A 1600 mm bookcase is built in two bays, never in one: past 800 mm of span, a 19 mm shelf loaded with books sags visibly within months. The centre divider is not an aesthetic choice, it is what holds the shelf up.
On its own this cabinet uses only 41 % of the 3 sheets it consumes. That is normal, and it is why the app nests the whole job in one go: one carcass's offcuts take the small parts of the others.
A 19 mm melamine shelf loaded with books — around 40 kg per linear metre — deflects roughly 2 mm over an 800 mm span. That is the limit of what the eye accepts. At 1000 mm the sag doubles, at 1200 mm it quadruples: deflection grows as the fourth power of the span.
Three remedies, in order of effectiveness: shorten the span, which is what the centre divider does; thicken the shelf; or glue a solid lipping to the front edge, which works like the flange of a beam. The first is free.
This model runs the side panels down to the floor instead of standing the carcass on an applied plinth. On a tall open piece that removes a horizontal line at the bottom of the front and carries the load straight to the floor.
The back is housed in a groove all round. On an open bookcase it is the back that stops the carcass racking into a parallelogram: a back stapled over the edges resists almost nothing.
800 mm for a book load in 19 mm melamine or MDF. In birch plywood you can hold 900 mm. Beyond that, add a divider, go to 25 mm, or glue a 30 mm solid lipping to the front edge, which triples stiffness without thickening the shelf.
320 mm is enough for paperbacks and novels. Allow 250 mm for paperbacks alone, and 350 mm if the bookcase has to take art books or A4 binders lying flat. Deeper than that, the shelf loses stiffness and the piece eats the room.
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