Drill a small pilot and read what comes out - the dust and the feel of the drill say more than tapping the surface.

Fine grey dust with steady resistance is usually concrete; red or brown dust is brick; a sudden drop in resistance after a short distance means a hollow, whether block cavity or stud partition.

Plasterboard gives white dust and almost no resistance, then either a void or a timber or metal stud behind.

A soft crumbly resistance with pale dust often indicates a lightweight aerated block, which needs its own anchor type entirely - ordinary expansion plugs spin in it.

Detectors help find studs and services but do not identify the material, so both are worth doing.

Where the substrate turns out to be different from expected, the answer is a different anchor rather than a longer screw.

Because expansion anchors work by being compressed inside the hole, and the hole diameter sets how much compression there is.

An oversized hole leaves the plug loose. It expands into free space instead of against the wall, and the fixing pulls out under modest load.

An undersized hole forces the plug in distorted, so it grips unevenly and often damages the surface as it is driven.

Hole depth is the second half of the rule. Most anchors need depth beyond the plug length so dust has somewhere to go; otherwise the plug bottoms out and never seats fully.

Worn bits drill oversize. A masonry bit that has done a lot of work no longer cuts the diameter printed on it.

Clearing dust from the hole before inserting the anchor matters most for adhesive and screw-in types, where dust prevents any grip at all.

A plug expands against solid material; a hollow-wall anchor develops its grip behind a thin skin with nothing solid beyond it.

In masonry the plug is compressed by the screw and presses outward against the hole wall. The wall itself carries the load.

In plasterboard there is no material to press against, so the anchor must spread, fold or thread into the back face of the board.

That is why a plug in plasterboard spins uselessly and pulls out with the board face attached.

Hollow-wall anchors need a known board thickness and a clear void behind, so hitting a stud or a service pipe changes the fixing.

Where a stud is present, a direct screw into it is stronger than any hollow-wall anchor and should be preferred.

It depends far more on the substrate than on the anchor, which is why capacities are always published per material.

The same anchor in dense concrete, in soft brick and in a lightweight block will give three very different results, and the weakest of them may be a small fraction of the strongest.

Failure is usually the substrate breaking out or crumbling rather than the anchor itself giving way.

Published figures assume correct hole size, correct depth, sound material and adequate distance from edges, none of which is guaranteed on an existing wall.

Load direction matters: tension and shear capacities differ, and most real fixings combine both.

For anything where failure matters, a pull test on the actual wall is the only reliable evidence, and is standard practice for safety-critical fixings.

Yes - both reduce capacity substantially, and a mortar joint is the weakest place in a masonry wall.

An anchor needs material all around it to resist the outward pressure it generates. Near a free edge that material is missing on one side and the corner breaks away.

Mortar joints are softer than the units they bond and are often incompletely filled. An expansion anchor in a joint may feel tight and hold very little.

Anchors placed close together interact, because their zones of stressed material overlap.

Perforated and hollow blocks add another failure: the drill breaks into a void and the plug expands into empty space.

Minimum edge distances and spacings are published with each anchor and are worth reading before setting out, not after.

It is a genuine advantage, because the wrong hole diameter is the most common reason a fixing fails.

A kit supplied with matched bits makes the correct pairing obvious and removes the guesswork when the packaging is long gone.

Bits wear, so the ones in a kit will eventually drill oversize and need replacing even though the anchors remain.

Some anchor types need a setting tool as well - certain hollow-wall anchors are deformed by a dedicated tool rather than by the screw.

Where no bits are included, marking the required drill size directly on each compartment is a five-minute job that keeps paying.

Rotary hammer and rotary-only drilling behave differently in soft and aerated materials, so the drill is as much a choice as the bit.

No. Overhead, structural and safety-critical fixings need specified anchors with published performance for that substrate.

General assortment anchors are unmarked, of unstated capacity, and often not traceable to a manufacturer once out of the packaging.

Overhead fixings are a distinct category in most anchor approvals, because the consequences of failure and the sustained tension are different.

Where an anchor is part of a designed connection, the design names a specific product and installation method, and substituting from a kit invalidates it.

Fire, seismic and cracked-concrete conditions each have their own approval routes, and a general anchor is assessed against none of them.

The kit's proper role is repair, fitting-out and light general fixing, where the consequence of a pull-out is inconvenience rather than injury.