Pick a timber bridge by the width of the gap and by what will cross it: a single log carries foot traffic over gaps up to about 8 m, a multi-beam deck carries carts over gaps of about 6-8 m, and a timber truss covers 8-15 m. Cut each log long enough to rest at least 60 cm on solid ground at both ends, and shorten the span by 30-40% when animals or carts will use a single log. Timber bridges are the fastest bridge to build, and they rot, so plan to inspect and replace them.
Part of Bridge Types & Construction
Timber spans built from felled logs and sawn planks, from a single log across a stream to a triangulated truss over a river.
The Gap And The Traffic Decide Which Timber Bridge To Build
Two questions choose the design. How wide is the gap, measured bank to bank across the open space? And what must cross: people on foot, loaded animals, or carts?
The parent article, Bridge Types & Construction, gives the working rules used here. They are the parent’s figures, not the result of a calculation on this page. The child pages Single Log Bridge and Multi-Beam Bridge check them against beam arithmetic.
| Gap | Foot traffic only | Animals or carts |
|---|---|---|
| Up to 3-4 m | Single log, 20 cm diameter | Multi-beam deck |
| 5-6 m | Single log, 30 cm diameter | Multi-beam deck |
| 7-8 m | Single log, 40 cm diameter | Multi-beam deck, large timber |
| 8-15 m | Timber truss | Timber truss |
The diameters in the table are the parent’s. A wider gap needs a much bigger log, and big logs are heavy to find, fell and move, which is why a truss takes over beyond about 8 m.
What You Need
- Logs — straight and sound, with no rot, large knots or sharp curves. Tap along the length: a dull thud over a hollow or soft spot means rot.
- Bearing ground — solid bank or stone abutments. See Abutment Building for the footing.
- Tools — an axe or adze to flatten the top face, rope for a handrail, pegs for the deck.
- A tape or marked cord — to measure the gap and the log.
Method: Fit The Bridge To The Gap
Step 1: Measure the gap Measure from the edge of firm ground on one bank to the edge of firm ground on the other. Do not measure to the water’s edge in a dry season; a flood will move that edge.
Step 2: Add the bearing Add 60 cm of bearing at each end, which is 1.2 m in total. Cut the log to the gap plus 1.2 m.
Step 3: Choose the type from the table If animals or carts will cross and the gap is above the single-log limit for your timber, move up one type rather than hoping the log is strong.
Step 4: Flatten the top and add a rail A round log is dangerously slippery when wet. Flatten the top face. Add a handrail at waist height for any crossing over water or a drop.
Worked Example: A 5 m Stream
Inputs: a gap of 5 m measured between firm banks; logs of 30 cm diameter are available; mules will cross as well as people.
- Log length: 5 m + 2 × 0.6 m = 5 m + 1.2 m = 6.2 m.
- Foot limit for a 30 cm log: 5-6 m, so the gap fits on foot.
- Reduced by 40% for animals: 5-6 m × 0.60 = 3.0-3.6 m. Reduced by 30%: 5-6 m × 0.70 = 3.5-4.2 m. Together that is about 3.0-4.2 m.
- The 5 m gap is wider than 3.0-4.2 m, so a single 30 cm log is not enough for the mules. Use a multi-beam bridge or larger logs.
Common Mistakes
- Measuring to the waterline — the span ends up shorter than the real gap in flood, and the log has no bearing. Measure bank to bank on firm ground.
- Leaving the log round — it rolls underfoot and is lethal when wet. Flatten it.
- Using the foot-traffic span for animals — the parent reduces it by 30-40% for animals or carts. Use the reduced figure.
- Never inspecting — rot starts where timber meets wet ground. Check the bearing ends.
Safety
Falls and sudden collapse
A timber bridge can fail without warning where a rot pocket sits at the end bearing. Probe the ends with a knife or spike before every season. Do not cross a flooded or ice-covered log. Post the load limit at each end.
What’s Next
- Learn next: Single Log Bridge — checks the span rules against beam arithmetic
- Combine with: Beam Bridge — a longer treatment of the beam type