Levelling a shelf in a room that is not square

A shelf can be level or it can be parallel to the ceiling, and in most rooms it cannot be both.

The decision has to be made before the first fixing goes in, because it changes where everything else sits. Level is the right answer where objects will roll, where a shelf continues a line set by something else already level, or where the run is short enough that the discrepancy stays invisible.

Parallel is the right answer when the shelf sits close under a ceiling or above a window head, where the eye has a strong reference line only a short distance away. A shelf two centimetres out of parallel under a ceiling looks wrong even when a spirit level says it is perfect.

Find out what you are dealing with before choosing. Mark a level line right across the wall with a long level or a water level, then measure from that line up to the ceiling at both ends and in the middle. The three numbers say everything. A difference of a few millimetres can be ignored; a centimetre or more over a couple of metres is the kind of thing that governs the decision.

Splitting the difference is a legitimate third option on long runs. Set the shelf half way between true level and true parallel, so neither error reaches the threshold at which it becomes noticeable. It feels like a fudge and it is the right call more often than either purist answer.

Ends need scribing whichever way the line runs. Hold the board in place, set a compass or a block of scrap to the widest gap, and run it down the wall so the pencil traces the wall's own shape onto the timber. Cut slightly outside the line and take the last of it off with a block plane, testing as you go.

Fix the back edge first and check for twist before the front is loaded. A shelf can be dead level along its length and still tip forward if the brackets are not in the same plane, which shows up the first time something round is put on it.

Finally, load it before deciding it is finished. Timber deflects, and a run that looked crisp empty can develop a visible sag in the middle bay under books. Adding an intermediate bracket is a ten-minute job at that stage and an afternoon once the shelf is painted in.