Shelf sag check and hardware geometry

Shelves that look straight on day one can bend visibly after a year of books. Kerfwise checks every shelf of a cabinet as a beam, including creep, and tells you what would fix it. The calculation is the same engine as the kerfwise-shelf-sag tool.

1. Open a bookshelf

Start from the Home office: desk + bookshelves template. Open the Objects tab on the left and select A · Bookshelf left. The inspector on the right shows Key dimensions and, below them, the Shelf sag card.

2. Read the shelf sag card

The card lists every shelf with its span and its long-term sag. For this bookshelf it says 6 shelves sag more than L/300 under 120 kg/m². The 762 mm adjustable shelves in 18 mm MDF sag 12.7 mm, over the limit of 2.5 mm. The first suggestion reads: Fixed shelf: use a 33 mm thick shelf, a solid front strip 33 mm high or a centre support (0.8 mm sag).

Bookshelves and open shelving assume a full row of books, 120 kg per square metre of shelf. Other cabinets assume 50 kg/m² of household goods. The sag includes creep: MDF and particleboard keep bending under a permanent load, which roughly triples the instant sag.

3. Try alternatives in the check

Choose Check shelf sag…. The dialog takes span, depth, thickness and material from the cut list. Change them to experiment; nothing in the cabinet changes.

• Under Typical contents, choose another preset, or type your own Uniform load (kg per metre of shelf).
• Type 38 into Front strip height (mm, empty = none). With a 19 mm red oak strip glued to the front edge, the sag drops to 1.61 mm, within the L/300 limit of 2.55 mm. The figure shows the deflected shelf, the limit line and the cross-section with the strip.
• Under Sag limit, choose L/600 (0.02 in per foot, “visible sag” rule) for a stricter look, or L/360.
• Supports switches between shelves on pins and shelves glued into housings.
• Custom modulus of elasticity replaces the typical value when your board maker publishes one.

When the shelf fails, Ways to meet the limit lists the thickness, the strip height, the longest span that passes and the sag with a centre support. The load is remembered per cabinet type and the limit for all cabinets; the inspector card uses them when you close the dialog.

4. Hardware geometry

Hinges, slides and shelf pins follow vendor-neutral 32 mm system conventions:

• hole rows 37 mm from the front edge, 32 mm pitch, Ø5 mm shelf-pin and plate holes;
• 35 mm hinge cups 13 mm deep, 22.5 mm from the door edge (boring distance K = 5 mm), with two Ø8 dowel holes 45 mm apart;
• side-mount slides with 12.7 mm (½″) clearance per side; undermount slides with an inside drawer width of opening − 42 mm.

For a cabinet with doors, Key dimensions now includes Door overlay on the side (negative = inset reveal). A full overlay door on an 18 mm side with a 3 mm gap covers 16.5 mm of the side. An inset door shows a negative value, the reveal. If your hinge maker lists overlay for a different boring distance or plate height, 1 mm more K adds 1 mm of overlay and a 1 mm higher plate removes 1 mm.

Good to know

• The check is about stiffness, not strength. Shelf pins, joints and knots are not checked.
• Melamine chipboard is checked with the EN 312 minimum stiffness for P2 furniture board, so it errs on the safe side.
• Stiffness values for solid woods come from the USDA Wood Handbook; panels use European standard values.