Support Preparation and Moisture Management
Chapter Introduction
Support preparation is the most critical step in any tiling project. A tile properly installed on an inadequate support will inevitably fail, regardless of the quality of the tile or adhesive. For the Red Seal exam, you must master the principles of support preparation, moisture requirements, leveling methods, and applicable Canadian standards. This chapter covers all the knowledge required to succeed on questions covering this topic.
2.1 Roles and Functions of the Support
The support is the surface on which the tile is installed. It must fulfill four essential functions:
7.Structural strength: support the weight of the tile, adhesive, and live loads.
8.Dimensional stability: minimize movement due to temperature and humidity variations.
9.Flatness: provide a surface sufficiently flat to meet installation tolerances.
10.Adhesion: allow for adequate mechanical and chemical bonding with the thin-set mortar.
2.1.1 Classification of Supports
| Support Type | Examples | Key Characteristics |
|---|
| **Rigid supports** | Concrete, mortar, brick, concrete blocks | Excellent stability, requires cleaning and sometimes a waterproofing membrane |
| **Semi-rigid supports** | Cement backer boards, gypsum boards, plywood | Good stability, requires installation in accordance with standards |
| **Flexible supports** | Acoustic underlayment membranes, radiant floors | Must be compatible with the chosen tile system |
2.2 Concrete Requirements
Concrete is the most common support for commercial and institutional installations. The following requirements apply:
2.2.1 Concrete Age and Moisture
Concrete must have a minimum 28 days of curing before tile installation.
The relative humidity of the concrete must be below 75% (measured with a probe hygrometer).
The moisture vapor emission rate (MVER) must be below 3.5 kg/100 m²/24 h for installations with standard thin-set mortar.
For installations with moisture-sensitive adhesives, the MVER must be below 1.5 kg/100 m²/24 h.
> MVER Calculation Formula: MVER = (mass of evaporated water in kg) ÷ (surface area in m²) ÷ (time in days) × 100
2.2.2 Surface Cleaning and Preparation
The concrete must be:
Free of laitance: laitance is a thin layer of cement and fine particles that forms on the surface. It must be removed by sandblasting, shot blasting, or mechanical scarifying.
Free of contaminants: oils, greases, waxes, paints, and curing compounds must be completely eliminated.
Free of active cracks: structural cracks must be repaired with a suitable product (injectable epoxy or polyurethane).
2.2.3 Flatness Tolerances
| Installation Type | Maximum Tolerance | Measurement Method |
|---|
| Tiles up to 300 mm | 3 mm under a 3 m straightedge | 3 m straightedge |
| Large-format tiles (> 300 mm) | 1.5 mm under a 3 m straightedge | 3 m straightedge |
| Natural stone | 1 mm under a 3 m straightedge | 3 m straightedge |
2.3 Cement Backer Boards and Gypsum Boards
2.3.1 Cement Backer Boards (Durock, WonderBoard, etc.)
Cement backer boards are used in wet areas and on floors. Their installation must follow these rules:
Minimum thickness: 12.5 mm for walls, 15 mm for floors.
Spacing between panels: 3 mm minimum, 6 mm maximum.
Perimeter spacing: 6 mm minimum from walls and obstructions.
Fastening: countersunk screws in stainless steel or galvanized, spaced 150 mm at the center of supports and 200 mm on the edges.
Joints: must be filled with thin-set mortar and covered with mesh tape (fiberglass).
> Important rule: Never use ordinary flat-head screws — they do not countersink properly into the board and create bumps.
2.3.2 Gypsum Boards (Drywall)
Regular gypsum board must never be used in wet areas (showers, baths, saunas). For dry areas, the requirements are:
Type X (fire-resistant): mandatory in certain commercial applications according to the building code.
Type H (water-resistant): required for residential wet areas, but does not replace a waterproofing membrane.
Joint finishing: joints must be finished with a moisture-resistant joint compound.
2.4 Plywood and Wood Supports
Plywood is used as a support in certain residential applications, but its use is strictly regulated:
2.4.1 Plywood Requirements
Minimum thickness: 19 mm (3/4 in) for floors.
Exposure: must be classified Exposure 1 or Exterior according to CSA O121 or CSA O151.
Spacing: 3 mm between panels to allow for expansion.
