Subfloor Preparation and Moisture Control

Introduction: Why This Step Is Critical

Subfloor preparation and moisture control represent the #1 cause of warranty claims in the flooring industry. A perfectly installed floor covering over an inadequate subfloor will inevitably fail. For the Red Seal exam, you must master not only installation techniques but also Canadian standards, flatness tolerances, acceptable moisture levels, and correction methods. This chapter covers all the knowledge required to succeed on questions related to this topic.

1. Roles and Functions of the Subfloor

The subfloor is the structural base upon which the finished floor covering rests. It must fulfill four essential functions:

7.Structural support: distribute loads (furniture, traffic) to the joists.

8.Installation surface: provide a flat, clean, and stable surface for adhesive or fasteners.

9.Moisture barrier: limit moisture migration from below (soil, crawl space).

10.Acoustic and thermal insulation (to a lesser extent).

1.1 Types of Subfloors

Type	Material	Typical Thickness	Common Use
Plywood	Cross-layered wood sheets	15.9 mm (5/8") to 19 mm (3/4")	Residential, light commercial
OSB (oriented strand board)	Oriented and bonded wood strands	15.9 mm to 18 mm	Residential, economical
Waferboard	Wood particles and resin	15.9 mm	Limited use (not recommended under resilient flooring)
Hardwood plank	Solid wood	19 mm to 38 mm	Older constructions
Concrete	Cast concrete or precast slab	100 mm minimum	Basement, slab-on-grade
Cement board (Durock, Hardibacker)	Cement and fibers	6.4 mm to 12.7 mm	Under ceramic tile

Exam Point: Exterior-grade plywood is the only acceptable type of plywood for subfloors. Interior-grade plywood will delaminate in the presence of moisture.

1.2 Minimum Structural Requirements

According to the National Building Code of Canada (NBCC) , wood subfloors must have a minimum thickness of:

15.9 mm (5/8") for joist spacing of 400 mm (16") on center.

19 mm (3/4") for joist spacing of 600 mm (24") on center.

These values apply to plywood and OSB. Panels must be installed with the load-bearing side (face grain) perpendicular to the joists.

2. Evaluating the Existing Subfloor

Before any installation, you must perform a systematic inspection of the subfloor. This inspection involves several components.

2.1 Flatness Verification

Flatness is measured using a 3 m (10 ft) straightedge or a laser level. Acceptable tolerances vary by floor covering type:

Floor Covering Type	Maximum Tolerance	Measurement Method
Vinyl (sheet, tile)	3 mm over 3 m (1/8" over 10 ft)	3 m straightedge
Hardwood (nailed)	6 mm over 3 m (1/4" over 10 ft)	3 m straightedge
Laminate (floating)	3 mm over 3 m	3 m straightedge
Ceramic tile	3 mm over 3 m (aligned joints)	3 m straightedge
Carpet (glue-down)	3 mm over 3 m	3 m straightedge

Slope calculation formula: ΔH / L × 100 = % slope. For example, a difference of 6 mm over 3 m gives: 6 mm / 3000 mm × 100 = 0.2%.

2.2 Moisture Verification of Wood Subfloors

Wood moisture is measured with a pin-type moisture meter or a non-invasive (pinless) moisture meter. Readings should be taken at multiple locations, particularly near exterior walls and water sources.

Acceptable moisture levels for wood:

Situation	Maximum Moisture Level
Wood subfloor (plywood/OSB)	12%
Hardwood (before installation)	6 to 9% (depending on region)
Concrete (before floor covering installation)	75% RH (relative humidity) or 3 kg/100 m²/24 h (calcium chloride test)

Golden Rule: The subfloor moisture content must not exceed that of the hardwood flooring to be installed by more than 2%. This difference is called the ΔH (delta H) .

2.3 Concrete Moisture Testing

Concrete is a porous material that releases moisture for months, even years. Three test methods are recognized:

33.In-situ hygrometer test (relative humidity probe): measures relative humidity within the slab. Acceptable if ≤ 75% RH.

34.Calcium chloride test (ASTM F1869): measures the moisture vapor emission rate (MVER). Acceptable if ≤ 3 kg/100 m²/24 h (5 lb/1000 ft²/24 h).

