P5 Chipboard: The UK Guide to Flooring Grade Chipboard, Squeak Prevention, and When NOT to Use It

Your builder lays the first floor before the roof goes on. It rains for a week. The chipboard goes down anyway because the schedule is tight. Six months after move-in the floor squeaks at every footfall and you find a soft spot near the bathroom door. P5 grade, 22mm thickness, glued T&G joints, and a protected variant if the roof is delayed: get those four right and the floor outlives the kitchen above it. Get any one wrong and you're lifting boards in year two.

What chipboard is and what it's for

Chipboard is an engineered wood panel made by compressing wood chips and sawmill residues with a synthetic resin binder under heat and pressure. Roughly 65-70% of the raw material is recycled wood (sawmill offcuts, forestry thinnings, post-consumer timber), which makes it the cheapest structural sheet material on the UK market and one of the more resource-efficient.

In domestic construction it does one job, and it does it well: P5 chipboard is the standard subfloor decking for upper floors, loft conversions, and floating floors over insulation. It sits on top of the floor joists, gets nailed or screwed down, and disappears under the carpet, vinyl, engineered wood, or laminate that goes on later.

The "P5" in the name is a grade classification under BS EN 312:2010 (the European particleboard standard). Seven grades exist (P1 through P7). Only P5 belongs in a domestic floor. More on why in the grade section below.

You'll see chipboard called "Caberfloor", "CaberDek", "Egger Protect", or "P5 chipboard" on site. CaberFloor and CaberDek are West Fraser brand names (made in Cowie, Scotland). Egger Protect is the equivalent product from Egger UK. They're all P5-grade boards. The differences come down to the surface treatment, which matters when the build is exposed to weather before the roof goes on.

Grades: only P5 belongs on a domestic floor

The grade confusion is the single most common reason chipboard floors fail. Most homeowners don't realise that "chipboard" covers seven different products, six of which have no business in a load-bearing or moisture-exposed application.

P5 is the answer for any UK domestic floor. NHBC Standards Chapter 6.4.19 explicitly requires moisture-resistant P5 chipboard to BS EN 312 for floor decking. Approved Document C requires moisture-resistant chipboard in any room where water might be spilled (kitchens, bathrooms, utility rooms). Your building inspector will check for the green identification stripe on the board edge during the floor inspection. No green stripe, no sign-off.

Sheet sizes and what to specify

The standard P5 flooring sheet in the UK is 2400 x 600mm tongue-and-groove (TG4). That covers 1.44 m² per sheet. You'll occasionally see 2400 x 1200mm square-edge sheets sold for situations where T&G isn't appropriate (loose-laid temporary boarding, repair patches), but for a structural floor the T&G version is the only sensible choice.

Two thicknesses matter for domestic work:

18mm is the minimum thickness NHBC accepts for joists at or below 450mm centres. It's the thinnest, lightest, and cheapest option. Each 18mm sheet weighs roughly 18-19 kg.

22mm is required for joists up to 600mm centres, which is the more common modern joist spacing. Each 22mm sheet weighs roughly 23-24 kg. Most extension floors use 22mm because joists at 400mm centres are unusual outside heavily-loaded zones.

If you don't know your joist centres, measure them before ordering. 18mm boards on 600mm-centred joists is the single most common dimensional mistake on a domestic floor. The result is a visibly bouncy floor that squeaks under foot traffic and may fail a building control inspection. The cost difference between 18mm and 22mm is around £11 – £16 versus £9 – £15 per sheet, so over a 30 m² floor (~21 sheets) the upgrade adds roughly the cost of two extra sheets. Not worth saving.

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Standard P5 vs CaberDek vs Egger Protect

This is the question every extension self-builder asks, and almost no competitor guide answers cleanly.

Standard P5 chipboard (CaberFloor and equivalents) is fine if it gets covered by a roof within a few weeks. The "moisture resistant" label refers to humidity cycling and brief wetting, not standing water or weeks of rain. Manufacturers specify a maximum exposure window of typically 42 days for standard CaberFloor before the warranty is void.

CaberDek is the same P5 board with a heavy-duty waterproof film bonded to the top face. The film peels off when the build is dried in. It carries BBA certification for up to 60 days of weather exposure when laid per the manufacturer's instructions (which include using CaberFix D4 adhesive on every joint and joist). The protective film is also slip-resistant, which matters on an upper floor in a half-built shell.

Egger Protect is the Egger competitor product. Same 60-day exposure window, but instead of a peel-off film it uses a hard-wearing anti-slip resin coating on both faces. The coating stays on the board permanently. Egger market it as the only P5 board that can be tiled directly with flexible adhesive (more on tiling below, where the picture is more nuanced).

