Curtain Wall, Cladding, and Architectural Metals

Curtain Walls, Cladding, and Architectural Metals

Chapter Introduction

This chapter covers the installation, anchoring, and weatherproofing of curtain walls, exterior cladding systems, and architectural metals. For the Red Seal exam, you must master load transfer principles, installation tolerances, the requirements of the National Building Code of Canada (NBCC), and the CSA A660 standard (curtain wall certification) as well as CSA S157 (strength criteria for aluminum). Questions often focus on the sequence of operations, thermal expansion calculations, and weatherproofing defects.

Definitions and Fundamental Principles

Curtain Walls

A curtain wall is a non-load-bearing façade system, attached to the building structure, that supports only its own weight and the wind loads transmitted to it. It supports no floor or roof loads. It is distinguished from the traditional load-bearing wall by this absence of vertical structural function.

The main components are:

Mullions (vertical members): transmit vertical and horizontal loads to the anchors.

Transoms (horizontal members): connect the mullions and support the glazing.

Glazing panels (glass, insulating glass units).

Spandrel panels (fillers, sill panels): often opaque glass, metal, or stone.

Anchors and attachments: connect the system to the building structure.

Gaskets and sealants: ensure air and water tightness.

Cladding

Cladding is the non-structural exterior envelope of a building. It can be:

Vented (rain screen): a continuous air space behind the cladding, allowing ventilation and drainage.

Unvented: attached directly to a backing or framing without a continuous air space.

Common materials: metal panels (aluminum, stainless steel, zinc, copper), composite panels (ACM), fiber cement, terra cotta, natural or manufactured stone.

Architectural Metals

This category includes visible and decorative metal elements: cornices, belt courses, parapets, sills, handrails, grilles, and ornaments. They must meet strict dimensional tolerances and finishing requirements (anodizing, powder coating, patina).

Calculations and Tolerances

Thermal Expansion

Linear thermal expansion is calculated using the formula:

ΔL = α × L × ΔT

Where:

ΔL = change in length (mm)

α = coefficient of thermal expansion (mm/m·°C)

L = initial length (m)

ΔT = temperature change (°C)

Material	α (mm/m·°C)
Aluminum	0.023
Steel	0.012
Stainless steel	0.017
Glass	0.009
Concrete	0.010
Zinc	0.022
Copper	0.017

Calculation example: An aluminum mullion 3.5 m long is installed at 20 °C. The maximum service temperature is 60 °C. The expansion is:

ΔL = 0.023 × 3.5 × (60 − 20) = 0.023 × 3.5 × 40 = 3.22 mm

This calculation justifies the need for expansion joints and slip connections between mullions.

Installation Tolerances

According to CSA A660 and recognized practices, typical tolerances are:

Parameter	Tolerance
Vertical alignment of mullions	± 1.5 mm over 3 m
Horizontal alignment of transoms	± 1.5 mm over 3 m
Overall façade flatness	± 3 mm over 6 m
Gap between glass panels	± 1 mm
Anchor deviation from theoretical position	± 6 mm

Rule of thumb: The anchor tolerance must be more generous than that of the curtain wall system. Slotted holes in the anchors are used to absorb deviations in the concrete or steel structure.

Wind Loads and Pressure

The design wind pressure is determined according to the NBCC, Part 4 (structural design). The sheet metal worker must understand that:

Positive pressure (windward side) and negative pressure (suction, leeward side) act on the curtain wall.

External pressure coefficients (Cp) and internal coefficients (Cpi) are provided in the NBCC.

Net pressure is the difference between external and internal pressure.

The simplified formula: p = q × Cp × Ce × Cg

Where:

q = dynamic wind pressure (kPa)

Cp = external pressure coefficient

Ce = exposure coefficient

Cg = gust coefficient

Anchors must resist pull-out (tension) and shear. Chemical or mechanical anchor bolts are sized according to manufacturer specifications and verified by the engineer.

Installation Procedures

Sequence of Operations for a Curtain Wall

52.Structure survey: verify actual dimensions, flatness, and squareness of the slab or framing.

53.Anchor installation: attach adjustable anchors to the structure. Use approved bolts, expansion anchors, or chemical anchors.

54.Mullion placement: install the vertical members, align them with laser or theodolite, then secure them to the anchors.

55.Transom installation: connect the mullions with horizontal members, respecting slip connections to allow for expansion.

56.Glazing panel installation: insert glass units into the frames, install gaskets and pressure plates.

57.Weatherproofing: apply sealants to joints, install expansion joints.

58.Final inspection: water penetration test (spray test), verification of tolerances.

Anchors: Types and Criteria

Anchor Type	Use	Advantage	Limitation
Mechanical expansion anchor	Concrete	Quick installation	Sensitive to cracks
Chemical anchor (epoxy)	Concrete, masonry	High capacity, vibration resistant	Cure time required
Structural bolt	Steel	Precise, removable	Requires steel structure
Welded stud	Steel	Fast	Weld quality control required

Common trap: Never use an expansion anchor in cracked concrete without engineering verification. Anchor capacity can be reduced by 50% or more.

Weatherproofing and Water Management

The fundamental principle is drainage and ventilation: water that penetrates must be able to drain to the exterior through weep holes and open joints (pressure-equalized design).

The three levels of protection:

65.External barrier: the glazing, panels, and gaskets.

66.Drainage cavity: air space behind the cladding, with openings at the bottom.

67.Air/vapor barrier: continuous membrane on the interior side.

