12 Steps for Building a Bridge

Materials Shopping List

Hardware & Supplies

• Galvanized 10-penny nails
• Galvanized or otherwise treated deck screws at least 3 in long
• Galvanized or otherwise treated deck screws 2¾ in long (for curb attachment)
• Hot-dip galvanized ½-in x 8-in lag bolts
• Metal hangers and brackets
• Concrete and forms (8-in diameter minimum), if pouring the footers

Lumber

• All wood should be ground-contact treated dimensional lumber
• 4 – 8-in diameter posts, 4-ft minimum (do not need if pouring concrete footers)
• 8 – 2 X 10 X 12
• 9 – 2 X 6 X 14 (Cut into 25 4’ deck boards plus the bottom support in Step. 9) 
• 2 – 2 X 4 X 12

Dimensions

• 4-foot-wide by 12-foot-long span
• 1,000-pound live load

Finished farm bridge

When you live out past where the pavement ends, you’re bound to encounter a small seasonal creek, drainage ditch or damp swale that you wish to cross with minimal fuss, not to mention dry feet. You might even want to get your wheelbarrow, lawn tractor or ATV to the other side without tearing up the ground or the equipment.

While you might be able to procure a culvert and haul in sufficient fill to create a path across the gap, in many cases a bridge is more practical and aesthetically pleasing—and it may be the only method permitted in the case of controlled waterways.

Don’t be afraid to modify the following plan to suit your situation, but be keenly aware of load-bearing capacities for the lumber you choose and the length of your span. Our bridge is plenty stout for a 4-foot-wide by 12-foot-long span that will handle a 1,000-pound live load.

Our project assumes that you don’t have to worry about swift water flowing over the banks of the ditch and that you don’t require a permit for your project. If the latter isn’t the case, you will want to consult with your local permitting agency and possibly an engineering firm before proceeding.

1. Site your bridge where the banks are relatively level and at roughly the same elevation. For our example, we are assuming a 7-foot creek bed (gap) with a 12-foot bridge length. The extra length is to get the footings at least 2 feet back from the creek’s banks.
2. Call 811 and consult your plat maps or any other records you may have to ensure that your intended bridge site is free of buried pipelines and cables. Level the areas where you want your bridge’s ends to land.
3. Measure, square and stake a rectangle that’s 4 feet wide and 12 feet long, centered lengthwise over the gap. You will install concrete or wooden footers just inside the four corners of your marked area.
4. Drill four 8-inch-diameter (or larger) holes, completely inside the rectangle’s corners, at least 3 feet deep with your tractor. Then fill with concrete or an 8-inch-diameter post. If using concrete, take care that each pair of footings is at the same level—slightly above grade is ideal, if you can find an appropriate form. If using posts, trim them level once you have backfilled the holes.
5. Sister two ground-contact treated 2″ x 10″ x 12′ dimensional boards by using 10 penny nails staggered every 8 inches or so to nail them together along their entire length. This creates what amounts to a 4″ x 10″ x 12′ dimensional timber. Place the timber on edge across the gap, with the ends supported by the footers. Set the center of the timber on the footer’s centerline.
6. Repeat step 5 and place the resulting timber centered on the other set of footers. Adjust the positioning to ensure the timbers are parallel and square.
7. Measure the distance between the two timbers and cut four pieces of 2″ x 10″ material to that length. Sister the 2″ x 10″ lengths by nailing them together in pairs to make crossbeams, and install them between the timbers (three 8-inch lag bolts per corner of the bridge), while resting centered on the footers. Attach timbers and beams to footers, using brackets and screws appropriate for wood or concrete footers.
8. Measure the distance between the cross beams, and cut two 2″ x 10″ x 12′ treated dimensional boards to length (should be approximately 11′). Attach them to the crossbeams with metal hangers and galvanized screws at two points that are roughly one-third and two-thirds of the distance between the timbers, respectively (much like floor joists).
9. Measure the distance between the outer edges of the timbers and cut a piece of 2″ x 6″ treated lumber to length. Attach it to the bottom of the bridge halfway across the span, screwing it to both outer timbers and the two interior “joists” to keep them from spreading or twisting.
10. Cut sufficient 2” x 6” x 4’ lengths (approximately 25) of treated lumber to deck the existing superstructure crosswise its entire length. Screw the decking down to the outer timbers and joists, leaving about a ¼-inch gap between the boards. The ends of the deck planks will extend beyond the outer timbers by about 2 inches on each side.
11. Install a curb on each side of the bridge by screwing a 2” x 4” x 12’ treated dimensional board flat on top of the decking, flush with the ends of the deck planks. This will keep the deck ends even, reduce warping and reduce the chance of you accidentally running a wheeled vehicle off the edge.
12. Grade the approaches to the bridge to offer a smooth transition from the ground to the end of the decking. Use crushed rock rather than soil if you need to raise the elevation more than a couple of inches.

Building a bridge is a big project that takes some time and commitment, but construction will go faster with the help of a Massey Ferguson® 2600 H Series utility tractor. Equip it with a loader for hauling material and/or a PTO-powered posthole digger and you’ll be done in no time. Add a rear blade to help level where your bridge’s ends land.