How the shelf sag checker works
Enter the span between the supports, the depth and thickness of the board, the material and what you will put on the shelf. The checker uses the standard engineering formula for a loaded beam to estimate how much the shelf will sag, both at first and after a few years under the load. It also tells you the longest span that will stay flat, and compares other materials and thicknesses.
Wood and boards do not just bend and spring back. Under a constant load, such as books, they slowly creep and sag further. The checker allows for this, and judges the sag on the long-term figure.
How much weight is on a shelf?
| What is on it | Typical load |
|---|---|
| Light decor | about 5 lb per foot (7 kg per metre) |
| Paperbacks | about 15 lb per foot (22 kg per metre) |
| Mixed books | about 25 lb per foot (37 kg per metre) |
| Hardcovers and binders | about 35 lb per foot (52 kg per metre) |
| Dishes, tools and heavy items | about 45 lb per foot (67 kg per metre) |
A full shelf of books is heavier than most people expect. A 30 in shelf of mixed books weighs about 60 lb.
Which materials sag the most?
- MDF and particleboard are the weakest, and sag a lot, even on short spans. They are fine for light decor.
- Solid pine and spruce are a good middle choice for shelves up to about 30 in with books.
- Plywood is a little stiffer than pine of the same thickness.
- Hardwood (oak, maple) is the stiffest of the common choices.
The thickness matters far more than most people expect. The stiffness of a board goes up with the cube of its thickness, so a 1 1/2 in board is 8 times as stiff as a 3/4 in board.
How to stop a shelf from sagging
- Add a support in the middle. It cuts the sag to almost nothing, because the span is halved.
- Use a thicker board, or a stiffer material.
- Add a front lip or edge strip, such as a 1 1/2 in strip of wood glued and nailed to the front edge, which makes the shelf far stiffer.
- Shorten the span by moving the supports in from the ends.
- Turn the board on edge. A board used on its edge is much stiffer, which is the idea behind a deep, thick floating shelf.
- Spread the heavy items over the shelf, and keep them near the supports.
Worked examples
A 30 in pine shelf, 3/4 in thick, 10 in deep, with mixed books
The shelf sags about 0.06 in at first, and about 0.09 in after a few years, which is 1 in 320 of the span. That is under the limit for a shelf that looks flat, so it works. The longest flat span for this board is about 31 in.
The same shelf at 36 in
The sag is about 0.13 in at first and 0.19 in later (1 in 185 of the span), so it will bend visibly. A support in the middle fixes it, as does a 1 in board, which brings the sag back to about 0.08 in.
The same 36 in shelf in MDF
MDF sags about 0.65 in over time, which is far too much. Use it only for light decor, or add supports every 16 in.
Common mistakes
- Using particleboard or MDF for books. They sag within weeks.
- Making the span too long. The sag grows with the fourth power of the span, so a small increase in length makes a big difference.
- Ignoring the weight of the books. The load is larger than it looks.
- Forgetting the supports. Weak brackets or screws can fail before the shelf bends.
- Using a “1 in” board as 1 in. Its real thickness is 3/4 in.
Tips
- Add a cleat or bracket in the middle of any shelf that is over 32 in long and holds books.
- Flip a shelf that has begun to sag. Reverse it, and the weight will bend it back over time.
- Edge banding or a solid wood face frame on plywood adds stiffness and hides the layers.
- Check the wall anchors too. A strong shelf is no use if it is held up by weak anchors.
