What to order for a bookcase: the cut list and the board count, worked through

The piece, and the rules it follows

One box, 900 mm outside, 2100 mm high, 300 mm deep, standing on a 100 mm plinth set back 50 mm, with four open shelves and no back. Our wall unit rules decide the rest. Boxes are 300 mm deep. Shelves are 290 mm, so they stop 10 mm short of the wall and sit flush at the front. Open shelves are doubled to 36 mm, two 18 mm layers glued face to face. An open cell has no back, so an 80 mm rail under the top of each cell keeps the box square and takes two screws into the wall. The top and bottom sit between the sides, so they are the outside width less 36 mm.

The plinth is a ladder: a front and a back the width of the box, and cross pieces 214 mm long, which is 300 less 50 of setback less the two 18 mm strips. It is levelled first, both ways, because everything above depends on it.

The part list

Codes as the workshop writes them: box M1, parts numbered, finished sizes in millimetres with banding on.

The cutting plan the engine returned

We fed that list to the same engine our workshop runs, with the grain switch on because the board is veneered. It laid everything on one board of 2800 by 2070 mm with a 4 mm kerf, 75 percent used, and said to order two: one to cut and one as the spare. The spare comes from the same batch, so a part cut again after a mistake matches the veneer of the rest.

The layout reads like instructions to the saw. First rip the board into strips across its width, widest first. Then cut each strip across, left to right. A part that sits on a wider strip is cut to length first and ripped to its own width after.

Why 36 mm, and why no back

Books are the heaviest thing most shelves ever carry. The panel association's shelving bulletin gives 19 mm particleboard and MDF a single span of roughly 510 to 580 mm under its heaviest listed load, about 2.4 kilopascals, at a sag limit of span over 240. Our 864 mm clear span is well past that in a single board, and comfortably inside it doubled, which is what the eight shelf layers are for. Spans beyond about 1200 mm add two steel flat bars between the layers.

The back is left off because the wall shows behind open shelves and is painted first. The 80 mm rail under each cell's top does the back's two jobs: it keeps the box square and it takes the wall screws, two per cell into a stud or a plywood strip fixed to the wall at the rail heights. The US safety commission's anchoring campaign exists because tall furniture with shelves tips when it is not fixed; this one cannot be left loose.

Source: Composite Panel Association: Particleboard and MDF for shelving, technical bulletin

If your board is 2440 by 1220

Retail boards here are mostly 2440 by 1220 mm, with the grain along the 2440 mm length. The 2100 mm sides still fit along it, and the 864 mm parts lie three to a strip as before, but the board is narrower, so the eight strips above no longer fit side by side. Expect two boards plus a spare, and lay them out before you buy. The rule does not change: parts along the grain, one kerf between each pair, widest strips first, and a spare from the same batch.

What is not on the list

Hardware and sundries are counted apart from the board. For this bookcase: 8 mm dowels and glue for the joints, screws for the plinth and the back rails, two wall screws per cell with plugs or a plywood strip, and 1 mm edge banding for the front edges, about 20 metres with the shelf fronts and the plinth. If a shelf is to move, add 5 mm pins and the two pin rows.

To see where these rules come from, read how we draw a box and edge banding and grain direction. If you would rather we drew and made it, start on our page for wall units and bookcases.

Source: U.S. Consumer Product Safety Commission: Anchor It!