Low Arched Bridge

We wanted to continue our garden path to the back lawn, and thought a low bridge over a dry creek bed feature would look good. So I designed and built one. This build was pretty straightforward and includes one unexpected discovery (for me!) that made the process MUCH easier - you can cut shallow curves with a circular saw very easily.

1. Two 10' × 12" × 2" pressure-treated boards
2. Five 16' × 6" × 3/4" pressure-treated decking
3. One 8' × 4" × 4" pressure-treated post
4. Deck screws (3" and 1 1/2")
5. Copper naphthenate solution (for added protection against rot)
6. Deck paint

Design was simple, a couple of curved rails from 10' pieces of 2×12" lumber and a decking platform (Fusion360 plans). I didn't want the curve to be too pronounced, to allow easy passage of a loaded wheelbarrow. I drew the curves using my 8 m (25') tape measure as a radius. I thought that cutting the curves might be difficult, but I just cut them slowly with an old carbide blade (24 teeth) in my circular saw, and that worked really well. I do not recommend trying this with a fine or thin-kerf blade. Take it easy and kick back won't be an issue. The video is sped up; the real duration of that cut was 1 minute.

A jigsaw is another option for cutting the curve but I loathe that tool for anything other than rough cuts, it suffers from too much blade drift.

I used four 4×4 posts 604 mm (24") in length to connect the two sides, lining them up with the top of the curve. I added extra supports in the middle, cutting them to length then trimming the curve by hand on my table saw (using the curve of the outside pieces to mark the line). Everything was screwed together with 3" deck screws.

I used copper naphthenate solution to treat the wood, as it would be in contact with the ground. I made some extra cut-outs in the side of the bridge to accommodate tree roots, and re-treated that area as well. I then painted all the visible parts of the bridge.

This was probably the trickiest part of the operation! I had to get the top end lined up with the lawn, and the bottom end level with the path, while avoiding tree roots and ensuring the bridge itself was level. Lots of digging and testing and digging some more. I rested the bridge on a mix of sand and stones rather than dirt to improve the drainage.

This was pretty quick, just make sure you measure the length accurately (including the gap between boards, I used 4 mm ~ 1/8") so you are not left with small planks at each end. This meant an even number of boards (22) for me, so I built out in both directions starting from the center line. I treated the ends of each board, then painted the whole thing. You're done! Enjoy your wee bridge. We created a dry river bed to "run" under the bridge and did some additional planting.