| 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 fails two ways: the bolt necks and snaps, or the threads strip out. Which depends on how deep it is threaded in, not just size or grade.
The bolt’s breaking load uses a smaller effective area (ISO 898-1, about the mean of pitch and minor diameter), not the full major diameter.
Minimum engagement for full strength: 1.0×D in steel, 1.25× stainless, 1.5× cast iron, brass and 7075, 2.0× aluminium 6061, 2.5× plastic.
On plasterboard the board is the weak point, not the fixing. A stud, timber or metal, is always the strongest option.
Axial load pulls a fixing straight out of the wall (a shelf, a TV bracket); shear pulls it sideways or down along the face (most picture hooks). Axial is the harder case, and the one plasterboard struggles with most. Packet ratings often describe shear.
400 mm is the maximum spacing for thinner 9.5 mm plasterboard, and many trades default to it regardless of board thickness because it gives a stiffer wall and more fixing points. 600 mm is only safe with 12.5 mm board.
A cavity fixing spreads load across a small area of a fairly weak material. A stud is solid load-bearing structure: no cavity-fixing trick beats bypassing the weak material entirely.
Manufacturer ratings assume perfect installation and even load distribution. Real brackets rarely share weight equally, so treat a multi-fixing total as an upper bound, not a number to plan right up to.
The breaking load is calculated from a smaller effective area, roughly the average of the pitch and minor diameters, because that is the minimum cross-section the threads leave behind.
Proof load is what a bolt reliably survives with no permanent stretch; yield is where permanent stretch begins; ultimate is where it breaks. Design loads are conventionally kept under proof load: near ultimate there is no real margin.
In a real engaged joint the first 2–3 threads can carry 70–80% of the tensile load before the rest share meaningfully, which is why short engagement concentrates stress and strips well before naive proportional scaling suggests.
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.
Releases in which this page changed, newest last. Derived from the archived copy of every release, not from notes written afterwards — so it reflects what actually shipped. Site-wide passes are left out; they are in the full changelog.