Decision rule: if the floor is going down before the roof, specify a protected variant. The premium over standard P5 is small. The cost of replacing a swollen, delaminated floor after the roof finally lands is enormous. If the roof is on first and the building is dried in, standard CaberFloor is fine.

For an extension build that follows the typical UK sequence (walls up, roof on, then first floor decking inside), standard P5 is the right call. For a build that decks the first floor early so trades can work on it before the roof is finished (common when the upper floor is a loft conversion or when scaffolding makes early decking practical), use CaberDek or Egger Protect.

Why chipboard floors squeak (and how to stop it)

This is the failure mode you'll find on every UK property forum: a P5 floor that squeaks at every other step within a year of laying. The root cause is almost always the same.

Chipboard floors squeak when the boards rub against each other or against the joist beneath them. The friction makes a noise. There are three points where movement happens: the T&G joint between adjacent boards, the contact between the underside of the board and the joist, and the gap around each fixing.

The fix is glue. Specifically, D4-rated PVA adhesive (or a polyurethane equivalent like CaberFix D4) applied to the inside of the T&G groove, plus a continuous bead on the top of every joist before the board goes down. That bonds the boards to each other and to the joists, eliminating the relative movement that causes squeaking.

The most common installation mistake is putting glue on the tongue surface instead of inside the groove. Glue on the tongue sits in the gap when the joint closes, smears along the top of the board, and contributes nothing to the bond. Glue inside the groove fills the cavity, locks the joint when it cures, and stays out of sight. Watch any West Fraser installation video and you'll see the bottle nozzle going into the groove, not on top of the tongue.

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The squeaking story doesn't end with glue. Fixing centres matter too. NHBC and the major manufacturers all specify ring-shank nails (3 mm minimum diameter, 2.5x board thickness in length, so 45 mm minimum for 18mm board and 55 mm minimum for 22mm) or screws (4.5 x 50-60mm chipboard floor screws) at every joist crossed, spaced 150-200mm along the joist line. Skip a fixing or stretch the spacing to 300mm and the board lifts at that point under foot traffic. Lift means rub. Rub means squeak.

If you're paying a flooring fitter, ask them to use screws, not nails. Screws are slightly slower but they pull the board down hard against the joist and stay tight. Ring-shank nails work too, but if a board ever needs lifting (for a plumbing repair, a future kitchen change), screws come out cleanly. Nails do not.

How much do you need

Each 2400 x 600mm sheet covers 1.44 m². Add 10% waste for cuts and offcuts on a typical rectangular floor. Add 15% if the floor has multiple bays, dog-legs, or notches around stair openings or services penetrations.

Sheet count rule: divide the floor area by 1.44 (the coverage of one 2400 x 600mm sheet) and add 10% for cuts and offcuts. Round up to the next whole sheet.

Worked example: A 4m x 5m extension upper floor needs 20 m² of decking. 20 / 1.44 = 13.9 sheets. Add 10% waste: 15.3 sheets. Order 16 sheets of 22mm P5 T&G at £11 – £16 per sheet for the chipboard line of the order.

For the adhesive: a 1L bottle of CaberFix D4 covers approximately 3 sheets when used on both joints and joists, or about 5.5 sheets if used on joints only. For 16 sheets of decking glued at joints and joists, budget 6 bottles at £6 – £10 per bottle. That's a small spend that prevents the most expensive failure mode.

For fixings: at 200mm centres along joists at 450mm spacing, a 4m x 5m floor takes roughly 20-25 screws per sheet, or 320-400 screws total. Two 200-packs of 4.5 x 50mm chipboard screws are enough, and a 200-pack runs around fifteen to twenty pounds at Toolstation or Screwfix.

Cost and where to buy

P5 chipboard is the cheapest structural sheet material on the UK market. That's the main reason new-build housing is built on it.

The lower end of each range is the trade price (Selco's CaberFloor on a registered trade account is the cheapest 18mm option). The upper end is high-street retail (Wickes own-label P5 is typical of the high-end-of-range pricing for both thicknesses). Builders' merchants like Travis Perkins and Jewson sit between the two on a trade account but rarely publish list prices online. Phone the trade desk at your local branch with the dimensions and quantity for an accurate figure.

Wickes runs frequent 10% off sheet materials promotions. If you're buying retail and don't have a trade account, time the order to a sale and the per-sheet price drops close to trade rates.

Protected variants carry a per-sheet premium of a few pounds over standard P5 (compare the CaberDek and Egger Protect prices to the standard P5 22mm price above). On a 16-sheet floor that's a low three-figure uplift in total. Negligible against the cost of replacing a swollen floor.

