Alcove shelving: getting both sides level when the chimney breast is not central

When a chimney breast sits off-centre the two alcoves are different widths, and shelves measured up from each floor finish at different heights. The fix is a single datum line marked right round the room at 1m with a laser, then everything set from that. In a Victorian terrace the floor can fall 15mm across one room, so measuring off the floorboards or the skirting guarantees the mismatch.

Alcove shelving is the most common joinery job in a Welsh terrace and it has one failure mode that everybody sees from the doorway.

Why are the two alcoves different widths?

Chimney breasts are rarely central and the two recesses either side are rarely the same. That is fine. Nobody notices a width difference of thirty millimetres.

What do people actually notice?

Shelves at different heights. If each side is set out from its own floor or its own skirting, and the floor falls away across the room, the left-hand shelves finish lower than the right-hand ones. Standing in the doorway with the fireplace between them, the eye reads that instantly.

How do you get both sides level?

The fix is to set a single level line across the whole wall, breast included, and work both sides from that one line. It means the bottom shelf may sit a little differently relative to the skirting on each side, which nobody sees, instead of the shelves reading out of level, which everybody does.

What else is worth doing at the same time?

Decide what is going on the shelves before they are built. A shelf sized for ornaments will bow under books or records, and the fix afterwards is a new shelf. Tell whoever is building it and the thickness and span get set accordingly.