The Complete Guide to Building a Wooden Bridge on Your Landscape
Adding a wooden bridge to your landscape is both attractive and practical. A bridge can span a stream, help you across a backyard pond, or provide a charming transition from one garden area to another. You may enjoy building it almost as much as walking over it when you’re finished.
From the simplicity of planks laid on timbers to the strength and energy of stone and reinforced concrete arches, to the graceful curves and lacy intricacy of suspension cables, bridges are fascinating and beautiful structures. In our world of high-speed, long-distance transportation where bridges carry multiple lanes of traffic, it is easy to forget how pleasing a small-scale footbridge is, both to look at and to use.
This project will bring you into contact with an essentially simple but clever design, one that is the result of refinements and improvements produced by craftsmen over many generations-the kind of evolution that results from practical experience.
Preparing to Build a Bridge
For a homeowner who enjoys construction projects, the idea of building a bridge is especially intriguing. There are some unusual, but not difficult, construction techniques to be learned. And there should be satisfaction in knowing that an arched bridge will be a unique landscaping feature.
How Long Will it Take?
You can build this 9-foot bridge in about four days if you want to devote yourself entirely to it. The work is not hard, but it will be a lot more enjoyable if you take your time to savor the completion of each stage and the satisfaction of mastering techniques such as laminating beams.
How Much Will it Cost?
The bridge is not a particularly expensive project. The wood and hardware specified could cost about $250 to $300, but that can differ greatly in various areas, or if you choose to use a different kind of wood. You don’t need any special tools, and the plans and instructions provided here will show you just what to do.
So go to work, and in a short time you’ll have the pleasure of using your bridge as well as a very justifiable pride in your accomplishment.
1 × 6 × 10′ clear redwood; beams
1 × 6 × 10′ clear redwood; handrails
4 × 4 × 10′ redwood; posts
2 × 6 × 4′ construction-grade redwood; decking
16′ ′ Flexible steel wire
4′ × 8′ × 3/4″ CDX plywood; beam form
2 × 4 × 8′ pine; beam form
2 × 4 × 10′ pine; beam form, blocks
1-1/4″ galvanized deck screws
3″ galvanized deck screws
2″ galvanized deck screws
1/4″ × 5″ lag screws
1/4″ × 2″ stove bolts and washers
12″ landscape spikes
Exterior-grade wood glue
10″ dia. × 24″ cardboard tubes; footing forms
5 60-lb. bags concrete mix; footings
Jigsaw (saber saw)
Drill with 1/4″, 7/16″, 7/8″, and screwdriver bits
Concrete mixing pan/board
Warning: This bridge is designed for walking use only. Do not use it for heavy equipment or more than four adults at a time. It is not a children’s play piece. The railings do not meet code requirements for attached decks.
Before starting to build your wooden bridge, you should understand its basic construction and the stages in which the work will be done. This will help you avoid making mistakes that would waste materials and will save you effort. It also will let you plan how much to do each day, whether you intend to build the bridge in a concentrated period or do it in two or three weekends.
The key elements in the design of this bridge are the arched support beams. The grace of their curves is accentuated by matching curved handrails. The plans call for using redwood for the beams, rails, posts, and decking because it has such a handsome appearance in its natural state. But you could use cedar instead, which is also a handsome-looking wood and weathers equally as well as redwood.
Choose materials according to your preference, as well as by cost and availability in your area. The beams are not only graceful, they provide strong support without a complex structure of cross-beams and braces, and they make it easy to assemble the bridge at its site. Their strength is a result of the way in which you build them: they are laminated from separate boards. This is much cheaper and easier than buying huge solid beams and trying to cut them to shape – an almost impossible task with homeowner’s tools. And a laminated beam is not likely to check or develop grain-line splits the way a large solid piece of wood often does.
The 1 × 6 (actually 5-1/2 inch) boards that you buy for the beams must be ripped to 5 inches wide. You could do that yourself, but it would be easier to have them cut at the lumberyard. To make a beam, you bend the boards on a bending form that you construct from 3/4-inch plywood and 2 × 4s. You build up the laminated piece by gluing and screwing each board layer to those below.
