Get that right and the cheapest board on the shelf is often the correct one. Get it wrong and you end up with a heaving floor, a squeaking deck, a fixing that pulls out of a board edge, or a sheathing board that was never graded for the job in the first place. This guide covers what each family is genuinely good at, how the grading systems work, and where each board belongs.
The Three Families At A Glance
| Board | How it is made | Strengths | Weaknesses |
|---|---|---|---|
| OSB | Rectangular wood strands, oriented in layers, bonded under heat and pressure | Consistent, no voids or delamination risk, low cost per m², strong in racking, high recycled and small-log content | Swells at cut edges when wet and does not fully recover, poor edge screw-holding, rough surface |
| Plywood | An odd number of veneers, each cross-laminated at 90° to the next | Best strength-to-weight, stable in both directions, holds screws and fixings well, machines and finishes cleanly | Most expensive, quality varies enormously by origin, hidden voids and defects in low-grade cores |
| Chipboard (particleboard) | Wood particles graded fine to coarse, resin-bonded, with dense faces and a coarser core | Flattest and most consistent surface, cheapest structural flooring, excellent load spread when tongue and groove | Weakest in bending, heaviest for its strength, worst moisture recovery, poor edge fixing |
For a floor deck, you want chipboard's flatness and its interlocking edges. For sheathing and racking, you want OSB's shear strength and price. For anything that gets cut, machined, screwed near an edge, or that has to carry a fixing, you want plywood.
Most board selection questions resolve against those three sentences.
Decoding The Grades
Every one of these boards carries a class marking, and the classes are not comparable between families. Here is what each system is telling you.
Service Classes, The Foundation Of All Three
Service Class 1 is dry, heated and internal. Service Class 2 is humid, which covers protected external positions and unheated internal ones. Service Class 3 is exterior and exposed. Every board grade below maps onto one or more of these.
OSB, To EN 300
Four classes. OSB/1 is general purpose and non load-bearing, for dry conditions only. OSB/2 is load-bearing in dry conditions. OSB/3 is load-bearing in humid conditions, covering Service Classes 1 and 2. OSB/4 is heavy-duty load-bearing in humid conditions.
OSB/3 is the most widely used class in Europe and the effective UK construction default, which is why it is the grade most merchants stock.
One caveat worth stating plainly, because "moisture resistant" oversells it. OSB/3 tolerates the wetting that happens during a build and the conditions in an unheated space. Under permanent water exposure, performance degrades over time and progressive swelling can reduce load-bearing capacity where the panel is not properly protected. It is not a weathering board.
Plywood, To EN 636
Three environmental classes: EN 636-1 for dry, EN 636-2 for humid, EN 636-3 for exterior. Each carries a suffix telling you whether the panel is structural (S) or general purpose (G, sometimes written NS for non-structural). Both labelling conventions appear in the field.
EN 636 also assigns bending strength and modulus classes derived from testing to EN 310, giving a designation such as F10/20 covering performance parallel and perpendicular to the face grain. Surface appearance is graded separately again, under EN 635.
That suffix matters more than most buyers realise. EN 636-2 S and EN 636-2 G are the same environmental class and very different products. Only one of them is graded for a structural position.
Chipboard, To EN 312
Classes run P1 to P7. The one that matters for construction is P5, load-bearing for humid conditions, which is the UK flooring grade covering Service Classes 1 and 2. P2 is furniture grade, P3 is non load-bearing moisture resistant, and P6 and P7 are heavy-duty.
The Cross-Reference
| Use | OSB | Plywood | Chipboard |
|---|---|---|---|
| Non-structural, dry | OSB/1 | EN 636-1 NS | P1, P2 |
| Structural, dry | OSB/2 | EN 636-1 S | P4 |
| Structural, humid | OSB/3 | EN 636-2 S | P5 |
| Heavy duty, humid | OSB/4 | EN 636-2 S, higher F class | P6, P7 |
| Exterior, exposed | Not suitable | EN 636-3 | Not suitable |
One Thing All Three Share
Boards intended for construction fall under the harmonised standard BS EN 13986, which means they must carry a UKCA mark for products placed on the market in Great Britain, and that mark requires a Declaration of Performance behind it. No Declaration of Performance means the board is not legal for construction use, whatever is printed on the face. It is a fair thing to ask a supplier for.
The Bond Class Trap
This is the most common misreading in the whole category, and it costs people money.
Plywood carries two separate moisture-related designations, and they describe different things.
