How is plywood made?

1. Hit a plain board in the right place and it splits along its grain. A plywood board does not split. Look at its edge: it is made of thin layers.
2. A soaked log spins against a long blade. The blade peels off one thin, never-ending sheet, like unrolling a roll of paper. The thin sheet is called veneer.
3. Wood is made of long fibers, all lined up like a bundle of straws. That line-up is the grain. Pulling along the fibers is very hard. Pulling them apart sideways is easy.
4. Glue is spread on the sheets, and they are stacked. Each sheet is turned so its grain crosses the sheet below. The top and bottom sheets always point the same way.
5. The stack slides into a hot press. Heat and a mighty squeeze set the glue in a few minutes. Now the layers cannot slide, and they act as one solid board.
6. Test the board in both directions. Whichever way it bends, some layers have their grain running the strong way. It also stays flat, because each layer stops its neighbors from swelling.

This five-layer board has been pulled apart so you can see every sheet. Tap each one.

• Face sheet. The top sheet is the face. It is the best-looking sheet, and its grain runs along the length of the board.
• Cross sheets. These sheets are turned a quarter turn, so their grain crosses the sheets above and below. They give the board its strength across the width.
• Middle sheet. The middle sheet is the core. The stack is built the same above it and below it, like a mirror.
• Glue lines. A thin layer of glue between every pair of sheets locks them together, so they cannot slide when the board bends.
• Back sheet. The bottom sheet is the back. Its grain runs the same way as the face, so the board is balanced and stays flat.
• Striped edge. On the edge of a plywood board you can count the layers as stripes. Light and dark stripes take turns, because you see the grain from the side, then from the end.

Tap a sheet to turn its grain. The press glues the stack, and two weights test the board in both directions. Find a stack that is strong both ways and flat.

Experiment: What happens if every sheet has its grain pointing the same way?

1. Turn sheets until all three point the same way. Watch both tests.
2. Now make the middle sheet cross the other two.
3. Try a stack where the top and bottom do not match. Does the board stay flat?

What we found: With every grain pointing the same way, the board is strong along the grain but snaps easily the other way, just like a plain plank. Crossing the middle sheet makes it hold both ways. If the top and bottom sheets do not match, the board is lopsided and curls.

Turn on JavaScript to try it yourself.

Turn the five sheets so the board is strong both ways and stays flat. Then press it and test it.

Wood is made of long, thin fibers that all run the same way, up the trunk of the tree. That direction is called the grain. Along the grain, wood is very strong. Across it, the fibers can be pulled apart, which is why a plank splits along its grain.

To make plywood, a log is first softened in hot water. Then it spins against a long blade that peels off a thin sheet of wood, the way paper comes off a roll. The thin sheets are called veneer. They are cut to size and dried.

Next comes the clever part. Glue is spread on the sheets and they are stacked so that each sheet's grain crosses the grain of the sheet below. A hot press squeezes the stack until the glue sets. Now, whichever way you push or pull, some of the layers have their grain running the strong way.

Plywood has another trick. Damp wood swells across its grain but hardly at all along it. In plywood each layer holds its neighbors still, so the board stays flat and does not crack. That only works if the top and bottom sheets match, which is why plywood has an odd number of layers. The woven mat comparison is close, but the sheets in plywood are stacked and glued, not woven over and under.

Key words

Grain

The direction the fibers run in a piece of wood.

Veneer

A thin sheet of wood peeled or sliced from a log.

Ply

One layer of a plywood board.

Cross-lamination

Gluing layers so the grain of each one crosses the next.

Balanced panel

A board whose layers are the same above and below the middle, so it stays flat.

Check your thinking

1. How is each sheet laid on the one below it?

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   Turned so its grain crosses the grain below. Crossing the grain means each layer covers the weak direction of the next.

2. Why does a plain plank split so easily one way?

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   Its fibers all run one way and can be pulled apart sideways. Wood is strong along its fibers, but the fibers separate easily from each other.

3. Why are the sheets pressed hard while the glue sets?

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   So the layers lock together and cannot slide. Loose sheets slide over each other like a deck of cards. Glued sheets act as one board.

4. Why does plywood have an odd number of layers, such as 3, 5 or 7?

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   So the top and bottom sheets match and the board stays flat. With an odd number, the stack is the same above and below the middle, so it does not curl.

5. How is a log turned into thin sheets?

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   It spins against a blade that peels off one long sheet. The spinning log is peeled like a roll of paper.