The glue must dry completely before the first beam can be removed from the form and the second one made. The beams are installed on concrete footings that are 12 inches in diameter and 24 inches deep. Landscape spikes in the centers of the footings anchor the beam ends so the assembled bridge cannot shift position. The walkway decking on the bridge is made of 2 × 6 boards screwed into the tops of the beams.
The handrail posts are notched to be fastened to the sides of the beams. Because they are all cut to the same length, their tops follow the curve of the beams. So when the handrails are screwed to the posts, they assume the proper curve; no prebending or shaping is required.
The actual work in building the wooden bridge falls into the following four stages:
1. Building the form. To make the curved beams you need to first build a bending form. The curve must be accurately marked and cut on the plywood sides of the form in order to shape the beams correctly.
2. Constructing the beams. Each beam is built up by gluing and screwing seven boards together in the form, one at a time, for an overall thickness of 5-1/4 inches. The glue should dry for at least 48 hours before a beam is removed from the form. During that time you could do some of the work for the next stage.
3. Installing the beams. At the bridge site you must dig holes and pour footings that are properly spaced and have their tops level with one another, so that the beams will be correctly aligned and firmly supported. Then you place the beams on top of spikes embedded in the footings.
4. Completing the bridge. In the last stage of construction you attach the posts to the beams, install the decking, and fasten the rails to the posts. Finally, you coat the structure with deck sealer to protect the wood.
Building the Form
The bending form that holds the boards in a curved position as you build up each laminated beam is built of two plywood sides separated by 2 × 4s. To mark the required curve, fasten a pencil to one end of a long piece of flexible steel wire and securely fasten the other end exactly 177 inches away. This will let you mark a curve with a radius of 14 feet 9 inches on a board for the first form side.
The curve must be centered on a 4 × 8-foot sheet of 3/4-inch CDX plywood. An easy way to do this is to fasten the end of the wire on the pencil exactly 4 feet out from a side wall and butt one end of the plywood against the same wall.
Another way is to draw a line dividing the plywood into two 4-foot halves and make a mark on that line 23-7/8 inches from one edge. That allows for the kerf wastage when you cut on the outside of the curved line you will draw.
Position the plywood so that the pencil is exactly at the center when the wire is pulled taut and swing the pencil right and left to draw a curve from one end of the plywood to the other.
Cut the curve with a jigsaw to make one side of the form, then trace its shape on the remaining piece of plywood and cut out the second side. Join the two sides of the form by screwing them to two 8-foot lengths of 2 × 4 (below right).
Use 2-inch galvanized screws spaced every 8 inches. Fasten one 2 × 4 along the bottom of the form, and the other exactly 2-1/4 inches below the corners of the curve at each end (see Bending Form plan, page 171). This spacing allows room for support blocks on the bottom of the beam.
Constructing the Beams
There are six steps in constructing each beam: mark the boards; attach support blocks; fasten the first board; laminate the boards; sand the sides; and cut the ends. Each step is described below:
Mark the Boards Stack seven 10-foot 3/4- × 5-inch redwood boards and align their ends and edges. Mark the middle of the stack down the edges on one side (Photo 1) and then mark the edges at 8, 16, 24, 32, 40, 47, and 58 inches from the middle toward each end of the stack. These will help guide screw placement later. The 47-inch marks show where to fasten support blocks.
Attach Support Blocks. To attach support blocks, you must install T-nuts (detail photo, opposite page) in the board that will be the bottom layer of the beam. At each end of the board, 47 inches from the center, drill two 1/4-inch diameter holes for the shanks of the T-nuts. Locate the center of each hole 1-1/4 inches from the edge of the board (see Detail 1 in the plans, page 171). With a 7/8-inch spade bit, mortise the holes 1/8-inch deep from the top of the board so you can countersink the bases of the T-nuts below the surface (Photo 2). They will remain in the finished beam.
Now drill matching 1/4-inch holes in two 10-inch 2 × 4 blocks. Locate the holes so the blocks will be centered in the width of the beam. Attach the blocks by driving 1/4 × 2-inch stove bolts with washers up into the T-nuts in the beam board.
Fasten the First Board. At each end of the bending form, clamp the bottom 2 × 4 to a sawhorse or other support that will put it at a convenient height. Then use the mark at the center of the beam board to center it on the form. Because it is wider, the board will rest on the curved edges of the form. Push one end down so the support block on the bottom of the board is between the form sides and fasten it with three 2-inch galvanized screws on each side (Photo 3). Push the support block at the other end of the board into the form and fasten it in the same way. You may want a helper to hold this end down as you drive the first screws.
