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A log bridge is a timber bridge that uses logs that fall naturally or are intentionally felled or placed across streams. The first man-made with significant span were probably intentionally felled .
The use of emplaced logs is now sometimes used in temporary bridges used for logging roads, where a forest tract is to be harvested and the road then abandoned. Such log bridges have a severely limited lifetimeNational Parks Conference 1915, p. 59. "The chief objection to a log bridge ... is the shortness of its life." due to soil contact and subsequent rot and Xylophagy insect infestation.
Longer lasting log bridges may be constructed by using treated logs and/or by providing well drained footingsNational Parks Conference 1915, p. 59. "If we can design the abutment so that no moisture can collect under the logs the life of the bridge is materially increased." of stone or concrete combined with regular maintenance to prevent soil infiltration. This care in construction can be seen in the French bridge illustrated below, which has well locked dry set stone and a footpath leveled with boards.
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