How to specify engineered wood flooring
What an engineered wood flooring clause has to say, the decisions behind it, the standards to cite, and how to write it in CAWS and in Uniclass.
Photo: FW Studio, CC0, via Wikimedia Commons.
How do you specify engineered wood boards?
To specify engineered wood flooring, name the species and grade of the wear layer, the core, overall thickness and board size, and cite BS EN 13489 as the product standard. Then state the fixing method, finish, reaction to fire class and any structural or fire role the boards play, plus the subfloor moisture and underfloor heating limits, and leave room locations to the finishes schedule.
On a sample board, an engineered oak plank with a 2 mm top layer looks much the same as one with 6 mm, and a board made to float on an underlay looks the same as one made to be glued or nailed. A clause that records the species, the grade and the finish leaves all of that open, and the floor that arrives may be one that can never be sanded, or one that cups over underfloor heating because the core was never meant for it.
This guide covers Uniclass Pr_25_71_97_93 Wood laminate strips and boards, which Uniclass also lists under the name engineered wood boards. Solid boards are a different product with their own code, Pr_25_71_97_87 Timber floorboards, and belong in a separate clause because they move and fix differently.
What does a engineered wood board clause need to cover?
Some properties are specific to engineered wood boards. Others belong in every product clause. A clause that leaves either out gives the contractor room to substitute.
Properties of engineered wood boards
- Material gradeWear layer thickness and core construction: plywood, softwood or HDF.
- Appearance classManufacturer's grade for knots, sapwood and colour variation.
- Nominal sizeBoard width and length set layout and movement.
- ProfileEdge profile: square, micro-bevel or bevel.
- Edge treatmentTongued and grooved or mechanical click joint.
- TolerancesDimensional and flatness tolerances from the product standard.
- FinishFactory lacquer or oil, sheen level and colour treatment.
- DensitySpecies density affects dent resistance and fire class.
- Moisture contentBoard moisture on delivery, within the product standard range.
- Moisture movementSets perimeter gaps and suitability over underfloor heating.
- Moisture resistanceNot for wet rooms; state if near kitchens or entrances.
- Bonding classGlue bond between layers suited to service conditions.
- Use classBS EN 335 use class 1 for internal dry floors.
- Structural performanceNever guessSpanning capacity where boards are fixed direct to joists.
- Reaction to fire classificationNever guessFloor Euroclass to BS EN 13501-1, e.g. Dfl-s1.
- Fire retardant durability classNever guessBS EN 16755 class if a fire-retardant treatment is relied on.
- Fire resistanceNever guessOnly where boards form part of a tested floor.
- Formaldehyde emission classE1 to BS EN 14342 for indoor use.
- EmissionsVOCs from adhesives, lacquers and oils.
- Timber source certificationFSC or PEFC with chain of custody.
- Third-party certificationIndependent evidence of product performance where required.
- Recycled contentFor sustainability targets and reporting.
- FixingsFloating, fully bonded or secret nailed.
- AccessoriesUnderlay, nosings, thresholds, skirtings and expansion covers.
What every product clause needs
- Manufacturer and product referenceNamed range, species and finish, or a performance description.
- Product standardBS EN 13489, with a BS EN 14342 DoP.
- Installation methodBS 8201 and the manufacturer's instructions.
- Technical submittalsData sheet, DoP, fire classification and chain of custody.
- Operation and maintenance informationCleaning, re-oiling or recoating, humidity range, sanding limits.
- MaterialWear layer species and core material.
- ThicknessOverall board thickness and wear layer thickness.
Uniclass Pr_25_71_97_93 Engineered wood flooring. Not every property applies to every project; where one does not, say so rather than leaving it blank.
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.
- 01
Wear layer and core
LifeThe hardwood top layer is the only part anyone walks on and the only part that can be sanded back. A thin layer may take one light sand in its life or none; a thick one can be refinished several times. The core decides how the board moves: multi-ply cores are generally more stable over underfloor heating than softwood-strip cores, and HDF cores behave differently again. Overall thickness says very little about either, which is why a clause that gives only "15 mm engineered oak" leaves the important choices to the supplier.
How to decide
- Agree with the client how long the floor should last and whether it will be sanded during that time.
- State the minimum wear layer thickness as a separate line from the overall thickness.
