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Outdoors > More outdoor > How to Build a Small Footbridge

Learn how to laminate beams and assemble decking and rails for a woodland footbridge.

Homeowners want a footbridge to help them cross a deep ravine on their property. When the area is dry, crossing the gulley at the bottom requires a hop, but rains and runoff can add slippery banks and rushing water as additional obstacles. Consequently, the bridge will be high enough to avoid the steep banks and any flowing water. Built of laminated four-by-six support beams, pre-cut posts, decking and side rails, the 12-foot length of bridge will span the gulley resting on heavy block piers.

Plan the dimensions of your project, purchase treated lumber or redwood in the required dimensions and then assemble tools and other materials. Our design uses four 12-foot four-by-six planks, two-by-six treads that are 36-inches long, four rails, and pre-cut posts. We apply wood preservative and coat any untreated ends that we expose by cutting. As you assemble tools, be sure to include safety glasses and a dust mask to protect yourself from flying debris launched by power tools and saw dust produced by cutting chemically-treated lumber.

Begin leveling four small patches on the ravine's slope, locating a pair on opposite sides of the gulley where the purchased block piers will rest as foundations for the bridge support beams. These patches delineate the corners of the long narrow bridge where it will span the gulley and must be aligned at true right angles to each other. Use shovels and hand tools to clear debris and dig out soil where necessary to make the areas the same elevation. Throughout the various stages of this project, you should confirm that each subsequent component is both level and "square."

Place the first pier on a leveled patch and use a torpedo/bullet level to ensure that the block itself is level. Be sure to position it squarely so that its fastening strap can attach to a support beam and its face will be aligned with the faces of the remaining three piers.

Work on the same side of the gulley, digging out the footing area and then placing and leveling the second pier. The distance between the first and second piers represents the width of the bridge. Measure to ensure this distance is correct based on your original plans. Keeping in mind the footprint of your bridge plan, position the second pier at right angles to the first pier and to what will be the locations of the third and fourth piers. In the coming steps, you will be making many tiny adjustments to position the bridge support properly.

Use a bar level to ensure the two piers also align at the same elevation. Once you have them properly positioned relative to each other and in accordance with your original plan, do not move them. Their current position is permanent and should not require any adjustment.

Set a 12-foot two-by-six plank on edge, spanning the gulley and resting on piers one and three. Place another across the gulley and resting on piers two and four. Placement of the planks is temporary so that their length indicates the positions for the third and fourth piers. Locate piers three and four in approximate position on the other side of the gulley, but be prepared to adjust them several times in the next few steps.

Clamp the ends of the planks to the metal fastening strap on their respective pairs of piers. One plank will span piers one and three, while a second spans piers two and four.

Ensure that the two planks are parallel. Measure the width of the gap between the two planks on the first side of the gulley and then on other side of the gulley. The measurements should be the same, so adjust piers three and four slightly, if necessary. Remove the clamps and planks from their temporary location without disturbing the position of any of the four piers.

Apply construction adhesive to the face of one two-by-six plank. This is the first step in laminating two of the two-by-six planks into a single support beam.

Set a second plank on top with the edges flush and clamp them together while the adhesive dries.

Pre-drill pairs of holes staggered slightly on either edge of the 6-inch face of the sandwiched planks. Space the holes every 24 inches (approximately) along the length, but avoid drilling at either end of the planks where you must attach them to the fastening strap on the piers.

Inject exterior grade silicon into each hole to coat them and prevent water from leaking in and rotting the planks from the inside out.

Hammer galvanized carriage bolts through the holes before the silicon sets and secure them with nuts to form a permanent four-by-six beam. The combined use of adhesive and carriage bolts will result in a strong, stable, and warp-resistant beam.

Add galvanized washers and nuts to the opposite ends of all the carriage bolts.

Tighten them with a socket wrench/ratchet until the washers just begin to compress the wood. This completes the construction of the first four-by-six beam. Follow the same process to construct one more beam for a total of two.

Position the beams against the fastening straps on the piers and clamp them in place.

Pre-drill holes through the beams that align with the holes on the metal fastening straps on the piers. Bore all the way through the beams.

Add silicon to the holes and drive galvanized carriage bolts through the straps and beams.

Add washers and nuts, tightening with the socket wrench until the washers barely compress the wood.

Position and temporarily clamp the pre-cut posts for the railing supports along the length of the beams. Position them at equal intervals along the beam so that their bases and the bolts that will attach them do not conflict with the bolts sandwiching the planks into beams. Additionally, each pair of posts should align across the gap between the beams. The posts were pre-cut with notches on the bottom and tabs on the top to fit into grooves on the underside of the rails.

Use a combination level to ensure the posts are vertical.

Pre-drill two offset holes in the base of the posts.

Add silicon to the holes, but do not let it set.

Drive in carriage bolts through the notched portion and into the beam. Secure them with washers and nuts on the opposite side.

Measure the width of the bridge between the outside edges of the support beam. We measure 34 inches as the basic width of the bridge floor, but allow two addition inches--an additional inch overhang on each side.

Cut the decking/treads in 36-inch lengths with a circular saw.

Position the first tread so that it overhangs the beams equally on both sides and is flush with the ends of the beams.

Secure the tread with rust-resistant deck screws, driving them through the tread and into one of the component planks in the beam beneath. Avoid positioning the screw in the crevice where the planks are joined.

Position the second tread on top of the first temporarily to determine where to notch it to fit around the vertical posts. Butt it against the forward edge of the first post on each side of the bridge.

Line up a speed square against the back edge of the tread and butt it against the inside of the post. Back off the post a fraction of an inch and trace a pencil line on the tread along the straight edge of the speed square. This is the first of two lines you will draw.

Add a 1/2-inch spacer between the first and second plank and reposition the second plank temporarily with its end butted against the inside face of the post. You will use 1/2-inch spacers between all the treads too allow space for moisture and debris to fall through.

Lay the straight edge of the speed square against the forward face of the post, back off a fraction of an inch, and use it to trace a line from the end of the tread to intersect the first line. These two lines mark a rectangular to be notched at the back corner of the plank. You will need to repeat this process to mark a similar notch on the other end of this second plank. Depending on the placement of the second tread, you may also need to mark notches on the third tread where it will butt against the back face of the post.

Cut out the notches with a jigsaw. You will need to perform a similar notching process along the floor of the bridge anywhere the treads meet a post.

Space the tread 1/2 inch from the previous tread and secure it with rust resistant deck screws. Add treads in this manner along the length of the bridge. You may have less than a full tread with once you reach the other end of the bridge. Narrow the last tread or several treads to distribute the distance in an attractive manner without making a tread so narrow that it is structurally weak.

Coat the cut ends of the treads with wood preservative and a paintbrush.

Walk across the bridge to test its stability and strength.

Position a handrail along lines marked earlier on the outside face of each post. We have decided to have two rails on each side of the bridge, fastened to the inside and outside of the posts.

Clamp the handrail in position.

Secure the handrail to each post with rust-resistant screws driven through the rail and into the posts. Add the other handrails in the same way.