Everyone would like an ornate arched bridge like they see in all the gardening magazines, but most do-it-yourselfers find that much more difficult to build than a flat one. It is difficult to make all the pieces fit properly for a solid bridge. If the bridge is at all wobbly or springy, people will feel uncomfortable standing on it to view your gardens.
Building a flat bridge is probably your best bet at this point in your DIY skill development. With ornate handrails, the proper selection of woods, and the right orientation of the decking, a flat bridge can be very attractive and look like a professional installation.
Before starting to build your bridge, consider all the ways you plan to use it. If the gully is deep and you are going to roll your lawn mower or wheelbarrow over the bridge, make sure it will be wide enough. Making it wider usually requires just a little more material and perhaps an extra center support joist.
If you are going to use it just for a walking bridge and not roll anything over it, you can have a small step up to it. In this case, it is basically a long, narrow deck. To roll things over it, make each end flush with the ground level; this will also give it a more professional look.
First, stretch some strings from the beginning to ending points for your bridge. Although it is not critical, it will look much better if the bridge is level from end to end. It is critical that you make it level from side to side, or it will look really odd and feel strange to walk on. Use a string-type hanging level on the taut strings to check for the proper end positions.
The best foundation method to use is a concrete grade beam. Other than dealing with a sore back from digging a trench (grade beam) and a few post holes (supporting piers), it is not particularly difficult. Make sure the pier holes extend down below the frost line in your area. Taper them out at the bottom to eliminate any chance of frost heave.
Before the concrete is set, insert 1/2-inch anchor bolts and make sure that they extend about 2 inches out of the beam. They should be located 32 inches on center and start about 8 inches from one edge of the beam. The lumber sill plates on each end, which the actual bridge is built on, are bolted to these anchor bolts.
If your bridge is less than 8 feet long, 2 x 8-inch joists on 16-inch centers (or less to space them evenly across the bridge width) should be adequate. For a bridge up to 12 feet long, you should use 2 x 10-inch joists, and if it’s up to 18 feet long, use 2 x 12-inch joists. Any longer than this, and you will need an intermediate support beam.
Now, all you have to do is install the decking and handrails. Since you prefer an ornate one, you might consider cedar or heart redwood. You can also trim the side of the bridge with matching lumber to hide the pressure-treated joists. Using turned cedar or redwood spindles for the railing, which can give the bridge a finished look.
