Built-in Plywood Bookcase with Uneven Floor
A customer wants a built-in bookcase 1800 mm high × 900 mm wide × 300 mm deep, with five shelves evenly spaced, made from 18 mm birch plywood. Each shelf must carry up to 25 kg of books. The back is 6 mm plywood, and the unit will be fixed to a plasterboard wall with two battens. The workshop has a biscuit jointer, a router, dowels and wood screws. The client wants a painted finish and no visible fixings on the outside. The floor is uneven by 8 mm over the 900 mm width, and the wall is slightly out of plumb. The workshop has no edge-banding machine, so exposed plywood edges must be treated before painting. The customer also wants the shelves to be adjustable if possible, but does not want the unit to feel flexible when books are loaded.
1. Recommend the carcass joints and shelf support method. Justify the choice for a painted built-in that must not feel flexible.
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Carcass: use 18 mm ply sides joined to the top and bottom with glued biscuits or dowels for alignment, plus screws in countersunk and plugged holes for strength. Biscuits alone are not a structural joint. For a painted unit, screws can be set below the surface, filled and sanded. Shelves: if adjustable, use 5 mm shelf pins with at least 12 mm of plywood around the hole, or metal shelf standards; if fixed, use 6 mm deep housings or glued-and-screwed cleats. A 6 mm ply back set in a rebate or groove and glued/screwed to the carcass gives racking resistance. Add a fixed top and bottom rail. For no visible fixings, fix through cleats from inside and plug the screw holes.
2. Check whether 18 mm birch ply shelves can carry 25 kg over the clear span. Show an approximate calculation or a recognised rule, and state the remedy.
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Clear span = 900 - 2 × 18 = 864 mm. For an 18 mm ply shelf, I = b h^3/12 = 864 × 18^3/12 ≈ 4.2 × 10^5 mm^4. Taking E ≈ 6,000 N/mm² for plywood and a 25 kg evenly distributed load, w ≈ 0.28 N/mm. δ ≈ 5wL^4/(384EI) ≈ 1 mm. That is within a normal L/300 limit, so 18 mm is not unsafe in the short term. However, plywood creeps under long-term load, the shelf is only 18 mm thick at the front edge, and adjustable pins concentrate the load. For a firm feel, specify 25 mm shelves, or keep 18 mm and add a 50 mm deep front stiffener or a centre divider. The centre divider also shortens the span to about 430 mm and is the strongest remedy.
3. Explain how you would deal with the 8 mm uneven floor, the plasterboard wall and the no-visible-fixing requirement.
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Uneven floor: do not force the carcass level. Build a separate base frame with adjustable feet, set the carcass level on it, then scribe a plinth to the floor and fix it to the base. An 8 mm gap can be hidden by a 10 mm scribe. Plasterboard wall: fix two horizontal battens to the wall studs with 75 mm screws; if there are no studs, use heavy-duty hollow-wall anchors and add more fixings. Pack the battens to bring them plumb. Fix the bookcase to the battens through the top and bottom rails from inside. No visible fixings: use internal cleats, screws from inside the carcass, and plugged or filled holes; keep the back panel in a rebate so it hides the wall gap.