How to specify plywood

How do you specify plywood?

To specify plywood, cite BS EN 636 with the service condition it must meet (dry, humid or exterior), then give the species, face grade, thickness, formaldehyde class and FSC or PEFC certification. Add what the location needs: a reaction to fire class for an exposed lining, a structural grade from the engineer for sheathing or decks, and a coating and edge-sealing regime for anything outside.

Plywood appears in specifications in three quite different jobs: as an exposed lining, as a hidden substrate or sheathing, and as a structural deck. Each needs different information, and the clause we most often find says "18 mm WBP plywood" and stops. WBP is an old trade description of the glue line. It tells the contractor nothing about the face veneer, the fire class of the sheet or where the timber was felled.

This guide works through an exposed birch lining, which Uniclass codes as Pr_25_71_97_91 Visual grade general veneer plywoods. Sheathing and decks use Pr_25_71_97_86 Structural veneer plywoods, and fire-retardant boards have their own code, Pr_25_71_97_30 Fire-retardant plywoods. The properties to cover are the same; the emphasis shifts from the face to the structural grade.

What does a plywood sheet clause need to cover?

Some properties are specific to plywood. Others belong in every product clause. A clause that leaves either out gives the contractor room to substitute.

Fields marked Never guess depend on the project and should never be filled from a default or a previous job. If the information is not available yet, the clause should say so with a TBC and a reason.

Which decisions does the clause depend on?

Most properties follow from the product you choose. These depend on the project, so they need a decision from the designer before the clause is written.

Which standards should the clause cite?

Cite the product standard the declaration of performance refers to, the code of practice for installation, and the test method behind each value you set. A value without its test method cannot be checked against a submittal.

BS EN 636

Plywood. Specifications

Requirements for general purpose and structural plywood in dry (Part 1), humid (Part 2) and exterior (Part 3) conditions, with bending properties and bond quality.

In the clause Cite it with the part for the service condition: "To BS EN 636-2".

BS EN 13986

Wood-based panels for use in construction. Characteristics, evaluation of conformity and marking

The harmonised standard behind the declaration of performance, including formaldehyde classes and the reaction to fire classes boards achieve without further testing.

In the clause Use it for the DoP and formaldehyde class: "Formaldehyde: Class E1 to BS EN 13986".

BS EN 314-2

Plywood. Bonding quality. Requirements

Bond classes 1, 2 and 3 for dry, humid and exterior conditions. It replaced the WBP and MR descriptions still seen in older clauses.

In the clause "Bond class 2 or better to BS EN 314-2."

BS EN 635-2

Plywood. Classification by surface appearance. Hardwood

Face appearance classes for hardwood plywood such as birch, defined by the defects and repairs allowed. Part 3 covers softwood faces.

In the clause Give the supplier's grade name with the BS EN 635-2 class it corresponds to.

BS EN 13501-1

Fire classification of construction products and building elements: classification using data from reaction to fire tests

The Euroclass system used for wall and ceiling linings in Approved Document B.

In the clause "Reaction to fire: Class B-s1,d0 to BS EN 13501-1."

BS EN 16755

Durability of reaction to fire performance. Classes of fire-retardant treated wood products in interior and exterior end use applications

DRF classes INT1 (interior, dry), INT2 (interior, humid) and EXT (exterior) for fire-retardant treated wood products, so the treatment is shown to last in the conditions of use.

In the clause "Durability of reaction to fire class INT1 to BS EN 16755."

BS EN 335

Durability of wood and wood-based products. Use classes: definitions, application to solid wood and wood-based products

Use classes 1 to 5 describing the exposure, which decide whether durability or treatment needs to be specified.

In the clause "Use class: 1 to BS EN 335" for a heated interior lining.

BS EN 1995-1-1

Eurocode 5. Design of timber structures. General. Common rules and rules for buildings

The design code the engineer uses where plywood acts as sheathing, a diaphragm or a deck.

In the clause Only for structural plywood: "Structural grade and fixing to the engineer's design to BS EN 1995-1-1".

Confirm the current edition of each standard at every issue. Avoice's State of UK Specifications 2026 found a withdrawn standard cited in 40% of specifications at contract issue.

