Domino Runs With Flat Planks: Spacing, Curves and Ramps

A domino run is the cheapest structure you can build with plank blocks: forty planks, five minutes, one satisfying collapse. It is also the most diagnostic, because a run that stalls tells you exactly which gap was wrong. This is how to get a run of a hundred planks to fall without a single stop.

Set the planks on edge, not flat

A plank standing on its narrow edge is unstable but that is the point: it is storing the energy that will knock over the next one. Standing on the wide face, it will not topple at all. Keep the run on a hard, flat surface — carpet absorbs the impact and the chain dies after ten pieces.

Spacing: the one number that matters

The gap between planks should be about half the height of the plank. If the gap is too wide, the falling plank does not reach the next one. If it is too narrow, the planks lean on each other and jam instead of falling.

Test with ten planks before building a hundred. Set the gap, topple the first, and watch the last one. If the run stops at piece six, the gap is too wide. If two pieces fall together, it is too tight.

Building a curve that does not stall

A curve works because the falling plank hits the next one slightly off-centre, which rotates the direction of the chain. Two rules keep that rotation consistent:

• Turn gently. Four or five planks around a quarter circle, not two.
• Keep the gap constant through the curve. The outside of a bend wants a wider gap and the inside wants a narrower one; resist it and keep the same spacing.

Most stalled curves are caused by one plank placed a few millimetres too far, not by the curve being too tight.

Adding a ramp

A run can climb a gentle slope if the fall from the previous plank is enough to push the next one over the lip. Two planks thick of book under the run is about the limit for plank dominoes.

Going down is easier than going up: a run that steps down a book will accelerate and travel further, so widen the gaps slightly after the drop.

Joining two runs

To join a run to a second run that starts at a different angle, place a single plank at the junction standing on its corner so it can fall in either direction. Then test the junction on its own before connecting both sides.

Branching: one plank knocks over two

Place two planks side by side, both within reach of the falling plank but not touching each other. The falling plank hits the top corners of both and the chain splits. Branch once per run at most; two branches make the whole thing fragile.

What the run teaches

Energy transfer, spacing calibration and cause-and-effect. A child setting a gap and watching the result is doing an experiment with one variable, which is more than most science worksheets ask for.

Storage and the morning after

A hundred planks standing on edge will not survive a passing cat. Build on the floor, photograph the run, and knock it down deliberately at the end — the collapse is the reward, and it saves you finding planks under the sofa for a week.

For the structural rules behind towers and bridges, see why plank structures collapse. For a set of timed building challenges, see the building tips section.