How to hang a shelf level.

Level the line, not the board.

The instinct is to hold the shelf up, put a level on it, and mark where it lands. It is the wrong order, for a simple reason: a shelf board is not straight.

Timber moves. A pine or oak board that left the timber yard flat will have taken up a bow within weeks of living in a heated room, and a melamine-faced chipboard shelf is often bowed from the factory. Put a level on the middle of a board with a 3 mm crown in it and you have measured the crown, not the wall. Fix the brackets to that reading and the shelf ends up genuinely out of level by however much the board was bent.

So the sequence is: establish a level line on the wall, set the brackets to the line, and let the board sit where it sits. If the board turns out to be visibly bowed, you can turn it over — crown up is better than crown down, because a load will flatten it rather than deepen it — or replace it. Either way you have not baked its defect into the fixings.

Nothing in the room is level.

Here is the thing nobody mentions until you are holding the pencil. In most houses the floor slopes, the ceiling slopes a different way, and the window head is out of true with both. A shelf set dead level in that room will not look level. It will look wrong, because the eye does not judge a shelf against gravity — it judges it against the nearest long line.

The resolution is not to abandon level, but to know when the eye wins:

• Level wins whenever things roll. Kitchen shelves, bathroom shelves, anything holding bottles, tins or round objects. A shelf that is visually pleasing and 0.8° out is a shelf you will be picking things up off the floor from.
• Level wins when the shelf is well away from a reference line. A shelf in the middle of a large wall has nothing nearby to be compared against, so true level reads as correct.
• The room wins for short shelves tight under a sloping feature. An alcove shelf 100 mm below a ceiling that falls 15 mm across the alcove will look badly wrong at true level, because the eye reads the closing wedge of shadow between the two. Splitting the difference — half the ceiling's slope — is the usual compromise, and it is what a good fitter does without discussing it.

Measure the room before deciding. Rest the phone on the floor, on the window sill, and hold it against the ceiling line, and write down what each one reads. If everything is within a couple of tenths of a degree, the argument does not arise and you simply build level. If the ceiling reads 1.2°, you now know what you are up against.

Marking the bracket line.

You want a straight batten a little longer than the shelf, a pencil, and the leveler with the top vial switched on and the range set to ±2°. Calibrate it first on a surface you trust.

1. Make one mark. Decide the height you want at one end and mark it. Resist measuring down from the ceiling at both ends — the ceiling is the thing you are trying not to copy.
2. Hold the batten to the mark. Rest one end of the batten's top edge on the mark and hold it roughly horizontal against the wall.
3. Lay the phone on the batten. Long edge along the batten's top edge, and pivot the free end up or down until the vial centres. Remember the bubble climbs toward the high end, so if it drifts right, drop the right end.
4. Draw the line. Pencil along the top of the batten while it is held there. A helper makes this much easier; a single screw driven lightly at the marked end, with the batten resting on it, works if you are alone.
5. Find the fixings. Now go looking for studs, noggins or masonry along that line. Do it in this order — a line placed to suit the fixings is a line placed to suit the wall's framing, not your shelf.

If the shelf spans more than about 1.5 m, check the line again after drawing it: put the phone at the left third and the right third of the line and confirm both read the same. A long batten with any bow in it will have quietly curved your straight line.

A run of several shelves.

The single biggest cause of a bookcase that looks drunk is measuring each shelf down from the one above it. Every measurement carries a small error, and measuring in a chain adds them all together in the same direction. Four shelves, each 1.5 mm out in the same sense, and the bottom one is 6 mm off — plainly visible.

Measure each shelf from a single datum instead. Draw one level line on the wall, then mark every shelf height down from that one line with a tape. Errors then stay independent and stop accumulating. Check the lowest and the highest with the phone once they are drawn, since those are the two that will be compared against the floor and the ceiling.

Sag, and setting the front high.

A loaded shelf deflects. An 18 mm board on brackets 800 mm apart, carrying a full run of hardback books, will visibly droop in the middle, and it will keep drooping slowly for months as the timber creeps. This is not a levelling error and no adjustment of the brackets will fix it — the answer is a thicker board, closer brackets, or a batten glued along the front edge to stiffen it.

What levelling can do is deal with the front-to-back angle. Many shelf brackets have a small deliberate upward kick at the nose, typically a degree or so, precisely so the shelf reads level once it is loaded. If you level such a bracket's top face rather than the wall line, you will end up with a shelf tipping forward under load. Fix to the wall line, then check the front-to-back reading with the phone once the board is on. A very slight lean back is fine and often desirable; any forward lean is not.

Checking it once it is up.

Put the phone on the shelf itself, running left to right, and read it. Then turn it end for end in the same spot and read again. If the two disagree, the difference is your instrument's error and the truth is halfway between — the technique is covered in the main guide.

Then turn the phone 90° and read front to back. Then move it to each end of the shelf in turn. Four readings takes a minute and catches the cases a single central reading hides: a twisted board, one bracket pulled out of the plaster, or a shelf that is level along its length but tipping forward.

A finished shelf inside ±0.2° is as good as the instrument can honestly tell you, and well beyond what anyone will see. Anything under 0.5° across a metre — about 9 mm/m — will look right in a normal room.

Pictures and TV brackets.

A picture on a single hook does not need levelling at all: it hangs from a wire and finds its own level, and the job is making sure it stays there. Two hooks, or a D-ring on each side, is what needs a level line, and the same batten method applies.

A television bracket is worth more care than a shelf, because a screen is a large flat rectangle that the eye compares directly against the ceiling and the skirting. Level the bracket's mounting rail to the wall line, hang the screen, and then read the top edge of the screen itself, since the screen's own mounting holes are not always square to its bezel. Tilt mounts should be checked with the tilt set where you actually intend to leave it.

For a screen, the room-versus-gravity argument bites hardest. A television is normally sat below a ceiling and above a unit, both of which the eye will use as a reference. If they disagree with level, favour the piece of furniture the screen sits above.