Laminate the Boards. Spread waterproof (exterior grade) wood glue on the surface of the board in the form and on the underside of the board that will be the next layer in the beam. Screw this board to the board in the form, starting at one end and working to the other (Photo 4). Keep the edges aligned, and have a helper hold up the free end of the board. Use 1-1/4 inch galvanized screws in this layer. Predrill the holes and countersink them so the screws will get a good bite and their heads will be below the surface. Space them according to the reference marks on the edge and countersink them 1/8 inch. Use two screws at each interval, set in about 1-1/4 inches from the edges. Continue gluing and screwing each layer, but use 2-inch screws in layers three through seven for better holding power. The bridge decking will conceal the screwheads in the top layer. Let the glue dry 48 hours before removing the beam from the form.
Sand the Sides. After removing the beam from the form, use a belt sander to remove the glue squeeze-out and edge irregularities on each side (Photo 5). You could wait to sand the first beam until you have laminated the second beam and it is drying in the form.
Cut the Ends. After both beams have been sanded, unscrew the stove bolts and remove the support blocks from the ends. Then mark the beams for cutting. Use a framing square resting on a 3/4-inch thick board spacer (Photo 6) to mark the side of the beam. Set the square so that the T-nut location in the beam is at the 9-3/4 inch mark on the inside edge of the framing square blade. Mark the inside horizontal and vertical lines. When you cut on these lines you won’t hit any screws or T-nuts in the beam. Make the cuts to square off the ends and the undersides of the beam using a circular saw, followed by a handsaw to cut through the part the circular saw blade doesn’t reach. Finally, give each beam a coating of deck sealer. You won’t be able to get at them once the bridge is completed.
Installing the Beams
- At the site where you will construct the bridge, clear away any interfering growth and drive stakes to mark the centers of the locations for concrete footings.
- The distance between stakes at opposite ends of the bridge span should be 102 inches (8 feet 6 inches).The spacing between the stakes at each end should be 39-1/2 inches. With a post-hole digger, dig 10-inch holes 24 inches deep.
- In firm soil you won’t need forms, but in sandy or crumbly soil insert a cardboard form tube-available at home centers-in each hole to keep the sides from collapsing and to shape the footing. Form tubes are also helpful in making height adjustments, because the tops of the forms must be level with each other.
- Fill the holes or forms to the top with concrete. You’ll need about 1-1/4 60-pound bags of concrete mix for each hole.
- While the concrete is still wet, insert a 12-inch landscaping spike in the center of each footing, extending 3-1/4 inches above the surface.
- Let the concrete set for at least a full day. Then measure the exact span distance between opposite spikes and drill corresponding 7/16-inch diameter holes in the flat undersides of the beams.
- Center the holes in the width of the beams.
- Install the beams by placing the holes over the spikes and pushing them down until they rest firmly on the footings.
Completing the Bridge
With the beams installed on their footings, mark the locations for the handrail along each side (below). Follow the spacing shown in the plans (Elevation View, page 170). The posts are 22 inches long.
Cut them to length, and then notch the bottom ends to fit over the beams.
Use a circular saw and handsaw to cut the notches. Predrill the posts and beams for 1/4-inch lag screws.
Offset the holes from one another, and countersink them 3/8 inch in the posts for the screw heads and washers.
Drive the lag screws with a socket wrench.
Attach a temporary spacer at each end of the beams to keep their outside edges 44-1/2 inches apart. Then cut and install the 2 × 6 decking (below center). Cut the decking to fit snugly around the posts, and fasten each end with 3-inch galvanized screws.
Predrill and countersik holes for the screws. The handrails are two layers of 1 × 6 boards ripped to 5 inches wide.
Screw the first board to the post tops.
Spread glue on the top of this board and the underside of the upper board.
Screw the upper board in place by driving 1-1/4 inch galvanized screws up from underneath.
Clamp the ends of the rails until the glue has dried.
Finally, sand the edges of the handrails to soften the sharp corners and remove any glue squeeze-out. After a few days, apply a coat of deck sealer to all accessible surfaces to keep the boards from cracking.