Golden rule: Water must always have an easier path out than in. Drainage openings must be at least as large as potential entry openings.

Sealants and Caulking

Sealant Type	Characteristics	Typical Use
Silicone	Excellent UV resistance, movement ± 25%	Glazing joints, expansion joints
Polyurethane	High mechanical strength, movement ± 15%	Concrete joints, sill sealing
Acrylic	Low cost, paintable	Interior joints, finishes
Butyl	Adhesion to metals, not paintable	Hidden joints, moisture barriers

Key requirement: Sealant must be applied to clean, dry, oil-free surfaces. A primer is often required on anodized aluminum or glass. The adhesion test (pull-off test) is mandatory before full application.

Applicable Codes and Standards

National Building Code of Canada (NBCC)

Part 3: Fire safety and occupant safety requirements. Curtain walls must meet flame spread and smoke requirements at floor levels.

Part 4: Structural design — wind loads, snow loads, seismic loads.

Part 5: Environmental separation — control of condensation, water penetration, and air leakage.

Part 9: For small buildings, simplified cladding requirements.

Article 5.4.1.1: Curtain walls must be designed to prevent rainwater penetration. The designer must choose a system that limits water accumulation or allows for its drainage.

CSA A660 — Curtain Wall Certification

This standard establishes certification requirements for curtain wall systems. It covers:

Structural performance (load testing).

Water performance (pressure rain test).

Air performance (infiltration test).

Component durability.

Typical test: The water penetration test is performed according to AAMA 501 or ASTM E331. The system is subjected to differential air pressure and water spray for 15 minutes. No water penetration into the interior plane is permitted.

CSA S157 — Strength Criteria for Aluminum

This standard provides allowable stresses for aluminum alloys used in construction. The sheet metal worker must know:

Common alloys: 6061-T6, 6063-T5, 6063-T6.

Allowable stresses in tension, compression, and shear.

Applicable safety factors.

Alloy	Temper	Tensile Strength (MPa)	Yield Strength (MPa)
6061	T6	290	240
6063	T5	185	145
6063	T6	240	205

Canadian Electrical Code, Part I

For curtain walls incorporating heating elements (glass defrosting) or motorized systems, the Canadian Electrical Code, Part I (CE Code) applies. Grounding and bonding requirements for metal frames must be respected. Metal mullions and transoms must be grounded if they are within 2.5 m of the ground or accessible.

Pitfalls to Avoid

95.Confusing curtain wall and load-bearing wall: The curtain wall supports no floor loads. Exam questions often test this distinction.

96.Forgetting thermal expansion: A 6 m aluminum mullion can expand more than 5 mm for a ΔT of 40 °C. Rigid connections cause buckling or anchor pull-out.

97.Neglecting structure tolerances: The concrete structure can vary by ± 25 mm. Anchors must be adjustable to absorb these deviations.

98.Reversing the sequence of operations: Always install anchors before mullions, and mullions before transoms.

99.Using an inappropriate sealant: An acrylic sealant on an aluminum expansion joint will crack quickly. Check the movement capacity (± %) of the sealant.

100.Ignoring drainage requirements: A curtain wall without weep holes will retain water and cause corrosion and infiltration.

101.Not verifying sealant adhesion: The adhesion test must be done before full application, not after.

102.Confusing the standards: CSA A660 is for system certification, CSA S157 for aluminum properties, NBCC Part 5 for environmental separation.

103.Forgetting grounding: Accessible metal frames must be grounded according to the Canadian Electrical Code.

104.Calculating expansion with the wrong unit: α is in mm/m·°C, so L must be in meters and ΔT in °C. A unit error gives a result off by a factor of 1000.

Summary

The curtain wall is non-load-bearing, attached to the structure, and must resist wind loads and its own weight.

Thermal expansion (ΔL = α × L × ΔT) is an essential calculation. Aluminum expands approximately twice as much as steel.

Adjustable anchors with slotted holes are essential for absorbing structure tolerances.

The drainage principle (weep holes, vented cavity) is the basis of water tightness.

Sealants must be chosen according to their movement capacity and compatibility with materials.

Key standards: NBCC Parts 3, 4, 5, CSA A660 (certification), CSA S157 (aluminum), Canadian Electrical Code, Part I (grounding).

Typical installation tolerances are ± 1.5 mm over 3 m for mullion alignment.

The water penetration test is mandatory for curtain wall system certification.

Slip connections allow expansion without transmitting stresses to the anchors.

Grounding of accessible metal frames is a non-negotiable electrical safety requirement.

Exam Tips

Memorize the expansion coefficients: aluminum 0.023, steel 0.012, glass 0.009. These values come up frequently.

Know how to identify diagrams: You will be shown a curtain wall section and asked to name the components (mullion, transom, gasket, weep hole, anchor).

Understand the logic of testing: The pressurized water test simulates wind-driven rain. The test pressure is often 300 Pa (approximately 30 kg/m²).

For expansion calculations: Read the units carefully. Temperature is in °C, length in meters, result in millimeters.

Questions on sealants often focus on product selection: silicone for glazing joints (high movement), polyurethane for concrete joints (mechanical strength).

Review the definitions: know how to distinguish curtain wall, cladding, storefront, and load-bearing wall. A storefront is a fixed glazing system for ground floors, often confused with a curtain wall.

This chapter covers the essential knowledge required to pass Red Seal questions on curtain walls, cladding, and architectural metals. Practicing expansion calculations and memorizing the standards are your best allies.