How To Build An Arched Bridge

Jenny Pike's San Francisco home boasts a large backyard, but Pike feels the lawn and deck need a little something extra to enliven the space. She likes the idea of adding a pond with a small footbridge and has hired landscape architect Louise Leff to implement the project.

Leff explains that this project isn't particularly difficult: homeowners with basic carpentry skills and the perseverance to do a lot of digging can complete the work in about two weekends. On a difficulty scale of 1 to 5, with 5 being the hardest, Leff rates this project a 3. Leff estimates that hiring the project out would cost around $3,500 (for bridge, pond, plants and design) but says a homeowner could purchase the materials for $1,000 or less.

Step One: Building the Bridge

To create a small footbridge that will lead from the deck to the garden and extend over the small pond, carpenter Larry McClary first cuts two large arcs from 2" x 12" beams. To create the guideline for the first arc, he uses a bender board held in place with nails, then traces its outline onto the wood. (In this case McClary is using wood that the homeowner happened to have on hand.)

After cutting along the curved lines with a circular saw to create the first supporting arc, McClary uses the arc as a template for the second one. Using a smaller circular saw — with a 6 1/2-inch blade — makes it easier to cut along curved lines.

Next McClary turns the two arcs on their sides and connects them on one end with an 18-inch-long 2" x 6" secured with nails. The opposite end of the bridge will be attached to the side of the deck. He adds a second 2" x 6" of the same length between the two arcs in the center for additional support.

With the frame complete, all that remains to complete the footbridge is the top decking. Here the decking is cut from lengths of redwood left over from the homeowner's deck. McClary cuts the wood to 24-inch lengths and lays them across the arched frame so there's an equal amount of overhang on both sides; he uses a nail as a spacer between boards.

He uses coated deck screws to secure each board of the decking to the arched frame below, spacing the screws evenly along each side. He leaves a small space at the open end of the bridge to allow more maneuverability when he attaches the bridge to the deck.

Step Two: Digging the Pond and Adding Plants

To get started on the pond, landscape contractor Jim Wilder outlines the area with landscaper's marking paint. He and the crew dig out the area, going three to four inches deeper than the desired depth of the pond — a total of about 15 inches.

Crew members remove all protruding roots and sharp objects that could puncture the liner and dig a two-inch-deep ledge around the pond's perimeter. Wilder explains that the ledge will provide a place to position the rocks that border the pond. After the ledge is dug, Wilder runs a string line from one side to the other and checks to make sure the ledge is level.

Then he cuts numerous strips of landscape fabric, which he lays along the edge of the pond so they overlap along the sides. The tops of the pieces are laid along the pond's border; the bottom edges of the extend into the bottom of the pond. The fabric too will help protect the liner from roots and rocks.

Wilder puts a layer of sand in the bottom of the hole and along the perimeter ledge (to hold the landscape fabric in place), then lays the large liner in place. The 10' x 12' square liner used for this project is fitted to the shape of the pond with pleats and folds so it will lie neatly along all the corners and ridges.

Wilder explains that although it's fine to cut away some excess to make the liner easier to manage, you must leave enough to cover the edges. He also recommends allowing the liner to warm in the sun before it's laid in place: this will ensure adequate flexibility and is particularly necessary for those working in cold temperatures.

While Wilder completes his work on the pond, Leff designs a planting plan that features varieties that grow naturally near water. Featured plants include

• Umbrella plant (Cyperus alternifolius), Zone 11
• Juncus effusus, Zones 6-9
• Carex morrowii, Zones 7-9
• Foxglove (Digitalis x mertonensis), Zones 3-8

Step Three: Placing the Bridge

Before setting the footbridge is set in place, the crew must position the stones around the pond's perimeter. Wilder has selected California water-wash stone, which has rounded edges and enhances the pond's natural look. For other regions, he advises selecting stone that is native to the area and has a smooth, rounded shape.

As he lays each stone around the ledge, Wilder works carefully to achieve a good fit between stones — a process he compares to working a puzzle. He allows the stones to hang slightly over the inside edge of the pond so they'll completely conceal the liner and keep the outline of the pond from looking contrived.

After all the stones have been laid, Wilder and the crew set the bridge in place to make sure it will fit in the designated area. They lay a piece of flagstone on the lawn where the end of the bridge will be set to prevent the wood from having direct contact with the soil, which would eventually rot the wood. Wilder adjusts the soil under the flagstone until the stone lies flat, then checks it with a level.

With the stones in position, Wilder begins mortaring each one in place. To ensure an especially strong hold, he uses mason's mix. He removes any excess from the sides of the stones with a trowel, then cleans the surface of each one with a damp sponge before the mortar has a chance to set.

Step Four: Bolting Down the Bridge

To secure the bridge to the deck, McClary nails a 2" x 6" to the side of the deck at the point where the end of the bridge will be attached. He uses a 3/8-inch drill bit to drill holes through the 2" x 6" and into the deck and threads three-inch carriage bolts through the holes for extra support.

McClary sets the bridge in place, checks its alignment and nails the ends of the bridge's arched frames to the 2" x 6", driving the nails in from the sides. Once the ends of the bridge have been anchored to the deck, McClary is ready to attach the last pieces of decking to the top of the frame with coated deck screws, filling the gap at the end of the bridge and completing the structure.

Finally, the crew trims the edges of the liner around the pond's border, making sure it's completely concealed under the stones. Leff adds half a dozen mosquito fish to the pond to prevent a mosquito problem. (Mosquito fish are usually available free from county water departments, and most ponds require only a few fish to keep mosquitoes from breeding in the pond.)

Leff also notes that a bubbler attached to a pump provides effective mosquito control because mosquitoes won't breed in moving water.