All good wood fences need help staying in good shape. When they're damaged, you must first decide whether to repair or replace. Armed with the techniques shown here, you can make a wood fence as good as new (or nearly so) with minimal need to replace sections.
Begin by removing any parts that get in the way of the repair. You can either remove the fencing with a pry bar or cut out the damaged section with a handsaw. When repairing a post, remove the fencing and rails connected to it.
Once the repair is complete, use new nails when replacing fencing and rails.
Tools and Materials
Cleaners
1×4, 2×4 and 4×4 posts
Nails
Deck screws
Lag screws
Carriage bolts
Fiber scrub brush
Wash cloth
Wire brush
Hammer
Screw gun
Ratchet and socket wrenches
Handsaw
Circular saw
Drill
Drill bits
Spade
Shovel
Cold chisel
3 lb. sledgehammer
Wood chisel
Clamps
Safety glasses
Work gloves
Pry bar
Cleaning the fence
Scrub wood fences with a fiber scrub brush and a mild detergent solution to remove most stains. For tougher stains, add 1 to 2 cups of bleach to a bucket of warm water. Wear gloves and safety glasses when working with strong cleansers.
Pressure washing
Rent a pressure washer for hard-to-reach crevices in basket-weave style and other wood fences. Spray about 6" to 10" away from the fence. Don’t use more than 1,000 PSI of water pressure or the force may damage the wood. Many companies make commercial deck cleaners that are also good for cleaning fences. Consult the manufacturer’s instructions before starting. Bracing wood rails
Step 1: Reinforce rail ends with 2×4 cleats fastened to the posts. Screw the cleats in place with #8 deck screws or nail with galvanized 10d (3") nails. Taller fences often have third rails in the center, which can be reinforced using this method as well.
Step 2: Add a sister rail to bolster a damaged rail. The sister rail can span the entire original rail or just part of it, depending on the damage. Clamp the sister rail under or on top of the original, then drill holes through both rails, bolt them in place with 3/8 × 4" carriage bolts, and remove the clamps.
Step 3: Add a third rail between the top and bottom ones if the fencing seems loose. Screw the rail to the posts with #8 deck screws or toenail with galvanized 10d (3") nails. Fasten the fence boards to the new rails with #8 deck screws. Replacing a rail
Step 1: Remove all of the fence boards from the damaged rail. Unscrew the boards, or pry off the boards and pull out the nails. Try not to damage the fence boards so you can reinstall them later. 
Step 2: Remove the rail. Pry out the nails or remove the hardware holding the old rail in place. Cut a new rail to fit between the posts. Nail or screw the rail in place, then reinstall the fence boards. Splicing a rail
Save yourself some work when you need to replace a short section of rail on a wood fence that has an exposed rail by splicing a new piece of rail in place. Remove the boards from the damaged section of rail, then cut off the damaged section at an angle. Put the new piece in place alongside the cut, and trace along the cut in the old piece to transfer the angles to the new piece.
Cut along each line, drill holes, and bolt the new piece in place with 3/8 × 4" carriage bolts.
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