Garden Bridge

This classic project will add a striking feature to your yard or landscape.

Project #SU063

You don't need to have a babbling brook in your backyard to find the perfect location for this simple, yet elegant, garden bridge; build it over a small pond, uneven terrain, or even a low patch. The bridge shown is 8 feet, 4 1/2 inches long, but it can span a maximum length of 11 feet, 10 inches with the 2 x 8 boards laid out at 16 inches on center. If you would like to build a longer bridge that is supported only at its ends, we suggest that you first check out the American Wood Council Web site at www.awc.org/technical/spantables to verify the size of the joists needed to support your desired span.

General Instructions

Cut all parts as you assemble the project, using the Cut List as a guide and adjusting as needed for fit. Note that it is best to purchase treated wood a few weeks before you plan to use it. Stack your treated boards, with spacers, in a dry place and turn on a fan. Allow the boards to air dry. Also note that even when it is no longer damp, pressure-treated wood tends to be quite heavy. So it's better to assemble this bridge on-site rather than to build it in the shop and then transport it to its final location.

Step-by-Step

Make the joist/box sill assembly.
- Use a framing square to verify and cut one end of each joist so that it is square. If any of the boards is out of square, trim only the amount necessary to form a right angle. (Boards usually measure slightly longer than labeled to allow for squaring. For example, an 8-foot-long board usually measures 96 inches plus.) Cut and square the other joist ends so that all measure 96 inches long.
- Use 16d galvanized finishing nails to attach the two box sills to the two outside joists (see Figure 1).
- Measure 13 inches from the inside faces of both outside joists for placement of the other two. Attach with 16d galvanized nails.
- Check the assembly for square by measuring diagonally at the corners, and adjust as needed until the two diagonal measurements are equal.
Prepare the location for installation of the bridge.
- Lay out, stake, and square the location of the four posts. The centers of the posts are 94 1/4 inches apart in length and 44 3/4 inches apart in width.
- At each marked corner, dig a hole that measures approximately 12 inches square and 22 inches deep. Add sand to the bottoms of the holes until they are 18 inches deep.
Secure the posts.
- Place the posts in the holes, and use a level to check for plumb both vertically and horizontally. Uses stakes and scrap lumber to temporarily secure each post, and add enough dry concrete mix to the holes to stabilize the posts. Tamp the concrete mix in place.
- Mix enough wet concrete per the manufacturer's instructions to fill the balance of the holes. Pour concrete into the holes until it is just above ground level, and then trowel the top of the concrete to slope it away from the posts in all four directions. Let the concrete cure overnight.
Install posts and cleats.
- Using 16d galvanized nails and lag screws, attach the posts to the joist/box sill assembly (see Figure 2). Drive in the top or bottom nail first, and then use a level to check each post for plumb (vertically level) before driving in the remaining nails.
- Use a framing square to scribe a line on the interior faces of each post at the point where the post meets the top of the joist; these lines will allow you to position wood to support the decking.
- Align the tops of the cleats with the scribed lines. Attach with construction adhesive and 2 1/2-inch outdoor wood screws.
Add the decking.
- Lay an end piece of decking against a pair of end posts with the decking overhanging the box sill and the outside joists by 3/4 inch on each side.
- Scribe the locations of the posts on the piece of decking (see Figure 3).
- Use a jigsaw to cut a 3 3/8-inch-deep notch for each end post (see Figure 3, Detail A). Repeat this process for the other end piece of decking.
- Use a drill/driver with the appropriate bit to attach the end decking to the joist/box sill assembly with 2 1/2-inch outdoor wood screws.
- Use 16d nails as spacers between the end piece and second piece of decking at both ends of the bridge. Attach each second piece of decking with outdoor wood screws.
- Install the two center pieces of decking next by repeating Steps 5a-3e. Cut the notches for the center posts to fit, about 111/16 inches deep on each center piece, as shown in Figure 3, Detail B.
- Lay the remaining 12 pieces of decking in place. Space them with even gaps, and then attach using outdoor wood screws. Remove the nails serving as spacers between the first and second pieces of decking.
Install the rails and handrails.
- Identify a centerline on each post to guide you when positioning the fence rail brackets. Also measure and mark a line on each post that's 8 inches up from the top of the decking; repeat at 19 1/2 inches and 31 5/16 inches above the decking (see Figure 4).
- Attach the fence rail brackets to the posts using Simpson Strong-Drive wood screws so that the bottom of each bracket aligns with your marks on each post and the bracket is centered on the centerline.
- Mark or scribe each rail, and cut to fit (approximately 42 1/2 inches from post to post). Use three Simpson Strong-Drive screws at each bracket to install the rails.
- Mark or scribe the handrail pieces, and cut to fit. Attach the handrails to the top railing with 16d galvanized nails.
Apply the finishing touches.
- Attach a copper postcap to the top of each post using a small amount of construction adhesive and 4d galvanized finishing nails.
- Apply the exterior stain of your choice.

Illustration

Cut List

Part	Material	Size (in inches)	Quantity
joists	2 x 8, TYSP	1 1/2 x 7 1/4 x 96	4
floor beams	2 x 8, TYSP	1 1/2 x 7 1/4 x 46 1/2	2
posts	4 x 4, TYSP	3 1/2 x 3 1/2 x 64 1/2	6
cleats	2 x 4, TYSP	1 1/2 x 3 1/2 x 3 1/2	14
decking	5/4 x 6, ChoiceDek	5/4 x 5 1/2 x 48	18
rails	2 x 4, TYSP	1 1/2 x 3 1/2 x 42 1/2	12
handrails	13/16 x 3 1/2, ChoiceDek	13/16 x 3 1/2 x 42 3/4	4
Finished Dimensions Height: 46 1/2 inches Depth: 48 inches Width: 100 1/2 inches

Total Votes: 12

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