Techtalk T-103: Substrate & Underlayment Requirements

General Note: Duradek does not supply framing sheeting and underlayment products. Always refer to manufacturer’s written instructions and local building code requirements. Below are Duradek’s recommendations. For complete and up-to-date installation procedures refer to Tiledek’s Installation manual. Ask your Duradek dealer for a copy.

Numbers on drawing above reference notes below:

1. Wood Deck Construction

• Structural Design: To governing building code requirements.
• Deflection: Roof decks must have a maximum allowable deflection of L/720 (as an expressed ratio of the clear span).
• Joist Spacing: 16″ o.c. maximum.
• Slope:  Minimum ¼” per foot (2%, 1 in 48) positive slope is required to prevent standing water.
• Door Openings: Sill of rough door openings to be 3″ to 6″ above elevation of finished tile.
• Framing Lumber: Install Kiln-dried lumber with crown sides up.

2.  Plywood Sub Floor Construction

Plywood requirements:

• Profile: T&G
• Grade: C-C Sheathing Grade
• Thickness: ³/₄” (18.5mm)
• No treated plywood to be used

Plywood Installation:

• Placement: Plywood panels to be installed with the surface grain at right angles to the joists. End joints shall be supported over 2″ wood joists. At unsupported joints install 2″ x 4″ blocking securely nailed between framing members.
• Lightly gap boards, do not force boards together.
• Fasteners: Recommend #10 exterior wood deck screw either stainless steel or coated with non-staining finish or hot dipped galvanized ring thread or spiral nails. Nails should be driven in at a slight angle to reduce nail pops. Fasteners to be of sufficient length to penetrate deck framing members 1″ (25 mm) min. 1.25″ (31.75 mm) max. Slightly recess all fastener heads.
• Fastener Spacing: Space fasteners 6″ (150 mm) o.c. along edges and 8″ (200 mm) o.c. along intermediate supports. Drive nails on slight angle. Slightly recess all fastener heads.
• Curbs & Upstands: Apply ¹/₂” min plywood (see plywood specs above) to vertical and horizontal surfaces to receive Tiledek membrane.

3. Underlayment

A. Cement Board Underlayment:
Recommended for All Applications and REQUIRED for Decks
Over Non-Condition Areas (Open or Storage Areas)

Cement Board Underlayment: USG (USA) or CGC (CAN) DUROCK® BRAND CEMENT BOARD WITH EDGEGUARD™.  ¹/₂” x 4 ft. x 8 ft.

Underlayment Adhesive: Polymer modified mortar, Mapei “Ultra-Flex 2” acceptable

Underlayment Fastener: 1-¹/₄” (32 mm) USG (USA) or CGC (Can) Durock® Tile Backer Screws or ITW #9 Buildex ROCK-ON® Cement Board Screws

Joint Compound: Mapei Planipatch, Latex-Fortified mortar or equal

Underlayment installation:

• >>>>>> Install CBU panels smooth side up <<<<<<
• Place panels with joints maximum stagger from plywood subfloor joints. End joints to be placed not over structural framing but at ¹/₄ span point (ie 4″ off center for a 16″ joist spacing). Fit panels closely but do not force together, leave a maximum ¹/₈” (3 mm) gap at panel edges and ends. Leave ¹/₄” (6 mm) gap along walls. Fasten panels to subfloor while mortar is wet with 1-¹/₄ (32 mm) wood screws spaced 6″ o.c. in both directions and placed at ¹/₂” (12.5 mm) from ends and edges. Do not fasten into wood structural framing.
• Laminate ¹/₂” CBU to ³/₄” plywood subfloor using polymer modified mortar.  Apply with ¹/₄” (6 mm) square notched trowel. Install CBU immediately. Laminate and secure one panel and at a time.
• Finished surface to be true to plane within ¹/₈” in 10 feet (3.2 mm in 3 m).

Underlayment Preparation:

• Ensure all screw heads are set at, or just below, surface of CBU
• Fill CBU joints and any damaged areas with polymer modified mortar.
• Lightly sand mortar – if required.

Protection:

• Protect underlayment from rain and snow after installation to prevent moisture pickup.

B. Plywood Underlayment:
⁵/₈” (15.9 mm) Plywood Underlayment Option for Applications
Over Conditioned Areas (Habitable Areas) Only

• See Plywood Sub Floor notes for plywood underlayment requirements.

Underlayment installation:

• Place panels with joints maximum stagger from plywood subfloor joints. End joints to be placed not over structural framing but at ¹/₄ span point (ie 4″ off center for a 16″ joist spacing). Fit panels closely but do not force together, leave a maximum ¹/₈” (3 mm) gap at panel edges and ends. Leave ¹/₄” (6 mm) gap along walls. Fasten panels to subfloor while mortar is wet with 1-¹/₄ (32 mm) wood screws spaced 6″ o.c. in both directions and placed at ¹/₂” (12.5 mm) from ends and edges. Do not fasten into wood structural framing.
• Finished surface to be true to plane within ¹/₈” in 10 feet (3.2 mm in 3 m).

Underlayment Preparation:

• Ensure all screw heads are set at or just below surface of plywood.
• Fill and any damaged areas with polymer modified mortar.
• Lightly sand mortar – if required.

Protection:

• Protect underlayment from rain and snow after installation to prevent moisture pickup.

For the full collection of Detail Drawing for the installation of Tiledek, please refer to duradek.com/tiledek-resources