Hang a shelf that holds

Work out what the wall actually is

Knock along the wall with a knuckle. A solid, dull, unresponsive sound suggests masonry. A hollow, drummy sound with a change of note every four hundred millimetres or so suggests plasterboard on studs. A hollow sound that stays hollow everywhere and feels slightly springy suggests board dabbed onto blockwork with a cavity behind.

Drill a small test hole somewhere that will be hidden by the shelf itself and look at the dust. Pink or grey gritty dust means masonry. Fine white or grey powder means plaster or plasterboard. The bit dropping suddenly through into a void tells you the rest.

That distinction matters more than anything else in this guide, because the fixing that is right for one is wrong for both of the others.

Check what is behind before you drill

Cables commonly run vertically and horizontally in zones from sockets and switches, and pipes run to radiators and up walls to bathrooms above. Treat the area directly above and below any socket, switch or radiator as occupied until proven otherwise.

Use a detector, sweep it in both directions, and sweep it again from the other side. Detectors are not infallible; they are a reason to move a fixing two hundred millimetres rather than a licence to drill.

If a bracket position cannot be moved away from a suspect zone, the honest answer is to move the shelf.

Mark, level, and mark again

Hold the shelf or bracket in place, get it level, and mark through the fixing holes with a bradawl or a pencil. Do not mark one hole, take the bracket away and measure the second one; small measuring errors put a bracket visibly out of true over a short span.

Check the level again after marking. A bracket held with one hand while marking with the other drifts, always downward on the far side.

On an old wall, remember that level and parallel to the skirting are different things, and the eye is drawn to whichever reference is closer. A shelf a hand's width above a sloping skirting will look wrong even when it is perfectly level; in that case, split the difference deliberately.

Drill and fix to suit the wall

Into masonry, use a masonry bit on hammer setting, drill to a depth that lets the plug sit fully in the brick or block rather than in the plaster skim, and choose a screw long enough to pass through the bracket, the plaster and well into the structure.

Into a stud partition, find the timber studs and fix into them. That is the only fixing in a partition that behaves like a proper one. If the shelf position and the studs genuinely cannot be reconciled, a purpose-made plasterboard anchor rated for the load is the alternative, used in numbers and never at the edge of a board.

Into dot-and-dab, remember there is a cavity behind the board. A plug that ends in the cavity has nothing to grip. The fixing must reach through the board and the dabs into the blockwork behind, which usually means a considerably longer screw than the bracket packet assumes.

Load it slowly and look at it again

Put weight on gradually and look at the bracket where it meets the wall after a day. A fixing that is going to fail usually announces itself first as a hairline gap opening at the top of the bracket.

Keep heavy things near the brackets rather than in the middle of the span. A shelf that sags is not always badly fixed; often it is simply loaded in the worst possible place.

Be honest about what the shelf is for. A run of paperbacks is a serious load, and it arrives gradually enough that nobody notices the moment it became one.