The keys to ensuring that your wooden gate won't sag are to build and reinforce the gate frame so it doesn't warp, to hang the gate plumb and to distribute the weight of the gate over as much area as possible. Building a strong wooden gate requires no special tools or advanced carpentry skills. The old-fashioned technique of using diagonal wooden braces in each corner of the frame takes a little longer than installing a single diagonal brace across the middle of the frame, but produces a durable gate that will resist warping for many years.
Draw your gate plan on paper. Include height and width dimensions of the gate. The gate will include a rectangular frame with mitered corners, wooden diagonal corner braces and infill material. Your chosen infill may be wooden planks, pickets, wire mesh, bamboo or nearly any durable material that you can attach to the frame. Refer to your sketch as you construct the gate.
Set the chop saw to make 45-degree cuts. Trim one end of a 2-by-4-inch plank by placing the plank on the table of the saw with the 4-inch surfaces of the plank horizontal. Hold the plank firmly against the guide fence when cutting. Measure and mark the plank on its longest edge to the length of your gate rails (horizontal framing members). Cut the rail to length. Make sure that the cut ends of the plank are divergent, not parallel. Measure, mark and cut a second rail to the same length as the first.
Refer to the dimensions on your sketch to measure, mark and cut the stiles (vertical framing members) from 2-by-4-inch planks. Trim one end of a 1-by-4-inch plank with a 45-degree cut. Divide the length of the shortest framing member by two to calculate the length of the wooden diagonal corner braces. Measure and mark this dimension on the longest side of the 1-by-4-inch trimmed plank. The plank will be oriented with its 4-inch surfaces horizontal on the saw table. Cut the first corner brace with the cut ends divergent, like the framing members. Measure, mark and cut three more corner braces to the same length as the first.
Arrange the rails and stiles on a flat surface so they form a rectangle with mitered corners, like a picture frame. Place an L-shaped steel corner bracket on the outside of the first corner of the frame. Center the bracket to the edge of the frame. Use a drill with a driving bit and 1 1/2-inch wood screws to attach the bracket to the mitered corner. Repeat to join each corner of the frame.
Place the wooden diagonal braces across the corners of the gate frame, aligning the cut-ends of the braces with the outside edges of the rails and stiles. Attach each brace to one framing member, with two evenly-spaced screws, 2 inches apart. Leave the opposite ends of the braces unattached. Lay a carpenter's square on one corner of the frame. Make adjustments to the positions of the framing members, if necessary, to square the corner. Screw the unattached end of the wooden brace to the corresponding framing member. Repeat to square and brace the remaining corners of the gate frame.
Turn the frame over so it rests on the wooden braces. Test fit the infill material by arranging it on the frame. Mark and trim the infill material if necessary. Use screws to attach wooden or bamboo infill material. The first and last pickets, planks or canes should be attached to both the rails and stiles of the frame. Interior infill material should be attached to opposite framing members with two evenly-spaced screws on each end. Your finished gate may be installed on the gatepost, using appropriate mounting hardware.