850 = 13.78 kg

5.3 Calculating Developed Surface Area of a Duct

For a round duct with diameter D and length L:

Surface Area = π × D × L

For a rectangular duct with dimensions a × b:

Surface Area = 2 × (a + b) × L

Example: Round duct 300 mm in diameter, 5 m long.

Surface Area = π × 0.3 × 5 = 4.71 m²

5.4 Thermal Expansion

Formula: ΔL = α × L × ΔT

Where:

Example: Aluminum duct 10 m long installed at 10 °C, exposed to 60 °C.

ΔL = 23 × 10⁻⁶ × 10,000 × 50 = 11.5 mm

Rule: For long ducts (> 30 m), expansion joints must be provided. Maximum expansion joint spacing is 30 m for steel and 15 m for aluminum.

6. Applicable Standards and Codes

6.1 National Standards of Canada

6.2 Specific CSA B149.1 Rules

CSA B149.1 (Rule 8.4) specifies requirements for gas appliance venting ducts:

6.3 Rule 8-200 of the Canadian Electrical Code, Part I

This rule addresses ventilation ducts for electrical installations:

7. Installation Procedures and Best Practices

7.1 Surface Preparation

Before applying any sealant:

7.2 Mastic Application

7.3 Tape Application

7.4 Leakage Testing

Ducts must be tested according to the required sealing class:

Leakage Formula: Q = F × S

Where:

Example: Duct of 20 m², Class B (1.5 L/s·m²).

Q = 1.5 × 20 = 30 L/s maximum.

8. Traps to Avoid

9. Summary

10. Self-Assessment Questions

Answers:

This chapter covers all the knowledge required for the Red Seal sheet metal exam regarding materials, fasteners, and sealants. Review the reference tables and formulas before the exam. Good luck with your preparation.