35.Plastic sheet test: qualitative method (presence of condensation after 24–48 h).

Caution: The calcium chloride test must be performed on a slab at least 28 days old (initial cure time). Multiple tests must be conducted (one per 100 m², minimum 3).

3. Correcting Subfloor Defects

3.1 Leveling and Patching

Low spots and high spots must be corrected before installation. Products used include:

Self-leveling compound: for low spots from 3 to 25 mm. Flows and self-levels automatically.

Patching compound: for low spots from 1 to 6 mm. Applied with a trowel.

Leveling underlayment: thin plywood sheets (6.4 mm) to correct major irregularities.

Leveling procedure:

45.Sand or grind down high spots (never sand waferboard).

46.Clean the surface thoroughly (vacuum, dust extractor).

47.Apply a primer if required by the manufacturer.

48.Pour the self-leveling compound starting in the deepest areas.

49.Use a notched trowel to spread, then a spiked roller to release air bubbles.

50.Allow to dry according to manufacturer specifications (generally 2 to 4 hours).

Exam Trap: Self-leveling compound must never be used to correct structural defects. It is only for surface flatness.

3.2 Reinforcing the Subfloor

A subfloor that flexes or squeaks must be reinforced. Methods include:

Additional screwing: add wood screws (#8 or #10 type) every 150 mm along the joists.

Adding joists: double up existing joists to reduce span.

Installing a plywood underlayment: add a 12.7 mm (1/2") layer of exterior-grade plywood, installed perpendicular to the existing subfloor, with joints offset by at least 300 mm from the joints of the first floor.

Important Rule: Never screw into joists within 25 mm of the top edge (risk of splitting).

3.3 Replacing Damaged Sections

Sections of subfloor that are rotted, warped, or delaminated must be cut out and replaced. The cut must be made:

At the center of the joist (never between joists).

With a 3 mm gap between panels (to allow for expansion).

Using exterior-grade plywood of the same thickness.

4. Moisture Control: Fundamental Principles

4.1 Sources of Moisture

Source	Example	Impact
Ground moisture	Concrete slab on grade	Capillary migration
Atmospheric moisture	Unventilated crawl space	Condensation
Leaks	Plumbing, roofing	Localized damage
Construction moisture	Fresh concrete, plaster	Vapor emission
Human activities	Kitchen, bathroom	High relative humidity

4.2 Vapor Barriers

A vapor barrier is a material that limits the diffusion of water vapor through a surface. It is installed:

On the warm side of insulation (interior side in Canadian climate).

Under wood subfloors above a crawl space.

Under concrete slabs (minimum 6 mil polyethylene).

Common materials:

Polyethylene (poly) 6 mil (0.15 mm): most common.

Aluminum foil: for reflective applications.

Self-adhesive bituminous membrane: for high-humidity areas.

Exam Point: The vapor barrier must be continuous and sealed at all joints and penetrations. Simply overlapping without sealing is insufficient.

4.3 Moisture Barrier

Do not confuse a vapor barrier with a moisture barrier:

Vapor barrier: limits vapor diffusion (perm rating ≤ 0.05 ng/(Pa·s·m²)).

Moisture barrier: blocks liquid water and vapor (perm rating ≈ 0).

For concrete slabs, an epoxy membrane or liquid membrane applied in two coats is often used. These membranes are mandatory if the calcium chloride test exceeds 3 kg/100 m²/24 h.

4.4 Crawl Space Ventilation

According to the NBCC, crawl spaces must be ventilated to the exterior at a rate of 1 m² of ventilation for every 150 m² of floor area (1:150 ratio). This ventilation must be distributed on two opposite sides to ensure cross-ventilation.

Alternative: A closed crawl space with a sealed vapor barrier on the ground and mechanical dehumidification is also acceptable.

5. Specific Preparation by Floor Covering Type

5.1 For Vinyl Flooring (Sheet or Tile)

The subfloor must be perfectly smooth (3 mm/3 m tolerance).

All imperfections visible through the vinyl must be corrected.

Subfloor joints must be sanded or covered with a skim coat.

The surface must be free of dust, grease, and old adhesive residue.

5.2 For Hardwood (Nailed or Glued)

The subfloor must be dry (≤ 12%).

