| Board thickness | Max centres | US equivalent | Studs per 1200mm board |
|---|---|---|---|
| 9.5 mm plasterboard | 400 mm | ~16 in OC | 3 |
| 12.5 mm plasterboard | 600 mm | ~24 in OC | 2 |
| Gauge | Typical diameter | Common use |
|---|---|---|
| No. 4–6 | 2.9–3.5 mm | Small hinges, light fittings, cabinet hardware |
| No. 8 | 4.2 mm | General DIY — shelving brackets, most fixtures |
| No. 10 | 4.8 mm | Heavier shelving, door furniture, decking |
| No. 12–14 | 5.5–6.3 mm | Structural timber connections, heavy brackets |
A threaded joint has two independent ways to fail under tension: the bolt itself can neck down and snap, or the threads it's engaged with can shear off and strip out — and which one happens depends entirely on how deep the bolt is threaded in, not just its size or grade. The bolt's own breaking load comes from its tensile stress area (ISO 898-1's formula, based on an average of the pitch and minor diameters, not the full major diameter) times its grade's ultimate tensile strength. The threads' stripping resistance grows with how much engagement length is actually cut — too little, and the threads give way well before the bolt ever gets close to its rated strength, no matter how strong that bolt is on paper.
An M10×1.5, grade 8.8 bolt has a tensile stress area of 58.0 mm² and an ultimate breaking load of 46.4 kN. Threaded a full 10mm (1.0×D) into steel, the bolt itself governs — the threads can take the full 46.4 kN before stripping. Thread the same bolt only 10mm into aluminium 6061 instead of steel — which needs 2.0×D for full strength — and the threads now strip at just 23.2 kN, half the bolt's real capacity, even though nothing about the bolt changed at all.
The same weight needs a completely different fixing depending on what's actually behind the surface. A timber or metal stud, found with a stud finder or the knock test, is always the strongest option — a wood screw driven straight into solid timber comfortably out-holds almost anything you'd hang in a home. Solid masonry needs a wall plug sized to the load. Plasterboard with nothing solid behind it is the trickiest case: the board itself is the weak point, not the fixing.
An 18 kg TV bracket on plasterboard with no stud nearby is a genuinely risky fixing even spread across multiple standard plasterboard plugs — independent destructive testing has found plasterboard itself typically fails around 60 kg of straight pull-out force regardless of fixing type, and trade guidance is to never load a fixing beyond roughly a third of that peak — meaning a safe practical ceiling of about 20 kg per fixing, at best. The right call here is a timber batten fixed across two studs, or finding the stud directly.
Packet ratings often describe shear load (force pulling down, parallel to the wall) rather than axial load (force pulling straight out) — a picture hook is mostly shear, a shelf bracket or TV mount is mostly axial, and axial is the harder case for any plasterboard fixing to resist.
An electronic stud finder detects the density change; a strong magnet will stick to the screws or nails already in the stud; tapping across the wall gives a dull "thud" over a stud versus a hollow ring in the gap between them. Combine two methods if you're hanging anything heavy enough to matter.
No — removing the screw drops the wings into the wall cavity permanently. A snap-toggle or a self-drilling plasterboard screw are the fixings to reach for if you might want to move the item later.
Standard plasterboard fixings only grip the board itself, which is often only glued to the masonry behind at intervals — for anything heavy, drill through to the masonry and use a wall plug rated for the actual load, long enough to reach solidly past the adhesive gap.
A tap drill only removes material for the thread's minor diameter — the tap itself cuts the rest of the thread profile as it advances. Drilling to the full major diameter would leave nothing for the tap to bite into.
Close fit keeps the bolt tightly aligned but demands accurate hole placement; loose fit tolerates real-world misalignment (weld fabrications, laser-cut sheet) at the cost of a looser-feeling joint and reduced bearing area under the head. Normal fit is the sensible default for most general assembly.
Aluminium's threads shear at a lower load than the steel bolt threading into them — engaging further spreads the same tensile pull across more threads, so the total stripping resistance catches up to what the bolt itself can take before breaking.
Proof load is the highest stress a bolt reliably returns from without permanent stretch — loading a real joint anywhere near the ultimate breaking load leaves no margin at all and risks permanent deformation even if it doesn't snap. Design working loads are conventionally kept under the proof load, with a further safety factor on top for anything critical.