For an installed cost (chipboard supplied and fitted by a flooring contractor or carpenter, on top of joists you've paid for separately):

That figure includes the chipboard and the labour to lay it, plus a small allowance for fixings and adhesive. The chipboard alone is roughly nine to eleven pounds per square metre (the per-sheet retail divided by 1.44 m² coverage). Labour on a straightforward rectangular floor is the bulk of the cost.

Where to buy

Selco (CaberFloor) and the builders' merchants (Travis Perkins, Jewson, Howdens) are the best value on volume orders. You'll need a trade account or a registered card for trade pricing. If you're a homeowner self-managing a build, ask your builder to order under their account and bill you the trade price plus a small markup. Most will, and it's still cheaper than retail.

Wickes stocks own-label P5 T&G in 18mm and 22mm at every branch. Walk-in availability, delivery options, no minimum order. Best for small repair jobs or if you only need a few sheets.

Specialist sheet material wholesalers (Sheet Materials Wholesale, Hanson Plywood, Arnold Laver) carry the full CaberDek and Egger Protect range and can deliver pallet quantities. Useful for larger extensions where you're ordering 20+ sheets.

How to lay it (the short version)

This is what your builder or carpenter should be doing. If you're paying for the work, watch for these steps. If they're missing, the floor will fail.

1. Acclimatise the boards. Stack them flat in the room they'll be laid in for at least 48 hours before installation. This lets the moisture content equalise with the building. Stacking on edge is wrong: the boards bow.

2. Confirm joist condition and spacing. Joists need to be level, at the correct centres, and dry. Any gap greater than 3mm between the top of a joist and a straight edge laid across multiple joists needs packing or the board will rock when fixed.

3. Run boards perpendicular to joists. Boards laid parallel to joists will sag between supports. The long edge of the board must cross the joists, not run along them.

4. Stagger end joints by at least 500mm. End joints in adjacent rows should never line up. Stagger them like brickwork.

5. Support all short ends on a joist or noggin. Where a board ends mid-bay, fit a 50mm timber noggin between the joists at that point so the short end is supported. An unsupported short end cantilevers, flexes, and squeaks.

6. Glue the T&G joints (in the groove) and the joists. D4 PVA in the groove of every long edge joint, continuous bead along the top of every joist before the board sits down.

7. Fix at 150-200mm centres along every joist crossed. 4.5 x 50-60mm chipboard floor screws or 3 x 45mm minimum ring-shank nails. Edge fixings minimum 10mm from the board edge.

8. Leave a 10mm expansion gap at all walls. The skirting board covers it. The expansion gap accommodates seasonal moisture movement (manufacturers specify 2mm per metre run, which works out to 10mm on a 5m run; 10mm is the practical minimum for any standard room).

9. Mark joist centres and pipe/cable runs on the upper face of the boards as you go. Future plumbers and electricians will thank you. So will you, when you need to lift a board to retrofit a fault.

10. Don't cover the floor until it's dry. If boards have been rained on, prod them with a screwdriver. If the screwdriver sinks in, the board has gone. Replace it. If the screwdriver doesn't penetrate, allow the board to dry fully before covering with carpet, vinyl, or kitchen units.

When NOT to use chipboard

This is the section every other guide skips, and the question every homeowner needs answered.

Tiling directly onto chipboard. Don't. The Tile Association, BS 5385, and every professional tiler in the UK will refuse it. Chipboard moves seasonally. The bond between rigid tile adhesive and chipboard cannot accommodate the movement. The grout cracks. The tiles loosen. The whole floor fails inside a year. Overboard with a minimum 6mm WBP plywood (or a cement backer board like HardieBacker for wetter areas) screwed at 300mm centres into the joists below, then tile onto that. Egger Protect markets itself as tileable with flexible adhesive: the manufacturer's own claim is that it can be tiled directly. Most professional tilers remain sceptical, BS 5385 has not been updated to specifically endorse the product, and tile adhesive manufacturers will not warranty installations on it. Treat the claim as the manufacturer's position and overboard with backer board if you want a guaranteed result.

Wet rooms (bathrooms, shower rooms, ground-floor utility rooms with regular spillage). P5 will deck the floor structurally. It will not tolerate standing water from a shower leak or a washing machine overflow. WBP plywood with sealed edges, or a cement-board overlay system, is the right substrate for any tiled wet area. Kronospan (one of the major chipboard manufacturers) explicitly stated they would not warranty their product in permanently damp conditions.

Underfloor heating (UFH) without a UFH-specific board. Standard chipboard works as a UFH substrate in floating-floor build-ups (where the chipboard sits over PIR insulation with the heating pipes embedded above), but it is not the right substrate for in-board heating where pipes run inside the deck. Specialist pre-routed P5 UFH chipboard exists with channels milled in for 12mm or 16mm pipework. If your plumber or UFH installer specifies in-board UFH, use the routed product, not standard P5.

