Dear James: I am planning to sell my house, so I want to improve the landscaping. I think a small decorative bridge would look nice over a gully near the flower gardens. How should I build it, and what material is best? — Ron M.
Dear Ron: Adding a bridge to your garden — or any location in your backyard — can enhance the landscaping and give it a professional look. You might consider moving soil to create a gully in the flower garden itself so the bridge can be integrated into it. Use the dirt to fill in the existing gully.
Another option is to install a pond under the bridge. With all the pond kits available at landscaping stores and home centers, installing a pond is not a major project. The most difficult part is digging the hole for the pond liner, especially if you have hard clay soil.
You really have two viable material options: pressure-treated lumber and vinyl. Using pressure-treated lumber offers the greatest design flexibility, particularly if you plan to build a decorative bridge. Vinyl materials, used for decks, can be used to build a flat bridge with a straight railing.
Since you indicated you would like a decorative bridge, select lumber as the primary building material. Arched bridges look the best, but building one requires a fairly high level of carpentry skills. Sticking with a flat bridge would be best. Adding a fancy railing will give the bridge a decorative appearance.
Before buying the lumber and getting out the saw, do some planning. Determine how the bridge will be used. If it is strictly decorative, it will only have to support the weight of a person or two, so its width is not critical.
If the bridge will be used to roll a lawn mower or wheelbarrow over it, then you must consider the bridge’s width and its strength. You don’t want to build a flimsy bridge and then have it collapse when the new owners roll something heavy over it.
Measure your garden tools and make the bridge at least 1 foot wider than the widest object you imagine will pass over it. Unless the bridge is extremely short, it would be wise to add vertical center supports on each side. These supports can be recessed from the edges so they are not noticeable.
When building a (strictly) walking bridge, it is acceptable to have a step at both ends. This actually looks more decorative. Obviously, if you are planning to roll items over the bridge, the ends must be flush with the ground. For flush ends, use pavers or natural stone at either end so the lumber does not contact the soil.
With your plan in hand, build the bridge in a manner similar to a long, narrow deck. Keep in mind that the bridge will look much better if it is level along its length and width. Locate level beginning and ending points and stretch level strings between them. You can use a standard carpenter’s level to check across the width.
For bridges less than 8 feet long, 2-by-8 joists on 16-inch centers (or less, to space them evenly) should be adequate. For bridges up to 12 feet long, use 2-by-10s, and for bridges up to 18 feet long, use 2-by-12s. Any longer than that and you should use an intermediate support beam. By James Dulley