Standard Plasterboard (12.5mm): The UK Homeowner's Complete Guide

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Your plasterer turns up, takes one look at the plasterboard stacked in the extension, and says "these are the wrong edge." You ordered square edge because it was what the merchant had in stock. He's taping and jointing, not skimming, so he needs tapered edge. The merchant won't take them back once they've been on site. That's 20+ boards at £9 – £13 each sitting in your garden while you order the correct ones and wait for another delivery slot. Knowing the difference between square edge and tapered edge before you order saves you a couple of hundred pounds and a week's delay.

What it is and what it's for

Standard plasterboard is a sandwich: a gypsum core (calcium sulphate dihydrate, the same mineral used in plaster of Paris) pressed between two sheets of heavy paper liner. The front face is smooth ivory or white. The back is grey. That's it. No exotic materials, no complex chemistry.

It goes onto every internal wall and ceiling in your extension. The boards get screwed to timber studs, ceiling joists, or metal framing, then either skim-plastered for a smooth finish or tape-and-jointed (where the joints are filled and sanded without a full plaster coat). Every room in a modern UK house has plasterboard behind the paint.

The standard thickness for walls and ceilings is 12.5mm. This has been the UK default for decades and is what your plasterer expects unless told otherwise. The standard sheet size is 2400 x 1200mm (roughly 8ft x 4ft), covering 2.88m2 per board.

That weight matters. A single board at roughly 24kg is manageable for one person on a wall but awkward solo on a ceiling. Ceiling boards need two people and ideally a board lifter (a ratchet prop that holds the sheet against the joists while you screw).

British Gypsum manufactures under the Gyproc brand. Knauf and Siniat are the other two major UK manufacturers. All three produce boards to BS EN 520 and perform identically in practice. Trade consensus is consistent: board freshness matters more than brand. A damp board from a merchant's open yard performs worse than any manufacturer's fresh stock. Tap the board before loading it. If it rings, it's dry. If it thuds, leave it.

Types: square edge vs tapered edge

This is the decision that catches people out. The boards look almost identical, cost the same, and come in the same sizes. But they're for different finishing methods.

The short version: ask your plasterer. If they're skimming (which is standard in UK domestic work), they'll almost certainly want square edge. If they're tape-and-jointing, they need tapered edge. Don't guess.

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Sizes and specifications

The 2400 x 1200mm board dominates. It's what every merchant stocks in depth and what's priced most competitively. But other sizes exist and sometimes make more sense.

Thickness options beyond 12.5mm:

• 9.5mm exists and is lighter (roughly 18kg for a 2400x1200 board). Some guides suggest it for ceilings. Be cautious. 9.5mm boards sag over ceiling spans greater than 400mm between joists. Most UK joists are at 400-600mm centres. Use 12.5mm as your default for everything unless your joists are confirmed at 400mm or closer.
• 15mm is used where enhanced fire or acoustic performance is needed. Heavier, more expensive, and unnecessary for standard domestic rooms.

One thing to be clear about: standard 12.5mm plasterboard does NOT achieve 30-minute fire resistance on its own. If your building control officer requires a 30-minute fire-resisting ceiling (common where the extension creates a new habitable floor above), you need fire-rated plasterboard (pink/FireLine) or two layers of standard 12.5mm with a plaster skim. Don't assume standard board will pass.

How to work with it

Three fixing methods

Screwing to timber or metal studs is the standard method for stud walls and ceilings. Use 38mm coarse-thread drywall screws (W-type) for timber studs, 32mm fine-thread (S-type) for metal stud framing. The screw needs to penetrate at least 25mm into the timber behind the board. With 12.5mm board, that means 38mm minimum. Not 32mm, despite what some guides say. 32mm only gives you 19.5mm penetration, which is below the 25mm minimum.

Screw centres: 300mm apart on walls, 230mm in the main field of ceilings with 150mm at board ends. At external angles (outside corners), tighten to 200mm centres. You'll use roughly 36 screws per standard 2400x1200mm board on a wall.