Double layer: recommended for natural stone installations (two layers of 15 mm minimum, with staggered joints).
2.4.2 Usage Limitations
Plywood must never be used as a support for:
Showers and bathtubs
Floors with electric radiant heating
High-traffic commercial installations
> Exam trap: Plywood is acceptable for residential floors provided that the joists are spaced a maximum of 400 mm on center and the plywood is grade CCX or better.
2.5 Waterproofing Membranes and Moisture Management
2.5.1 Types of Membranes
| Membrane Type | Application | Advantages | Disadvantages |
|---|
| **Liquid membrane** (epoxy, polyurethane, acrylic) | Shower walls and floors | Easy application, no seams | Requires multiple coats, drying time |
| **Sheet membrane** (PVC, PE, rubber) | Floors, walls | Continuous waterproofing, durable | Seams need welding, more complex |
| **Uncoupling membrane** (matting) | Floors with movement | Reduces crack transmission | Higher cost |
| **Acoustic insulation membrane** | Multi-storey floors | Reduces noise transmission | Requires flexible thin-set mortar |
2.5.2 Waterproofing Requirements for Showers
According to the National Building Code of Canada (NBC), all showers must have a waterproofing membrane:
Minimum height: 2 m above the shower floor, or up to the ceiling if the shower has no door.
Extension beyond the shower: 150 mm minimum beyond the shower opening.
Waterproofing test: the membrane must be tested for 24 hours with a water height of 25 mm before tile installation.
2.5.3 Vapour Barrier vs. Moisture Barrier
It is essential to distinguish between:
Vapour barrier: prevents the diffusion of water vapour through the assembly (6 mil polyethylene).
Moisture barrier: prevents liquid water from penetrating (waterproofing membrane).
> Red Seal rule: The vapour barrier must be placed on the warm side of the assembly (interior side in Canadian climate). The waterproofing membrane, however, is placed on the side where water is present.
2.6 Leveling and Support Compensation
2.6.1 Self-Leveling Compounds
Self-leveling compounds are used to correct flatness defects:
| Compound Type | Minimum Thickness | Maximum Thickness | Application |
|---|
| Standard self-leveling compound | 3 mm | 25 mm | Interior floors |
| High-strength compound | 3 mm | 50 mm | High-traffic floors |
| Fast-drying compound | 3 mm | 20 mm | Projects with tight deadlines |
2.6.2 Application Procedure
78.Clean the support: remove dust, grease, and laitance.
79.Prime: apply a primer to improve adhesion.
80.Mix: follow the manufacturer's exact water-to-powder ratio.
81.Pour: pour the compound and spread it with a squeegee.
82.Deaerate: use a spiked roller to eliminate air bubbles.
83.Allow to dry: respect the drying time before tile installation.
> Quantity calculation: Volume (L) = Area (m²) × Thickness (mm) × 1.5 (waste factor). For example, for 20 m² at 5 mm: 20 × 5 × 1.5 = 150 L.
2.6.3 Tile Leveling Systems
Leveling systems (clips and wedges) are mandatory for large-format tiles (> 600 mm):
Clip spacing: 300 mm maximum along the edges.
Clip type: to be used according to tile thickness (3-6 mm, 6-12 mm, 12-20 mm).
Removal: clips must be removed after the initial curing of the thin-set mortar (typically 24 hours).
2.7 Radiant Floors and Tiling
2.7.1 Specific Requirements
Electric or hydronic radiant floors impose particular constraints:
Maximum surface temperature: 28 °C for ceramic tiles, 25 °C for natural stone.
Temperature gradient: must not exceed 5 °C per hour.
Start-up: the system must be started gradually (increase of 5 °C per day) before tile installation to eliminate residual moisture.
2.7.2 Thin-Set Mortar for Radiant Floors
Use a flexible thin-set mortar (class C2 or higher according to ISO 13007).
Minimum thin-set mortar thickness: 3 mm above the cable or tubing.
Never use gypsum-based thin-set mortar on a radiant floor.
