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Explanation

DOCS / HOW-TO GUIDES / FURNITURE

Check a shelf for sag

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KerfCraft estimates how much a shelf bends under its load, now and after years. The check is advice from simple beam sums and estimated loads. It does not tell you that a shelf is safe.

Set the load on a shelf#

  1. Select a shelf.
  2. In the Properties panel, under Manufacturing, find Shelf load.
  3. Select what the shelf holds, for example Paperbacks or Vinyl records.
  4. If no preset is correct, select custom kg/m and type the load in kg per metre.

Each preset shows its estimated load in kg/m. In a project in feet and inches, the loads are in lb/ft.

Read the result#

  1. Open the Cut list tab of the manufacturing panel.
  2. Find the line for the shelf below the cut list.
  3. Click Fixes and assumptions to read the details.

If the shelf is fine, there are no fixes, so click Assumptions instead. A line starts with ✓ when the shelf is fine, and with ⚠ when it is not. It gives the sag now and in time, as a fraction of the span. The fixes are in order, from the smallest change:

  • a thicker board,
  • a centre support,
  • a stiffer material of the same thickness,
  • a hardwood lip on the front edge. KerfCraft does not calculate this one, so check it yourself.

If KerfCraft has no stiffness and strength data for the material, the line says "Can't check this material".

Try a shelf here#

This calculator uses the same check as the app, for a sheet-material shelf. It takes the thickness from the material.

Shelf
Load

Visible sag: sags 1.1 mm now, 2.1 mm in time (L/387). Bending stress 1.56 N/mm², 10% of what we allow.

  1. Thicker board: 21 mm or more of the same material (now 18 mm).
  2. Add a centre support: two 400 mm spans sag 0.1 mm in time (fine).
  3. Stiffer material: none we list at this thickness gets to fine on its own; combine it with a support or a thicker board.
  4. Glue a hardwood lip on the front edge (say 20 × 40 mm on edge): it stiffens the shelf a lot, but we do not calculate it, so check it yourself.
Assumptions
  • Span 800 mm (given), 280 mm deep, 18 mm thick; a plain rectangular board.
  • Simply supported at both ends (shelf pins, brackets, housings): conservative, since fixed or screwed ends sag less.
  • Load: Hardback and reference books, 30 kg/m (estimate: 160 × 240 mm at about 650 kg/m³ is 25 kg; coated-paper reference books are heavier). Converted to newtons with g = 9.81 m/s².
  • Birch plywood 18 mm: E = 10048 N/mm², bending strength 40.2 N/mm² (UPM DoP UPM007CPR (birch plywood 18 mm)), face grain along the span.
  • Long-term sag = instant × (1 + k_def), k_def = 0.8 for plywood (EN 1995-1-1 Table 3.2, indoors), the whole load sitting there for years.
  • Fine is at most L/600 long-term (Sagulator's 0.02 in per foot), visible sag up to L/300, worse will bow badly.
  • Strength: the load × 1.35 against k_mod 0.6 × strength / γ_M 1.2 (EN 1995-1-1, permanent load).
  • Front lipping, edging and back cleats are ignored: they stiffen the shelf, so it should do a little better than this.

What the check assumes#

  • The shelf is a plain board, supported at both ends. Fixed ends sag less, so the result is on the safe side.
  • The load sits on the shelf for years. Long-term sag comes from Eurocode 5 creep factors for heated rooms indoors.
  • "Fine" is a long-term sag of at most 1/600 of the span. "Visible sag" is up to 1/300. More than that "will bow badly".
  • A separate strength check gives a warning when the bending stress is too high.
  • Front lipping and back cleats are not included. They make the shelf stiffer.

For a generated shelf, the span is the length of the shelf. The real span between the pins is a little shorter.

Tall units#

A freestanding carcass or PAX-size frame more than twice as tall as it is deep shows "Fix this to the wall so it can't tip". KerfCraft does not calculate stability.