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Introduction, navigation & wood sizes

This bridge was constructed entirely on the back lawn, showing that you do not always need an elaborate workshop or shed to undertake such a project.

Lumber sizes

The measurements throughout this project are given in both Metric (mm) and Standard/Imperial inches.
The metric measurements (mm) are given first followed by the imperial measurements (inch) in brackets ().
There are only two stock sizes required for this project, both being common stock sizes.
They are…
90mm x 45mm (1 1/2″ x 3 1/2″) and 90mm x 19mm (3/4″ x 3 1/2″).

90mm x 45mm means wood that is 90 millimeters wide by 45 millimeters thick.
That is called the actual size. The nominal size of the wood relates to its size prior to being dressed (planed, surfaced, smoothed). For example, wood that is 100mm x 50mm (nominal size), becomes 90mm x 45mm (actual size) once it has been dressed (planed, surfaced, smoothed).

And the equivalent in standard…
1 1/2″ x 3 1/2″ means wood that is 1 1/2 (one and a half) inches thick by 3 1/2 (three and a half) inches wide.
That is called the actual size. The nominal size of the wood relates to its size prior to being dressed (planed, surfaced, smoothed). For example, wood that is 2″ x 4″ (nominal size), becomes 1 1/2″ x 3 1/2″ (actual size) once it has been dressed (planed, surfaced, smoothed).

Note: The 90mm x 19mm (3/4″ x 3 1/2″) boards used for making the arches must be premium and knot free. If there are any deformities in the boards, then they may snap when bending pressure is applied.