- State the core construction, and check the manufacturer's approval for underfloor heating where it is present.
- Ask for a cut section of the board with the sample, so the wear layer can be measured.
Write it asConstruction: European oak wear layer not less than 4 mm on multi-ply birch core, 15 mm overall. - 02
Reaction to fire class
Never guessUntreated wood floorings are commonly declared Dfl-s1, and the declared class depends on the species, density, thickness and how the board was mounted for the test. Approved Document B's lining classes apply to walls and ceilings, not floors, so a flooring class normally comes from the fire strategy or the client's own standard. If the strategy asks for something better than the product declares, the answer is a treated or different product with its own evidence, not a note in the clause asking for a higher class.
How to decide
- Check the fire strategy for any stated flooring class, especially on escape routes and in common areas.
- Specify the class the selected board actually declares, so that a substitute cannot fall below it.
- Where a better class is needed, ask the manufacturer for a classification report covering the exact board and fixing method.
Write it asReaction to fire: Class Dfl-s1 to BS EN 13501-1, as declared for the board and fixing method supplied. - 03
Fire-retardant treatment
Never guessIf a better reaction to fire class is reached only with a fire-retardant treatment or coating, the clause has to say so and state how the treatment is expected to last. BS EN 16755 gives durability classes for fire-retardant treated wood products in interior and exterior use. A floor is sanded, cleaned and recoated over its life, which is the opposite of what most treatments are tested for, so ask the manufacturer directly whether the class survives the maintenance regime.
How to decide
- If the board is untreated, say so; the field then closes.
- If treatment is relied on, specify the BS EN 16755 durability class and the treatment product.
- Ask how sanding and recoating affect the class, and write any limits into the operation and maintenance information.
Write it asFire-retardant treatment: Not used. Reaction to fire class achieved by the untreated board. - 04
Boards that span joists
Never guessFloated and fully bonded engineered boards sit on a continuous subfloor and carry no structural load. Some thicker boards are made to be secret nailed directly to joists or battens, and at that point the floor finish is also the deck. Joist centres, board thickness and loading then have to be checked by the manufacturer and the structural engineer. A brochure statement that a board "can be nailed to joists" does not tell you at what centres or for what load.
How to decide
- Decide the fixing method first: floating, fully bonded or nailed.
- For floating or bonded floors, state that the boards are not structural.
- For boards on joists or battens, get the maximum centres for the floor loading in writing from the manufacturer and have the engineer confirm them.
Write it asStructural performance: Not structural. Boards fully bonded to a continuous screed base. - 05
Fire resistance of the floor
Never guessFire resistance is a property of the whole floor construction, not the finish. In most buildings the structure and the ceiling below provide it and the wood flooring plays no part. Where a tested or assessed floor build-up does include the boards, typically over timber joists in a compartment floor, the clause has to name that test or assessment and stop anyone swapping the board for a different one that was not part of it.
How to decide
- Check the fire strategy and the floor build-up drawings for the fire resistance requirement and how it is achieved.
- If the boards are not relied on, say so in the clause.
- If they are, name the test or assessment and make the board and its fixing part of the approved build-up.
Write it asFire resistance: Boards not relied on. Fire resistance provided by the floor structure and ceiling as the fire strategy.
The values under "Write it as" are examples. The right value depends on the location, the use and the fire strategy.
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 13489
- Wood flooring and parquet. Multi-layer parquet elements
The product standard for engineered boards, covering construction, dimensions, tolerances, moisture content and top layer thickness.
In the clause Cite it as the product standard: "Product standard: BS EN 13489".
- BS EN 14342
- Wood flooring and parquet. Characteristics, assessment and verification of constancy of performance and marking
The harmonised standard behind the declaration of performance, including reaction to fire and formaldehyde class.
In the clause "Submit declaration of performance to BS EN 14342, including formaldehyde class E1."
- BS 8201
- Code of practice for installation of flooring of wood and wood-based panels
UK code of practice for subfloors, moisture checks, conditioning, fixing methods and expansion gaps for wood flooring.
In the clause Use it for workmanship: "Laying: to BS 8201 and the flooring manufacturer's instructions".
- BS EN 13501-1
- Fire classification of construction products and building elements. Classification using data from reaction to fire tests
The Euroclass system; floorings carry the fl suffix, for example Dfl-s1.