Drive screws so the head sits just below the paper surface without breaking through. If the screwdriver bit punches through the paper, the screw has lost its holding power. The gypsum core is too soft to grip a screw head. A dedicated drywall screwgun with a depth-stop clutch prevents this. If you're using a standard cordless drill, go slowly and stop the moment the head dimples the paper.

Dot and dab (adhesive bonding) fixes boards directly to masonry walls without studs. Blobs of plasterboard adhesive (Gyproc Dri-Wall adhesive or equivalent) are applied to the wall in a grid pattern, then the board is pressed into position and tapped level. Each 25kg bag of adhesive covers roughly two standard boards (5.76m2). Prime the wall with diluted PVA (1 part PVA to 5 parts water) first.

Dot and dab is fast and avoids the need for battening. But it creates a void behind the board, typically 10-25mm deep. This matters in kitchens: if your kitchen units are wall-mounted and the wall behind them is dot-and-dabbed plasterboard, the fixings go through the board into thin air, then (hopefully) into masonry. Standard wall plugs won't work. You need either spring toggles, heavy-duty cavity fixings, or (the better approach for a kitchen extension) ask for wet plaster on the walls where units will hang. A solid plaster backing coat provides a continuous fixing substrate with no voids.

Battening and screwing involves fixing timber battens (typically 25x50mm or 25x38mm) to masonry walls, then screwing the plasterboard to the battens. This method is useful when walls are very uneven (old properties), when you need a services void for cables, or when you want to add insulation behind the boards. It takes longer than dot and dab but gives you a solid fixing for anything you hang on the wall later.

Cutting

Score the ivory face with a sharp Stanley knife along a straight edge, snap the board along the score, then cut through the back paper. Clean and simple. No power tools needed for straight cuts. A plasterboard saw (a short, jabbing saw with coarse teeth) handles cuts for sockets and switches. A hole saw on a drill makes perfect circles for downlight cutouts.

Board orientation

Fit boards horizontally on walls where possible (long edge running left to right). This reduces vertical seams and makes the plasterer's life easier. Stagger the joints between rows so that board edges don't line up vertically. Aligned joints crack. Staggered joints don't.

On ceilings, run boards perpendicular to the joists. Again, stagger the end joints so they land on different joists.

Leave a 10mm gap at the floor. Don't try to get the board tight to the slab. The skirting board covers this gap, and it prevents moisture wicking up from a concrete floor into the gypsum.

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How much do you need

Measure the total area of walls and ceilings to be boarded in square metres. Divide by 2.88 (the coverage of a standard 2400x1200mm board). Add 10% for waste, cuts, and offcuts.

A worked example for a typical 4m x 6m single-storey kitchen extension with 2.4m ceiling height:

• Ceiling: 4 x 6 = 24m2
• Walls: perimeter is (4 + 6 + 4 + 6) = 20m. At 2.4m height, that's 48m2 of wall. Deduct roughly 6m2 for a window and a pair of bifold doors. Net wall area: 42m2
• Two internal stud walls (say 3m and 2.5m long, 2.4m high, one side each): 13.2m2
• Total area: 24 + 42 + 13.2 = 79.2m2
• Boards needed: 79.2 / 2.88 = 27.5 boards
• With 10% waste: 30 boards minimum

Your plasterer will have their own view on quantities. They do this every week and will usually be accurate. But running the calculation yourself means you can sanity-check their figure and spot if they've over- or under-ordered. Over-ordering by 2-3 boards is normal (better than running short mid-job). Over-ordering by 15 boards means someone hasn't measured properly.

Cost and where to buy

Standard 12.5mm plasterboard (2400x1200mm) costs £9 – £13 per board at UK retailers, depending on supplier. This is the full-size board. Smaller 1800x900mm boards run £9 – £9 each, which is worse value per square metre but easier to handle.

Trade accounts and bulk pallet orders bring the price down to around £7 – £8 per board. Your plasterer will buy at trade prices. If they're supplying materials, expect to pay less per board than retail but check what they're charging as a markup.

