The cover photo for the July/August 2003 issue of HANDY features a large garden bridge we built to span a landscape stream. The accompanying story (“Bridge Club,” p. 32) offers some useful information on the general topic of designing and building garden bridges. For a more detailed glimpse into the process, we are providing this step-by-step description of how we made the cover story bridge, along with some technical information that could not be included in the magazine because of space constraints.
Materials: We built the entire bridge with ACQ-treated Southern yellow pine. The 10 segments that make up each of the three support beams are face-joined with polyurethane glue and 3/8 x 3-1/2 in. galvanized carriage bolts. The posts are fastened to the outer support
beams with 5/16 x 5-1/2 in. carriage bolts. The decking is attached with 2-1/2 in. deck screws. The finish is Redwood Naturaltone semi-trans¬parent deck, fence and siding stain from Behr.
Dimensions and sizes: Overall length is 12 ft.; width is 3 ft.; total rise is 20 in. The posts are 42 in. tall, with 1-1/2 x 8-in. half-laps cut on the lower ends (except on the end posts, which are shorter because they have smaller half-laps). To make the posts appear lighter, we ripped the 4x4 post stock down to 3 x 3 in. The rails and handrails are ripped to 3 in. wide from 2x4 stock (the handrails are eased with a 1/2-in. roundover on all edges). The balusters are 2x2; the spread¬ers (8) are from 2x4 scrap. The deck boards are cut to 32 in. long from 5/4 decking, and the posts are topped with premilled, chamfered caps.
STEP-BY-STEP INSTRUCTIONS
Step 1: Cut pieces of 2x12 to rough length then lay out the seg¬ments for the support beam components using a carpenter’s square and straightedge.
Step 2: Cut out the segments with a circular saw and jig saw then dry-assemble each support with deck screws only, making sure the seams between seg¬ments do not align on the inner and outer plies.
Step 3: Draw cutting lines at the top of the inside layer of each outer support assembly. Set your saw to cut 1-1/2 in. deep. Trim along the cutting lines, creat¬ing a ledge for the deck boards (inset photo).
Step 4: Set the support assemblies upright on a flat, level surface then screw the spreaders between the supports to assemble the undercarriage.
Step 5: Cut half-lap joints at the bottoms of the posts that will rest on the flat section of the bridge. Gang these posts together to make the shoulder cuts.
Step 6: After tacking the flat-bottom posts to the supports, clamp the angled-bottom posts in position and mark a cutting line for the half-lap shoulder cut onto each post.
Step 7: With all the posts trimmed, plumb and tacked to the supports, drill guide holes for carriage bolts through the posts and supports. Wait until you are working on-site to install the carriage bolts.
Step 8: Make a 1/2-in. roundover on all the edges of the 2x4 stock you’ll use for the railings. Cut the railing sections to rough length (a few inches too long).
Step 9: Hold the railing sections in position against the inside faces of the adjoining posts, then scribe trim lines at the ends of each rail section, following the edges of the posts.
Step 10: Predrill pilot holes for the screws that will attach the deck-boards to the support ledges. Make a drilling template from hardboard to ensure that the screwheads align.
Step 11: Prepare the bridge site, making sure the landing areas are flat and well drained (compactable gravel is a big help here). Assemble and finish the bridge, check¬ing the components for square as you work.