In the clause "Reaction to fire: Class Dfl-s1 to BS EN 13501-1."
- BS EN 16755
- Durability of reaction to fire performance. Classes of fire-retardant treated wood-based products in interior and exterior end use applications
Durability classes for fire-retardant treated wood products, used only where a treatment is relied on for the fire class.
In the clause Only for treated boards: "Fire-retardant treatment: durability class to BS EN 16755 as manufacturer's declaration".
- BS EN 335
- Durability of wood and wood-based products. Use classes: definitions, application to solid wood and wood-based products
Defines use classes by exposure; internal dry flooring is use class 1.
In the clause "Use class: 1 to BS EN 335."
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, or search the Avoice Regulations library.
How do you write it in CAWS?
Engineered wood flooring is written in CAWS section K21, which covers timber strip and board flooring and fine linings. Each flooring type gets a type clause in the 100s, typically 110 for the first, and that clause carries the substrate, the preparation, the board, its species and grade, the finish, the fixing and any special requirements for that floor.
Workmanship and protection, site conditions, heating, the moisture test on the base, fixing and expansion provision are written once in the later K21 clauses and referred to from the type clause. A bonded oak board in the apartments and a herringbone in the reception are two type clauses, 110 and 120, sharing the same general clauses. Underfloor heating is usually specified in another section and cross-referenced from the type clause.
- Substrate
- Sand and cement screed with underfloor heating, as section M10
- Preparation
- Moisture content of base as clause 260, flatness to manufacturer's tolerance
- Manufacturer
- Submit proposals
- Product reference
- Submit proposals
- Product standard
- BS EN 13489, declaration of performance to BS EN 14342
- Construction
- European oak wear layer not less than 4 mm on multi-ply birch core
- Board size
- 15 x 190 mm, lengths 1900 mm
- Appearance class
- Manufacturer's select grade, to approved sample
- Edges
- Tongued and grooved, micro-bevelled
- Finish
- TBC Client choice between hardwax oil and matt lacquer outstanding. Resolve before ordering.
- Reaction to fire
- Class Dfl-s1 to BS EN 13501-1
- Moisture content at time of laying
- Within manufacturer's stated range
- Fixing
- Fully bonded with manufacturer's flexible flooring adhesive
- Expansion gaps
- As clause 360, concealed by skirting
- Timber source
- FSC or PEFC certified, with chain of custody evidence
- Accessories
- Matching threshold strips at door openings
Illustrative, not from a real project. One property is left as a TBC with a reason so it can be tracked and closed before issue.
Clauses the type clause relies on
- K21/210
- Workmanship generally Boards kept dry before fixing, and the finished floor protected from dirt and damage until practical completion.
- K21/220
- Environment No laying until the building is weathertight and dried out, with temperature and humidity held close to in-use levels.
- K21/230
- Heating How the heating is run up to completion so the boards do not take up or lose moisture too quickly.
- K21/260
- Moisture content of base Hygrometer test to BS 8201 on concrete and screed bases; no laying until every reading is 75% RH or less.
- K21/350
- Fixing boards A flat, true surface, with allowance for timber movement over the humidity range expected in use.
- K21/360
- Expansion provision Perimeter and intermediate expansion gaps, sized to the width of floor and hidden by the skirting.
Clause numbers vary between NBS versions and practice masters. The numbers here follow a flooring manufacturer's NBS-format K21; check the titles and numbers in your own K21 before cross-referencing.
For more on clause structure and checks, see NBS clause writing: common mistakes.
How do you write it in Uniclass?
Uniclass 2015 writes the floor as a system and the board as a product. For a board glued to a screed the system is Ss_30_20_95_25 Direct-glued wood floor systems, which describes the base, the adhesive, expansion gaps and how the finished floor behaves. The board is a separate Pr_25_71_97_93 clause with its own properties, called up from the system by code and type mark.
The choice of system follows the fixing method. A floating floor on underlay is Ss_30_20_95_95 Wood floating floor systems, and boards nailed to joists come under Ss_30_20_95_20 Direct-fixed wood floor systems. If the same board is used in two ways on one project, two systems point to one product clause, which keeps the species, grade and finish consistent.