- EN 314-2 bond class describes the glue line only, in classes 1, 2 and 3. Class 3 is broadly comparable to what the trade used to call WBP, weather and boil proof, under the now-withdrawn BS 6566 Part 8, although the old requirement for phenolic-type glues no longer applies.
- EN 636 class describes the whole panel: glue and timber together.
A board can carry a Class 3 glue line and still only be an EN 636-2 panel, because the veneers themselves are not durable. The glue will survive the weather. The wood will not. A Class 3 bond on an EN 636-2 sheet does not license leaving that sheet exposed.
So What About WBP?
WBP is not a current standard and has not been for years. The bond test in use is similar in principle, which is why the term persists in the trade. When someone asks for WBP plywood, what they usually want is EN 636-2 with a Class 3 bond, which is a sound specification as long as the board ends up covered.
"Hardwood Faced" Plywood, And What The Name Means
Worth being straight about this, because the honest answer is more useful than the evasive one.
On a hardwood faced board, the face veneers are hardwood. The core usually is not the same species. Using poplar internally with a different, more decorative species on the outside is common practice across the industry rather than anything underhand, although suppliers do carry a responsibility to describe what they are selling correctly.
Does It Matter?
For most of what these boards are bought for, no. TRADA's position is that the majority of exterior quality plywood used in UK construction sits in situations that are generally dry with only occasional wetting, during the build or in service: internal flooring, wall sheathing, roof sheathing. That is a Service Class 2 situation, EN 636-2 is the appropriate specification, and wood durability is of limited importance there. Plywood built from veneers classified as not durable has been used in exactly these positions in the UK for over fifty years without a problem.
Two things a buyer should know all the same. First, unless durability is declared otherwise, all plies should be assumed to be not durable, and a manufacturer is not obliged to declare the durability class of the plies as part of the mark. Second, where the specification genuinely matters, third-party certification is the answer: the BM TRADA Q-Mark scheme was created for this problem, requires individual board marking, and makes a species declaration and durability rating a mandatory part of the panel label.
Where things go wrong is grade selection rather than species. Plywood of the wrong type, or of poor quality, turning up in roof decks and wall sheathing is a documented and recurring failure.
Fire: The Default Is D
Wood-based panels are classified without further testing under EN 13986. Chipboard produced to EN 312 at a density of 600 kg/m³ or above and 9mm thick or more automatically falls into D-s2, d0. Across the family, most wood-based panels land in D-s2, d0, or Dfl-s1 for flooring applications.
D means combustible, with significant smoke production, and sits well below the A2 and A1 thresholds required for the external walls of relevant buildings under Regulation 7(2). That is precisely why fibre cement boards get specified as plywood replacements in fire-rated and multi-occupancy work.
Flame-retardant panels do exist and reach higher. FR-treated OSB/3 is marketed at Euroclass B-s2, d0 for wall, ceiling and roofing applications, with Bfl-s1 for flooring. Note the split: a panel's classification is tied to its field of application, so a wall figure is not a floor figure.
Reaction to fire is a board property. Fire resistance is a property of a complete tested assembly. The two are not interchangeable and no sheet material has a fire resistance period on its own.
Where Each Board Goes In Insulation And Drylining Work
Loft Boarding Over Insulation
The commonest domestic use, and the commonest mistake. Boarding straight onto the joists compresses the insulation underneath and destroys its performance in exactly the area you have boarded. Either use a raised loft boarding system that keeps the full insulation depth clear, or accept the thermal loss as a deliberate decision. Narrow 2400 x 600mm P5 flooring boards are the sensible choice up there, because they are manageable by one person in a confined space.
Floating Floors Over Insulation
Board laid over rigid insulation over the slab, not fixed to the substrate. Deflection under point load, edge support and the perimeter expansion gap all become critical, and the governing specification is the insulation's compressive strength rather than anything about the board.
Sheathing In Timber Frame
OSB/3 or EN 636-2 S plywood fixed to the outside of the studs, breather membrane over, then cavity and outer leaf. The board is carrying racking loads, so the fixing schedule is structural and not a matter of preference.
Sarking And Roof Decking, With A Warning Attached
This one is non-obvious and catches people out.
Where a Type LR breathable underlay is fully supported on a sheet deck that resists the passage of air and vapour, such as plywood, OSB, chipboard or tongue and groove sarking boards, NHBC guidance is that the underlay and sarking together should be treated as a Type HR underlay for roof ventilation purposes.
In plain terms: putting a sheet deck under a breathable underlay cancels the breathability, and the ventilation you have to provide changes as a result. Worth checking before the deck goes down rather than after.