For the regulations behind these choices, see Approved Document B explained, Fire strategy reports explained, or search the Avoice Regulations library.

How do you write it in CAWS?

An exposed plywood lining belongs in CAWS section K13, the fine linings and panelling section, not K11, which is for sheathing, decking and linings that will be covered. K13 is written as type clauses in the 100s, one for each panelling type, and each type clause names the substrate, the board, its finish and how it is fixed.

The general clauses later in K13 hold what every panelling type shares: wood-based sheets generally, samples, the environmental conditions before fixing, accuracy and fixing. If the project also has sheathing behind cladding or a plywood deck, those go in K11 as separate clauses, even when the same board is used.

Clauses the type clause relies on

K13/105

Wood-based sheets generally Moisture content and handling of all board types used in the section.

K13/220

Material samples Sheets showing the range of colour and figure, approved before ordering.

K13/260

Environmental conditions Building dry and at service conditions before the lining is fixed.

K13/310

Accuracy of fabrication Squareness and joint widths for panels cut off site.

K13/350

Fixing linings and panelling Fixings, centres and alignment of exposed screw heads.

K13/480

Clear finished wood Preparation and coats of the clear finish, applied as tested for fire.

Clause numbers vary between NBS versions and practice masters; the numbers here follow an NBS-based K13 seen on a public project, so check them against your own K13.

How do you write it in Uniclass?

In a Uniclass specification the lining is a system, Ss_25_25_75_94 Wood panel lining systems, and the system clause holds the build-up and the performance of the finished wall, including its reaction to fire class with battens and finish. The plywood itself is a separate product clause under its Pr code.

That split earns its keep when one board appears in several places. The same birch sheet used for joinery, a wall lining and a soffit is written once as a product, and each system clause points to it. A fire-retardant version is a different product, so give it its own type mark rather than a note in the system clause.

In NBS Chorus the Uniclass product clause also carries an NBS reference (for visual grade plywood, 45-45-95/487); use whichever your masters use, but not both systems in one specification.

What goes in the spec, and what goes in the schedule?

The specification defines each type once. The schedule and drawings say where each type goes.

What are the common mistakes?

Most problems are a missing property, a property filled with the wrong kind of value, or a decision left silently open.

• !
  "WBP plywood" and nothing else. It describes a glue line, not the board. Give the BS EN 636 part, the face grade and the fire class.
• !
  D-s2,d0 assumed to be enough. That is the class of untreated plywood fixed tight to a non-combustible background. Most rooms and every circulation space need better, so check the fire strategy room by room.
• !
  Finish left out of the fire test. A lacquer or paint added after a fire-retardant treatment can change the class. Make the finish part of the classified build-up.
• !
  Exterior bond read as exterior durability. A class 3 bond keeps the sheet together. It does not stop the veneers rotting or the face checking outside.
• !
  One hand sample for a feature wall. Birch varies between batches. Approve a set of sheets and keep them for comparison.
• !
  "Sustainably sourced" with no scheme named. Name FSC or PEFC and ask for the chain of custody number on the delivery note.

How Avoice helps

Avoice's spec writer drafts plywood clauses from the joinery and finishes information already on the project, and marks what it cannot settle.

1. 1
   Drafts from your drawings and schedules

   It picks up lining types such as PL01 from the finishes schedule and elevations, chooses K13 or the Uniclass lining system, and fills in the board, thickness and fixing from what you have given it.

2. 2
   Leaves a TBC where the information stops

   If the finish or the fire class for a corridor has not been decided, the clause says TBC with the reason, and the item joins the project TBC register to close before tender.

3. 3
   Writes in your format and your style

   CAWS or Uniclass, built from your practice's own K13 and product masters rather than generic text.

4. 4
   Checks the clause against everything else

   QA/QC flags a lining shown in a circulation space on the drawings while the clause gives class D, a clause still citing WBP, or a PL02 on the schedule that the specification never defines.

5. 5
   Keeps product data in one place

   The DoP and the fire classification report sit in the Product Room linked to the clause, so a substitute board can be checked against the class and mounting it was tested in.

Related guides: How to specify OSB boards, How to specify timber cladding, How to specify acoustic panels.

Software can draft and check. The designer still chooses the product and is responsible for the specification.