Outdoor or exposed structural use. Never. Even P5 is not weatherproof. Long-term exposure causes irreversible delamination. CaberDek and Egger Protect extend the construction-phase exposure window to 60 days. They are not outdoor materials.

Suspended ground floors over a damp void. Possible but not ideal. P5 over joists above a ground-bearing crawl space needs good airflow (working airbricks), insulation between the joists, and a damp-proof membrane between the joists and the chipboard. Kronospan and the other manufacturers consider this an edge case rather than a primary application. WBP plywood is the safer call for a suspended ground floor where damp is a known concern.

Health and safety

Chipboard dust is hazardous. The Health and Safety Executive classifies wood dust (including chipboard dust) as a hazardous substance under COSHH. Long-term exposure causes asthma and is linked to nasal cancer.

For domestic-scale cutting (a few sheets, occasional use), wear an FFP3-rated dust mask (the FFP2 paper masks sold for general DIY are inadequate for wood dust), use a circular saw or jigsaw with dust extraction connected, and cut outdoors or in a well-ventilated space. For sustained cutting (a full extension floor, multiple days of work), Class M dust extraction at the tool is the minimum HSE specification.

The resin binder in standard chipboard contains formaldehyde. All UK-supplied chipboard meets the E1 low-formaldehyde standard to EN 120, which is the building-regs requirement for indoor use. E0 (super-low formaldehyde) products exist if you have a specific sensitivity but cost more and offer marginal benefit in a normally-ventilated home.

Alternatives

Structural plywood

Plywood is the premium upgrade. WBP-grade or marine-grade plywood is more rigid across its width, holds fixings better at edges, doesn't swell permanently when wet (it dries back to size), and accepts tiles directly. It is significantly more expensive than chipboard, typically four to six times the per-sheet cost of an equivalent-coverage P5 board at high-street retail. For wet rooms, tile substrates, and UFH substrates, plywood is the right call. For a standard upper-floor decking that's getting carpet or laminate on top, plywood is overspec.

OSB3

OSB3 is structural, moisture-resistant, and cheaper than plywood. It's the standard sheet for flat-roof decking and timber-frame wall sheathing. As a floor deck it's less popular than P5 chipboard because the surface is rougher (visible strand pattern), the edges swell more aggressively when wet, and T&G OSB3 panels are less widely available. For a domestic extension floor, P5 wins on cost, smoothness, and merchant availability.

Engineered timber flooring as a structural deck

Some self-builders skip the chipboard layer and lay engineered timber flooring directly on joists. This works only if the engineered timber is specified for direct-to-joist installation (most are not), and it forfeits the cost and acoustic benefits of a chipboard deck under a separate finished floor. Not recommended unless your designer has specifically engineered the build for it.

Where you'll need this

• Flooring - P5 T&G chipboard is the standard subfloor decking over joists or over PIR insulation in a floating floor build-up
• Insulation - laid over PIR insulation in a floating floor build-up, common in loft and garage conversions
• Underfloor heating - decking layer above PIR insulation and UFH pipework in a floating floor build-up

These applications appear across any extension or renovation project that involves a new or rebuilt timber-suspended floor.

Common mistakes

Ordering 18mm boards for joists at 600mm centres. The single most frequent dimensional error. Results in a bouncy floor that fails inspection and squeaks for life. Match thickness to joist centres: 18mm for 450mm or below, 22mm for 600mm.

Glue on the tongue, not in the groove. Covered above. Watch for it during installation. If your fitter is dribbling glue along the top of each tongue, stop them and ask them to apply it inside the groove instead.

Skimping on fixings. 300mm fixing centres save five minutes per sheet and cost you a squeaking floor. Stick to 150-200mm centres along every joist crossed.

No expansion gap at the perimeter. Boards butted against the wall buckle when humidity rises. The gap is invisible once the skirting goes on. Leave it.

Covering wet boards. A common rush job when the builder is behind schedule and the floor finish is booked in. Wet chipboard covered by laminate or carpet rots in months. If boards have been rained on, dry them out properly. The screwdriver test (push the tip into the surface) tells you whether the board has structural integrity left.

Buying generic P5 from an unfamiliar source. Some imported "P5" boards perform below the standard despite the marking. CaberFloor, Egger Protect, CaberDek, and Kronobuild are the brands consistently endorsed by experienced UK builders. Trade pricing on branded product is competitive with generic retail.

Not staggering end joints. Aligned end joints in adjacent rows create a continuous weak line across the floor. Stagger them by at least 500mm, like brickwork.

Running boards parallel to joists. Boards must run perpendicular to joists so each sheet bridges multiple supports. Parallel-laid boards sag between joists and fail.