For 30 boards (our kitchen extension example above), the board cost alone runs £270 – £390 at retail. Add drywall screws (roughly £15 – £20 for a box of 1,000, which covers 25-30 boards), scrim tape (£3 – £5 per roll), and plasterboard adhesive if dot-and-dabbing (£8 – £12 per 25kg bag, one bag per two boards). The total materials bill for boarding a typical kitchen extension is £350 – £500 before plastering materials.

Delivery is worth thinking about. Plasterboard is heavy and fragile. A pallet of 36 boards weighs over 860kg. Most online suppliers offer free delivery over a threshold (typically £250 – £350), and a 30-board order usually clears that. Have the delivery dropped as close to the extension as possible. Carrying boards from a street-parked lorry through a house takes longer than you expect and damages boards in the process.

Alternatives and specialist boards

Standard plasterboard isn't always the right choice. The alternatives cost more but serve specific purposes.

Moisture-resistant plasterboard (green board/MR board) has a water-repellent additive in the gypsum core and moisture-resistant paper liners. Use it in bathrooms, en-suites, and behind kitchen sinks, anywhere that gets regularly splashed. It costs roughly double standard board (around £20 – £25 per 2400x1200mm sheet at Wickes). Don't use standard plasterboard in a bathroom. It will absorb moisture, swell, and the plaster will eventually blow.

Fire-rated plasterboard (pink board/FireLine) contains glass fibres in the gypsum core that maintain the board's integrity at high temperatures. Required by Building Regulations where you need 30-minute fire resistance (between floors, in garage conversions, around boiler cupboards). Around £20 per board.

Insulated plasterboard bonds a layer of PIR insulation to the back of a standard or moisture-resistant board. It's more expensive per board (£15 – £30 depending on insulation thickness) but saves labour because you're fixing insulation and plasterboard in one operation. Standard plasterboard with separate PIR insulation boards is cheaper on materials but takes longer to install, which is why it's the budget alternative.

Acoustic plasterboard (SoundBloc) is denser than standard board and reduces sound transmission through walls. Used for party walls and home cinema rooms. Niche product, not needed for a standard extension.

For a typical kitchen extension, you'll use standard plasterboard on most walls and ceilings, with moisture-resistant board in any wet areas. Your building control officer will tell you if fire-rated board is required anywhere.

Common mistakes

Ordering the wrong edge type. Square edge for skimming, tapered edge for tape-and-joint. Confirm with your plasterer before ordering. Merchants rarely accept returns on plasterboard because it damages easily in transit.

Using 32mm screws instead of 38mm. The 12.5mm board plus 25mm minimum penetration into timber equals 37.5mm. Round up: 38mm. The 32mm screws that some merchants sell for "plasterboard" are for 9.5mm board into timber, not 12.5mm.

Not staggering joints. Boards lined up edge-to-edge on the same stud or joist create a continuous weak point. When timber moves seasonally, the crack follows that unbroken joint line. Stagger board ends onto different studs or joists. This is the single most common cause of cracked plaster on newly built walls.

Overdriving screws. When the screw head punches through the paper liner, it's useless. The gypsum core won't hold the screw. If you break through, drive another screw 50mm away and move on. A drywall screwgun with a depth-stop clutch prevents this entirely.

Boarding before first fix is complete. All electrical cables, plumbing pipes, and any other services that run inside the walls or above the ceiling must be in place and ideally signed off before boards go on. Ripping off freshly fixed plasterboard to route a forgotten cable is a waste of time and money that happens on extension builds regularly.

Mixing plasterboard waste with general skip waste. UK regulations (in force since 2009) ban plasterboard from going into landfill mixed with biodegradable waste. The gypsum reacts with organic material to produce hydrogen sulphide gas. Your skip company will charge extra (or refuse the load) if plasterboard is mixed in. Keep plasterboard offcuts separate. Many skip companies offer dedicated plasterboard bags or skips. Budget for this.

Where you'll need this

• Plastering - fixed to ceiling joists and internal stud walls before skim plastering, typically 10-15 boards for a standard extension