Inspired by a helpful YouTube guide. This walk-through draws from the video "Building a Wooden Arched Garden Bridge - Part 1", which covers the arched stringer design in good detail. It is a more complex shape than a flat bridge but looks far better in a garden setting. Worth watching before you decide which approach to take.
1. Plan the span and style
Most garden bridges are between 1.2m and 2.4m long - enough to span a modest stream or dry feature without needing very heavy timber. Keep it under 1.8m if this is your first attempt; the longer the span, the heavier and more complex everything gets.
Decide early whether you want a flat deck or an arched one. A flat bridge is easier to build and more practical if people need to walk over it with a wheelbarrow or mower. An arched bridge looks more traditional and suits a garden pond or decorative feature, but requires curved stringers which take a bit more work to cut and shape.
2. Choose the right timber
Pressure-treated softwood is the practical choice for most garden bridges - available from any timber merchant or large DIY store, takes paint and stain well, and lasts many years if the end grain is treated. For the stringers (the main side beams), use 150mm x 50mm or 175mm x 50mm treated timber.
Hardwood - oak or sweet chestnut - is more durable and looks better unfinished, but it costs significantly more and is harder to work with basic tools. If budget allows and the bridge is a prominent feature, it is worth considering. Use stainless steel or galvanised fixings throughout - ordinary steel screws will rust in contact with treated timber.
3. Cut and fit the stringers
For a flat bridge, the stringers are simply two lengths of timber laid parallel at the correct width apart - typically 800mm to 1000mm for a pedestrian bridge. Cut a bird's mouth notch or flat bearing at each end so the stringer sits solidly on the bank without rocking.
For an arched bridge, mark the curve on the stringer using a long flexible batten bent to the desired arc and clamped at each end. Cut along the line with a jigsaw. The two stringers need to be identical - clamp them together and cut both at once if possible.
Bed the ends on paving slabs, concrete pads, or timber bearers set into the bank. The feet of the bridge should not sit directly on soil - they will rot far faster if they do.
4. Lay the decking boards
Use 75mm x 38mm or 100mm x 38mm treated decking boards across the stringers, fixed with galvanised or stainless screws. Leave a 5mm gap between boards for drainage and expansion - a 5mm spacer between each board keeps it consistent. Pre-drill near the ends to avoid splitting.
Trim the board ends flush with a circular saw or jigsaw once all boards are fixed. This gives a much cleaner finish than trying to cut each board to length individually before fitting.
5. Fit handrails
For a bridge over a pond or stream, handrails are sensible. Fix upright posts to the outside of the stringers - 75mm x 75mm or 100mm x 100mm timber at 600mm centres works well. A top rail at around 900mm height and a bottom rail at mid-height is a simple, tidy arrangement.
For an arched bridge, the uprights need to follow the curve. Cut each one to the correct height as you go rather than assuming they will all be the same - the curve means each one is slightly different.
6. Treat and finish
Apply a quality exterior wood oil or stain to all surfaces, particularly any cut ends. Even pressure-treated timber benefits from an oil finish - it slows the grey weathering that untreated wood develops after a couple of seasons outdoors.
Check fixings annually and apply a fresh coat of oil every two to three years. A well-maintained timber bridge should last fifteen to twenty years without needing structural work.
When to call a handyman
Call Richard for the bank preparation and footing work if the ground is soft or unstable, or if you want the whole bridge built and fitted. The stringers are the bit most people find awkward - getting two arched ones cut identically and set level across a real stream bank rather than a flat workshop floor is a two-person job.
Garden carpentry in Sandwich and East Kent
The Sandwich Handyman builds outdoor timber structures including bridges, gates, decking, and garden features across Sandwich and the surrounding area.
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