Waterproofing a Plywood Deck: What Building Codes Actually Require

Why Plywood Decks Are Different

A plywood deck that functions as a walking surface over occupied space, serves as a roof covering, or is part of a multi-family or commercial building is not the same thing as a backyard deck you built over the weekend. Building codes govern what materials can be used, how they must be tested, and who can install them.

When plan checkers review your project, they need to verify that the waterproofing system meets code requirements for waterproofing performance, fire classification, impact resistance, wind resistance, and durability. Consumer deck paints and sealers cannot provide this documentation because they haven't been tested for it.

The waterproofing system you specify determines whether your project passes plan check, passes inspection, and holds up for the life of the building. That decision starts with understanding what the codes require.

What Building Codes Require

For plywood decks that serve as walking surfaces and roof coverings, the International Building Code (IBC) and International Residential Code (IRC) require systems that have been independently tested and evaluated for:

• Fire classification — Class A or Class B roof covering per ASTM E108, plus surface burning characteristics per ASTM E84
• Waterproofing performance — the system must prevent water penetration through the deck assembly
• Durability — resistance to weathering, UV exposure, and foot traffic over the system's service life
• Impact resistance — ability to withstand normal pedestrian use without damage to the waterproofing membrane
• Wind resistance — the system must remain adhered under design wind loads

The accepted way to demonstrate compliance is through an ICC-ES evaluation report based on AC39 (Acceptance Criteria for Walking Decks). AC39 is the standard that ICC-ES uses to evaluate walking deck and roof covering systems. An evaluation report (ESR) confirms the system meets code when installed as specified.

ICC-ES Evaluation Report ESR-3672

Deck Flex Walking Deck and Roof Covering Systems are evaluated in ICC-ES Evaluation Report ESR-3672, issued by ICC Evaluation Service. This is the document that building officials, plan checkers, architects, and engineers use to verify code compliance.

ESR-3672 confirms that both the W.M. and W.F. systems comply with the International Building Code (IBC) and International Residential Code (IRC) when installed as specified. The report covers:

• Evaluation criteria: AC39 — Acceptance Criteria for Walking Deck and Roof Covering Systems
• Code editions: IBC 2006 through 2021, IRC 2006 through 2021, California Building Code (CBC) 2019, Los Angeles Building Code (LABC) 2020
• Fire testing: ASTM E84 (surface burning characteristics), ASTM E108 (roof covering classification) — confirming Class A (W.M.) and Class B (W.F.) fire ratings
• Wind resistance: Tested to 130 mph basic wind speed, Exposure B, for structures up to 40 feet
• Substrate requirements: Minimum 5/8" exterior-grade plywood (both systems); concrete substrate also accepted for W.F.
• 1-hour fire-resistance-rated assembly: W.M. system qualifies per IBC Table 721.1(3) when installed with specific framing and gypsum wallboard below
• Installation conditions: Ambient temperature 45°F to 100°F, maximum deck slope 1/4" per foot, installation by approved applicator only

Plywood Substrate Requirements

Before any waterproofing system goes down, the plywood substrate must meet specific requirements. These aren't suggestions — they are conditions of the ICC-ES evaluation. Per ESR-3672:

• Minimum 5/8-inch thick (15.9 mm) exterior-grade plywood
• All joints over framing members or blocked with minimum 2-by-4 blocking
• Free of contaminants — no water, curing compounds, hardeners, bond breakers, or paint on the surface
• Structurally sound — no delamination, rot, soft spots, or damage
• No projections or depressions — the surface must be smooth and even
• Sloped for drainage — standing water accelerates wear on any waterproofing system

If the existing plywood doesn't meet these requirements, it needs to be repaired or replaced before the waterproofing system is installed. There are no shortcuts here. Applying a waterproofing system over compromised plywood defeats the purpose — water will get through, and you'll have a bigger problem than you started with.

Two Systems Listed in ESR-3672 for Plywood

ESR-3672 covers two Deck Flex walking deck and roof covering systems, both evaluated under AC39. Both are designed for plywood substrates and both include fiberglass reinforcement, but they serve different applications based on fire rating requirements.

Why Consumer Products Don't Meet Code

If you search for "waterproof deck coating for plywood," you'll find consumer products — deck paints, elastomeric coatings, rubber sealers — that claim to waterproof your deck. For a ground-level residential deck with no building code requirements, some of these products may work fine.

But if your plywood deck is any of the following, consumer products are not an option:

• A roof covering (the deck is over occupied space below)
• Part of a multi-family building (condos, apartments, townhomes)
• Part of a commercial building
• Subject to a building permit for waterproofing replacement
• In a jurisdiction that requires fire-rated deck assemblies
• Subject to SB 721 or SB 326 inspection requirements in California

Here's why:

A product without an ICC-ES evaluation report has no independent verification of performance. Building officials have no way to confirm it meets code, and they will not approve it.

Choosing the Right System

The choice between the W.M. and W.F. system comes down to what your project requires. Your architect, engineer, or building official makes this determination — not the coating contractor.

Specify the W.M. system (Class A) when:

• The building code requires Class A fire rating for the roof covering
• The deck is over occupied living space
• The project is multi-family residential or commercial
• A one-hour fire-resistance-rated assembly is required
• The building official specifies Class A
• When in doubt — Class A satisfies any application where Class B would also be accepted

The W.F. system (Class B) may be acceptable when:

• The project is single-family residential
• Building code does not specifically require Class A
• The building official approves Class B for the application
• You need a system that also works on concrete substrates (W.F. is evaluated for both plywood and concrete)

If you're not sure which fire rating your project requires, ask your building department before specifying a system. Getting this wrong means resubmitting plans, delays, and potentially tearing out work that doesn't meet code. For a detailed explanation of fire ratings, see our guide on Class A vs Class B fire ratings.

What to Expect From a Professional Installation

A trained Deck Flex applicator follows the installation procedures specified in ESR-3672. Here's what that process looks like in practice:

1. Substrate inspection and preparation. The plywood is inspected for structural integrity, moisture content, contaminants, and proper slope. Any damage is repaired. The surface is cleaned and prepped per Section 4.1 of the ESR.
2. Joint treatment. For the W.M. system, metal lath is fastened over the entire deck surface. For the W.F. system, galvanized steel seams are installed over all plywood joints and covered with patching compound.
3. Cementitious layer (W.M. only). TetraCrete is mixed and troweled into the metal lath to a minimum 1/8" thickness. This layer cures for 24 hours at 45-100°F before the next step.
4. Fiberglass reinforcement. Fiberglass mat is laid into wet base coat, creating a continuous reinforced membrane across the entire deck surface.
5. Waterproofing layers. Base coat, texture coat, and top coat are applied at specified coverage rates with required drying times between each layer.
6. Inspection. The completed installation is inspected to verify compliance with the ESR requirements.

The entire process takes several days due to cure times between layers. There are no shortcuts — skipping steps, reducing coverage rates, or applying in the wrong conditions invalidates the system's fire rating and code compliance.