Fixings by substrate
What actually holds in what. The fixing that fails is almost never the wrong size — it is the right size in the wrong wall.
Identify the wall first
Every fixing decision follows from what the wall is made of, and you cannot tell by looking at it. Two tests, thirty seconds:
- Knock it. A sharp, solid sound with no resonance is masonry. A hollow drum is plasterboard on studs or on dabs. Knock in several places — the sound changes over a stud.
- Drill a small pilot hole and look at the dust. Grey and gritty is concrete. Red or orange is brick. Fine, pale grey is a dense block. Very fine, almost white and extremely easy to drill is aerated block. White powder for the first 12 mm then nothing is plasterboard.
That last one matters most. Aerated concrete block and plasterboard are the two substrates where a standard fixing feels fine when you install it and pulls out six months later.
The table
| Substrate | Use | Typical safe load | Notes |
|---|---|---|---|
| Solid brick | Nylon plug + screw, or frame fixing | 25–50 kg | Fix into the brick, not the mortar joint — mortar is much weaker and often perished. |
| Dense concrete block | Nylon plug + screw | 25–50 kg | Drill on rotary only, no hammer, or you will blow out the hole. |
| Aerated / lightweight block | Purpose-made aerated-block plug, or chemical anchor | 10–25 kg | Standard nylon plugs spin in the hole and fail. This is the most commonly misjudged substrate. |
| Concrete | Nylon plug, or sleeve/wedge anchor for heavy loads | 50–100 kg+ | Strongest substrate you will meet domestically. Hammer drill and a sharp bit. |
| Plasterboard on timber studs | Screw directly into the stud | 30–50 kg | Always the best answer where a stud is in the right place. Nothing beats solid timber. |
| Plasterboard, between studs | Self-drill anchor (light), spring toggle (heavy) | 5–25 kg | Load is limited by the board tearing, not by the fixing. Spread load over several points. |
| Plasterboard on dabs | Long spring toggle, or fix through to the masonry behind | 5–20 kg | There is a void of 10–30 mm behind the board. A short fixing finds nothing. |
| Tile over any of the above | As the substrate beneath, one size longer | as substrate | The tile carries nothing. Drill through it without hammer, then switch. |
| Timber | Wood screw, pilot-drilled | 50–100 kg | Pilot hole about 70% of the screw's core diameter in hardwood; often none needed in softwood. |
Loads in the table are per fixing, in shear — hanging straight down a wall. Anything that pulls away from the wall, like a shelf with weight at the front or a wall-hung cabinet, applies leverage and the effective load is a multiple of the actual weight. For those, use more fixings, fix into the strongest substrate available, and do not work close to the limit.
Drilling: the part that determines whether it holds
Match the bit to the material
- Masonry bit — tungsten-carbide tip, for brick, block and concrete. Used with hammer action except in aerated block and dense block.
- Twist bit — for timber and metal. Never use hammer.
- Tile or glass bit — spear-pointed or diamond, for the glazed surface only. No hammer, ever.
Get the hole right
- Depth: plug length plus 10 mm, so the dust has somewhere to go. A plug bottomed out on drilling dust does not expand properly.
- Diameter: exactly the plug's stated size. A hole 1 mm oversize loses most of the fixing's capacity, and you cannot see the difference once the plug is in.
- Clean it out. Blow or vacuum the dust. This is the step everyone skips and it makes a measurable difference, especially for chemical anchors, which fail outright in a dusty hole.
- Keep it straight. An angled hole means the plug only grips on one side.
Drilling through tile
Start with no hammer and a tile bit. Mark the spot with masking tape to stop the bit wandering on the glaze. Go slowly, with light pressure, and keep the tip cool. Once you are through the tile and into the substrate, stop, change to a masonry bit, and only then switch hammer on if the substrate needs it. Hammering through a tile cracks it, and there is no repair for a cracked tile in a finished wall.
Finding studs
Worth the effort, because a screw into a stud beats every hollow-wall fixing ever made.
- Studs are usually at 400 or 600 mm centres, so find one and the others are predictable.
- There is almost always a stud beside a door opening and in each corner.
- An electronic detector finds the edges; the centre is halfway between. Check for cables and pipes at the same time, and check on both settings.
- Knocking works: the hollow note goes dull over a stud.
- Verify with a fine pilot hole before committing. A 1.5 mm hole is nothing to fill; a 10 mm hole in the wrong place is a repair.
Never drill blind near a socket, a switch, or directly above or below one — cables run vertically and horizontally from those points, and that is exactly where people put shelves.
When to use a chemical anchor
Resin injected into the hole, with a threaded stud or a perforated sleeve. More work and more cost, and the right answer in four situations:
- Aerated block carrying anything meaningful
- Old, soft or perished brick where a plug finds nothing solid
- Close to an edge, where an expanding plug would split the masonry
- Anything structural — a deck ledger bolted to a house wall, a handrail, a gate post
Resin does not expand, so it puts no stress into the substrate — which is exactly why it works where expansion fixings cannot. It also fills the irregularities of a rough hole, so it tolerates the imperfect holes you get in old masonry.
Common mistakes
- Fixing into the mortar joint because it drills more easily. That is why it drills more easily.
- Overtightening. A screw driven too hard in plasterboard strips the anchor and you lose the fixing entirely. Stop when it is firm.
- One heavy fixing instead of several light ones. Spreading the load across four points in plasterboard beats one point every time.
- Reusing an old hole. It is the wrong size, it is full of dust, and the substrate around it is already damaged. Move 50 mm and drill a fresh one.
- Assuming the whole wall is the same. Plenty of walls are masonry to waist height and stud above, or brick with a blocked-up opening in the middle of them.
Related
- Drywall calculator — screw counts and spacing for boarding itself
- Decking calculator — deck screws and frame fixings
- How to measure a room properly
Last reviewed 2026-09-18. Load figures are indicative planning values for domestic work. The fixing manufacturer's data sheet is the authority, and anything structural or safety-critical should be specified by a qualified person.