Nails or staples must penetrate at least 25 mm into the subfloor or joists.

For glue-down installation, the subfloor must be clean and absorbent (lightly sand to open the pores).

5.3 For Laminate (Floating)

The subfloor must be flat (3 mm/3 m).

An underlayment (foam, cork, felt) is required to:

Absorb minor irregularities.

Reduce impact noise.

Act as a vapor barrier (if the subfloor is concrete).

A 10 mm expansion gap must be left around the perimeter for expansion.

5.4 For Ceramic Tile

A wood subfloor must be covered with a cement board (6.4 mm minimum) or an uncoupling membrane (such as Ditra).

Cement board is fastened with cement board screws and joints reinforced with mesh tape.

The concrete slab must be clean, free of laitance, and have sufficient porosity for thin-set mortar adhesion.

5.5 For Carpet

The subfloor must be clean and dry.

Joints must be planed or sanded to prevent them from telegraphing through.

A carpet cushion (pad) may be required depending on the installation method.

6. Essential Calculations and Conversions

6.1 Moisture Unit Conversions

The calcium chloride test yields a result in kg/100 m²/24 h (metric) or lb/1000 ft²/24 h (imperial). Conversion:

1 kg/100 m²/24 h = 0.205 lb/1000 ft²/24 h

1 lb/1000 ft²/24 h = 4.88 kg/100 m²/24 h

Example: A result of 2.5 lb/1000 ft²/24 h is equivalent to 2.5 × 4.88 = 12.2 kg/100 m²/24 h. This result far exceeds the 3 kg/100 m²/24 h limit.

6.2 Calculating Area and Quantities

Area of a rectangle: L × W = A (m²)

Area of a circle: π × r² (π ≈ 3.1416)

Quantity of self-leveling compound:

Volume required (L) = Area (m²) × Average thickness (mm) × 1.5

The 1.5 factor accounts for waste and product density.

Example: For a 20 m² room with an average thickness of 5 mm:

20 × 5 × 1.5 = 150 L of compound.

6.3 Calculating Slope for Drainage

Slope (%) = (ΔH / L) × 100

For a bathroom, the minimum slope toward the drain is 1% (10 mm per meter).

7. Applicable Canadian Standards and Codes

7.1 National Building Code of Canada (NBCC)

The NBCC (2020 edition) contains structural and safety requirements. Relevant sections:

Section 9.23: Wood floors and subfloors.

Section 9.25: Moisture and condensation control.

Section 9.29: Floor coverings.

7.2 CSA Standards (Canadian Standards Association)

CSA O121: Softwood plywood.

CSA O325: Construction sheathing (OSB).

CSA A23.1: Concrete and related materials.

7.3 ASTM Standards (Referenced in Canada)

ASTM F1869: Calcium chloride test for concrete.

ASTM F2170: In-situ relative humidity test in concrete.

ASTM F710: Concrete preparation before floor covering installation.

7.4 Canadian Electrical Code, Part I

Although this code concerns electrical work, it is relevant for electric radiant floor heating installed under floor coverings. Rule 8-200 requires that heating cables be installed in accordance with manufacturer instructions and be protected by a ground fault circuit interrupter (GFCI) . The cable must be embedded in a mortar bed or self-leveling compound with a minimum thickness of 6 mm.

8. Step-by-Step Procedures for the Exam

8.1 Complete Subfloor Evaluation Procedure

149.Visual inspection: look for stains, warping, cracks, signs of rot.

150.Sound test: tap the surface to detect hollow areas (delamination).

151.Flatness measurement: 3 m straightedge in multiple directions.

152.Moisture measurement: pin-type moisture meter for wood, calcium chloride test for concrete.

153.Fastening verification: ensure panels are properly screwed or nailed.

154.Documentation: record all measurements and observations.

8.2 Procedure for Correcting a Damp Subfloor

156.Identify the moisture source (leak, capillary rise, condensation).

157.Eliminate the source (repair the leak, install drainage, improve ventilation).

158.Dry the subfloor (dehumidifier, forced ventilation).

159.Re-measure moisture after drying (wait 48–72 h).

160.If moisture persists, install a moisture barrier or vapor barrier.

161.Proceed with floor covering installation only when values are acceptable.

