The gate in the Houston house's back yard became unhinged during Hurricane Ike a few weeks before we moved in, and the previous owner did such a lousy repair job that I had to replace the replacement within a couple of years. The replacement gate fit fine and I did a good enough construction job that it doesn't sag, but the BigBox lumber I used to build it began to warp seemingly within minutes, causing a serious case of gaposis near the top. I decided that a set of eye bolts, some cable and a turnbuckle would straighten it out, so I returned to my local BigBox store where I'd bought the lumber (blue, not orange). There on the hardware aisle I found a National Zinc Anti-Sag Gate Kit (V-852) and brought it home.
The kit consists of a two zinc-plated steel support brackets that fit over opposite corners of the crosspieces plus screws to install them; several feet of 1/8-inch galvanized steel cable, plus a pair of galvanized steel rope clips (also called cable clamps); and a ¼" turnbuckle with hook-end bolts. The turnbuckle is aluminum and the bolts are zinc-plated steel.
Installation
1) Use the included screws to install one corner bracket at the hinge end of the top crosspiece and the other at the post end of the bottom horizontal. I did it the other way, since the gate didn't sag: it was warped outward at the top.
2) Open the turnbuckle as far as it will go and place a hook through the eye hole hole in the top bracket.
3) Make a smallish loop at one end of the cable and secure with a rope clamp. Place this loop on one of the hooks in the turnbuckle. If your turnbuckle has only one hook, thread the cable through the eye screw before you secure the loop.
4) Thread the other end of the cable through the eye-hole on the bottom bracket. Make a second loop and pull it as tight as possible, then secure it with another rope clamp.
5) Screw down the turnbuckle until the cable lifts the sag out of the gate or, as in my case, unwarps the warp.
The manufacturer says this kit can reach across a diagonal up to 7½ feet, which is about the size of a 5’ x 6’ gate. My gate is smaller (3’ x 6’) and I ended up with extra cable. If you have bolt cutters, you can trim this; otherwise it's fairly easy to ignore. The tools needed for installation were a cordless drill, a ratchet and 9/32” deep-well socket for the rope clips, and an adjustable wrench for the turnbuckle. In all it took about 30 minutes to install. One thing I didn't like about the kit is that the brackets are designed to fit crosspieces that are set on edge; mine are flat to match orientation of the fence rails. It's stayed secure for a couple of years; and the zinc-plated steel parts haven't shown any rust yet. In fact, I only recently tightened up the
turnbuckle to take out some fresh warp.
An anti-sag kit is an easy-to-install and economical solution for problem that's altogether too common. This or a similar kit is sold under several labels, such as the Stanley 76-0828 and the Ace Hardware 5299029. Some have an eye bolt instead of two hooks on the turnbuckle, but as long as the parts are zinc they're pretty much all the same.