Flat Roof Decks
The deck sits under the AVCL in a warm roof, and over the ventilated void in a cold roof. A moisture-tolerant grade is essential either way, because the deck will very likely get wet during construction before the waterproofing goes on.
Pattress And Noggin Boards Behind Plasterboard
Plywood, every time, for anything that has to carry a fixing: televisions, radiators, grab rails, wall-hung units. This is where plywood's edge and face fixing hold earns the price difference over OSB.
Substrate In Wet Areas
Plywood or OSB as a base for tile backer board, but the backer board manufacturer's substrate requirements govern what is acceptable.
Getting It Down Properly
Published guidance varies. The common positions are 22mm P5 for wider centres and 18mm for narrower ones, but sources disagree on where the line falls, and UK convention is often quoted at 450mm rather than 400mm. Take the figure from the board manufacturer's datasheet rather than from a rule of thumb.
Leave a minimum of 10mm to 12mm around the full perimeter of the room and at every rigid obstruction, including pipes. Wood-based panels move with moisture content. There is no way to stop that, only to allow for it, and a missing gap is the most expensive failure in this list to put right.
Ring-shank nails or carcass screws at roughly 200mm centres along supported edges and 300mm on intermediate joists, with every head driven around 2mm below the surface so nothing grins through the finished floor covering.
Gluing T&G edges improves joint strength and absorbs a degree of joist variation. It is the difference between a silent floor and a squeaking one.
Bring panels into the space and let them equilibrate. Laying boards straight off a cold, wet lorry into a heated house guarantees shrinkage gaps later.
The factory edge seal on P5 flooring is a real part of the product, and cutting removes it. Treat field-cut edges with a suitable wood sealant wherever they may see moisture, near plumbing or an external doorway in particular.
OSB is directional. The face strands run along the panel length, and the panel should span across the supports in that direction. On a deck, the rougher mesh-marked face goes up for grip during construction.
Ten Ways It Goes Wrong
- Edge swell on OSB after wetting. Swelling at a cut edge does not fully recover, so the joint telegraphs through a finished floor.
- Floor heave from a missing expansion gap. Common, and costly to correct.
- Squeaking. Un-glued tongue and groove, under-fixing, or fixings that missed the joist.
- Delamination in low-grade plywood. Voids, overlaps and gaps in the core, and glue lines that fail on wetting despite the declared bond class.
- Wrong grade in a structural position. Non-structural EN 636-2 used as roof deck or sheathing. A documented cause of investigated failures.
- Compressed loft insulation under boarding. A thermal failure rather than a structural one, and completely invisible.
- Deck laid before the building is weathertight. Every family tolerates incidental wetting. None tolerates standing water.
- Chipboard in a genuinely wet area. P5 handles humidity and construction wetting. It does not survive a leak.
- Fixings into board edges. Chipboard and OSB hold poorly in the edge. Anything carrying load needs plywood or a timber pattress.
- Species or grade not as declared. Which is why the Declaration of Performance matters more than the stamp on the face.
Handling, Dust And Formaldehyde
Full 2440 x 1220mm sheets in the thicker gauges are a two-person lift. The 2400 x 600mm flooring boards are deliberately sized to be handled by one person, which is a large part of why loft and floor work is easier with them.
Wood dust is a respiratory sensitiser, and hardwood dust is a Category 1 carcinogen. Cutting composite panels releases resin dust alongside it. Use on-tool extraction, and FFP3 protection where extraction is not workable. This applies to OSB and chipboard just as much as to hardwood ply.
The resins that bind these boards release a small residual amount of formaldehyde into indoor air, and this is regulated. Everything sold in the UK meets the E1 class. From 6 August 2026 the EU limit for wood-based articles tightens, with no additional grace period, although panels manufactured before that date may be sold until stocks run out provided they meet the current E1 class.
Great Britain is not bound by the new limit. Northern Ireland is, as are GB exporters supplying into the EU. In reality the supply chain is converging anyway, because dual-stocking two formaldehyde grades adds cost and complexity nobody wants. Worth adding that this is a tightening of a precautionary limit rather than a response to any failure of compliant E1 board.
Still Deciding?
Work out what the board has to do before you look at the price. If it carries a fixing, use plywood. If it is racking a timber frame, use OSB/3. If it is a floor deck, use P5 tongue and groove. If it is going anywhere permanently exposed to weather, none of the standard grades is the answer.
For help matching a board to a specification, call the team on 03003 034 578, or browse the full timber sheet materials range online.