2.8 Applicable Canadian Standards
2.8.1 Main Standards
| Standard | Title | Application |
|---|
| **CSA A23.1/A23.2** | Concrete: constituents and execution of work | Concrete preparation |
| **CSA O121** | Softwood plywood | Plywood supports |
| **CSA O151** | Hardwood plywood | Plywood supports |
| **CAN/CGSB-51.34-M** | Gypsum boards | Gypsum supports |
| **ISO 13007** | Grouts and adhesives for tiles | Classification of thin-set mortars |
| **ANSI A108** | Tile installation | Installation procedures |
| **TCSB (Terrazzo, Tile and Marble Association of Canada)** | Installation manual | Complete reference guide |
2.8.2 National Building Code (NBC)
The NBC 2020, article 9.29.5.2, requires that tile supports be:
Stable: capable of supporting the intended loads.
Flat: with a tolerance of 10 mm under a 3 m straightedge for walls, 8 mm for floors (before leveling).
Dry: with a relative humidity below 75% for concrete.
2.9 On-Site Testing and Inspections
2.9.1 Concrete Moisture Testing
Probe hygrometer method (most reliable):
114.Drill into the concrete to a depth of 40% of the slab thickness.
115.Insert the probe and seal the opening.
116.Wait 72 hours for stabilization.
117.Read the relative humidity percentage.
Calcium chloride method (water vapour test):
119.Place a calcium chloride container on the slab.
120.Seal with a plastic dome.
121.Leave in place for 72 hours.
122.Weigh the container before and after to calculate the MVER.
2.9.2 Adhesion Test
The pull-off adhesion test is used to verify the bond between the thin-set mortar and the support:
Minimum value: 0.5 MPa for ceramic tiles.
Minimum value: 1.0 MPa for natural stone.
Number of tests: minimum 5 tests per 100 m² area.
2.10 Pitfalls to Avoid
130.Installing on concrete that is too new: concrete must have 28 days of curing. Younger concrete releases moisture that will destroy the adhesion of the thin-set mortar.
131.Confusing vapour barrier and waterproofing membrane: the vapour barrier prevents vapour diffusion, the membrane prevents liquid water. They are not interchangeable.
132.Ignoring cement laitance: laitance prevents adhesion. It must be removed mechanically, not simply swept.
133.Using regular gypsum board in a shower: this is a violation of the NBC and a guaranteed cause of failure.
134.Neglecting perimeter expansion spacing: without a minimum 6 mm spacing, tiles will lift due to compression.
135.Applying self-leveling compound too thick: beyond the manufacturer's maximum thickness, the compound will crack and lose its adhesion.
136.Forgetting the membrane waterproofing test: the 24-hour test is mandatory before tile installation.
137.Using standard thin-set mortar on radiant floors: only flexible thin-set mortar (class C2) can absorb the dimensional variations caused by heat.
138.Not priming the support before leveling: primer is essential for the adhesion of the self-leveling compound.
139.Installing large-format tiles without a leveling system: this causes unacceptable flatness defects.
2.11 Summary
The support must be stable, flat, dry, and clean before any tile installation.
Concrete must have 28 days of curing and a relative humidity below 75%.
Cement backer boards are required in wet areas; regular gypsum board is prohibited in showers.
Plywood is acceptable only for residential floors with joists spaced a maximum of 400 mm on center.
Waterproofing membranes must be installed in all showers with a minimum height of 2 m and tested for 24 hours.
Self-leveling compounds correct flatness defects but have maximum thicknesses that must be respected.
Radiant floors require flexible thin-set mortars (class C2) and gradual start-up.
Canadian standards (CSA, NBC, ISO 13007) define the technical requirements for each type of support.
Moisture and adhesion tests are mandatory on professional job sites.
The flatness tolerance is 3 mm under 3 m for standard tiles, 1.5 mm for large formats.
2.12 Exam Tips
Memorize the key values: 28 days, 75% RH, 3.5 kg/100 m²/24 h, 2 m membrane height, 24-hour test, 3 mm under 3 m.
Understand the distinctions: vapour barrier vs. membrane, cement board vs. gypsum, standard vs. flexible mortar.
Visualize the procedures: exam questions often focus on the order of operations (cleaning → priming → leveling → membrane → tiling).
Review the standards: know the CSA, NBC, and ISO designations and their respective applications.
Practice the calculations: MVER, self-leveling compound volume, fastener spacing.
Mastery of support preparation is the key to success on the Red Seal exam. Questions on this topic represent a significant proportion of the assessment, and the principles learned in this chapter apply to all other sections of the trade.