- Description
- Engineered oak boards fully bonded to screed with underfloor heating
- Base
- Sand and cement screed, as the screed system clause
- Moisture before laying
- Hygrometer test to BS 8201, within manufacturer's limit
- Boards
- Pr_25_71_97_93 Wood laminate strips and boards (F03)
- Adhesive
- Manufacturer's flexible flooring adhesive, full bed
- Expansion gaps
- Perimeter and intermediate gaps to manufacturer's recommendations
- Underfloor heating
- Commissioned before laying; surface temperature within manufacturer's limit
- Structural performance
- Not structural
- Manufacturer
- Submit proposals
- Product reference
- Submit proposals
- Standard
- BS EN 13489, declaration of performance to BS EN 14342
- Construction
- European oak wear layer not less than 4 mm on multi-ply birch core
- Nominal size
- 15 x 190 mm, lengths 1900 mm
- Appearance class
- Manufacturer's select grade, to approved sample
- Edge profile
- Tongued and grooved, micro-bevelled
- Finish
- TBC Client choice between hardwax oil and matt lacquer outstanding. Resolve before ordering.
- Reaction to fire
- Class Dfl-s1 to BS EN 13501-1
- Formaldehyde emission class
- E1 to BS EN 14342
- Timber source
- FSC or PEFC certified, with chain of custody evidence
Illustrative. Performance of the finished system sits in the system clause; properties of the product sit in the product clause.
In NBS Chorus the product clause also carries an NBS reference (for wood laminate strips and boards, 45-45-95/370); use CAWS or Uniclass throughout a specification, not both.
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.
In the specification
What the product is and how it performs and is installed. Written once per type.
In the schedule or drawings
Where each type goes and how much of it. Changes with every plan revision.
Write each flooring type once with a type mark (for example F01) and let the finishes schedule and drawings say where it goes. A herringbone version of the same board is a different type, because it changes the product, the fixing and the waste.
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.
- Overall thickness without a wear layer. A 14 mm board can have a thin veneer or a sandable top layer. Specify the wear layer, because that decides whether the floor can be refinished.
- No fixing method. Floating, bonded and nailed floors need different subfloors, accessories and acoustic details. Choose one per type.
- Underfloor heating not mentioned. Boards over heating need the manufacturer's confirmation, a surface temperature limit and a commissioned, dried screed before laying.
- No moisture check. Wood floors laid on wet screeds cup and gap. Require a hygrometer test to the manufacturer's limit, as BS 8201 describes.
- Assuming a fire class. Wood flooring does not reach a better class just because the fire strategy asks for one. State the class and get the evidence.
- Room names in the clause. "Oak to living room and hall" goes out of date at the next plan revision. Use a type mark and the schedule.
How Avoice helps
Avoice's spec writer drafts engineered wood clauses from what the drawings and schedules already say, and flags what is still open.
- Drafts from your drawings and schedules
It reads the wood flooring types from the finishes schedule and plans, sees where underfloor heating is shown, and writes the K21 type clause or the Uniclass system and product clauses with the properties your information supports.
- Leaves a TBC with a reason
If the client has not chosen between oil and lacquer, or nobody has confirmed whether the boards go over joists, the line says TBC and why. Each TBC goes into one register for the project to close before tender.
- Works from your own masters
Choose CAWS or Uniclass. Avoice adapts your practice's existing K21 clauses and wording instead of replacing them.
- Checks the spec against the drawings
QA/QC flags a herringbone type on the plans with no clause, a fixing method that disagrees with the floor build-up detail, a reference to a K21 clause that is not in your master, or a withdrawn standard.
- Links product data to the clause
Data sheets, chain of custody certificates and declarations of performance live in the Product Room against the clause. Change a linked product once and every clause using it follows. Approved Document B guidance is to hand in the Avoice Regulations library.
Related guides: How to specify screeds, How to specify vinyl flooring, How to specify stair nosings.
Software can draft and check. The designer still chooses the product and is responsible for the specification.
For more on the evidence behind product choices, see the role of manufacturer product data in specification writing.
Draft your engineered wood board clause in Avoice
Upload your drawings and schedules. Avoice drafts the clause in CAWS or Uniclass, cites the standards and flags anything your information does not settle.
This guide is for general education and is not professional advice. Check current editions of standards and Approved Documents, the manufacturer's data, and the requirements of your project before relying on any clause.