9. Pitfalls to Avoid

Here are the most frequent errors made by Red Seal exam candidates:

165.Confusing vapor barrier and moisture barrier: the vapor barrier limits diffusion; the moisture barrier blocks liquid water. They are not interchangeable.

166.Forgetting concrete cure time: a moisture test on a slab less than 28 days old gives falsely elevated results.

167.Neglecting expansion joints: wood subfloors require a 3 mm gap between panels. Floating floor coverings require a 10 mm perimeter gap.

168.Using self-leveling compound on an unprimed surface: this causes uneven drying and cracking.

169.Ignoring crawl space ventilation requirements: an unventilated or poorly ventilated crawl space leads to excessive moisture.

170.Not checking ambient relative humidity: subfloor moisture must be evaluated in relation to the building's ambient humidity (ideally 30 to 50% RH).

171.Sanding waferboard: this damages the surface and releases harmful particles. Use a skim coat or underlayment instead.

172.Forgetting joint offset: when installing underlayment, joints must be offset by at least 300 mm from the joints of the existing subfloor.

173.Using screws that are too long: screws that penetrate through the subfloor can damage conduits or cables below.

174.Not following manufacturer instructions: tolerances and requirements vary by floor covering manufacturer. Manufacturer instructions take precedence over general practices.

10. Summary

Key Element	Value / Rule
Minimum wood subfloor thickness	15.9 mm (joists at 400 mm); 19 mm (joists at 600 mm)
Maximum flatness (vinyl, laminate, tile)	3 mm over 3 m
Maximum flatness (nailed hardwood)	6 mm over 3 m
Maximum wood moisture content	12%
Maximum ΔH between subfloor and hardwood	2%
Maximum concrete relative humidity	75% RH (in-situ test)
Maximum concrete vapor emission	3 kg/100 m²/24 h (ASTM F1869 test)
Minimum concrete age before testing	28 days
Vapor barrier under concrete slab	Minimum 6 mil polyethylene
Crawl space ventilation	1 m² per 150 m² of floor area
Expansion gap for floating floor coverings	10 mm around the perimeter
Gap between subfloor panels	3 mm
Minimum slope toward drain	1% (10 mm/m)

Key Retention Points:

Subfloor preparation is mandatory and must never be skipped.

Moisture is enemy #1: always measure, document, and correct if necessary.

Flatness tolerances vary by floor covering type: the thinner and more rigid the covering, the stricter the tolerance.

Canadian standards (NBCC, CSA, ASTM) provide the reference values. Know the numerical limits.

The calcium chloride test and the in-situ relative humidity test are the two recognized methods for concrete. The plastic sheet test is qualitative only.

11. Self-Assessment Questions (Exam-Type)

186.A 15.9 mm plywood subfloor is installed on joists spaced at 600 mm. Is this compliant with the NBCC? (No — 19 mm is required for 600 mm spacing.)

187.A calcium chloride test gives 4.2 kg/100 m²/24 h. What must you do before installing vinyl flooring? (Install an appropriate moisture barrier or vapor barrier, as the value exceeds 3 kg/100 m²/24 h.)

188.What is the maximum moisture difference between a plywood subfloor and a maple hardwood floor? (2% — the ΔH must not exceed 2%.)

189.You find a 8 mm low spot over 2 m in an OSB subfloor. What product do you use? (Self-leveling compound, after verifying that the depth is within the manufacturer's recommended range.)

190.A crawl space has an area of 120 m². What is the minimum ventilation area required? (120 / 150 = 0.8 m², distributed on two opposite sides.)

12. Normative References (For Further Study)

National Building Code of Canada 2020 — Sections 9.23, 9.25, 9.29.

CSA O121-17 — Softwood plywood.

CSA O325-16 — Construction sheathing (OSB).

ASTM F1869-21 — Calcium chloride test.

ASTM F2170-19 — In-situ relative humidity test.

ASTM F710-22 — Concrete preparation.

Canadian Electrical Code, Part I — Rule 8-200 (radiant floor heating).

This chapter prepares you for Red Seal exam questions on subfloor preparation and moisture control. Review the numerical values in the summary table before the exam — they form the basis of many multiple-choice questions.

Subfloor Preparation and Moisture Control