A gate is a lever, and the post is the fulcrum
Every gate is trying to fall over. Its weight acts at the middle of the leaf, which is half the leaf width away from the hinges, and the hinges have only their own vertical spacing to resist that. The result is a couple: the top hinge is pulled outward from the post, the bottom hinge is pushed inward, and the leaf tries to rotate about its bottom corner. A 45 pound gate three feet wide on hinges five and a half feet apart pulls the top hinge outward with about 15 pounds. Double the width and you double it. Halve the hinge spacing and you double it again.
That is why the first two rules of a gate that stays square have nothing to do with the gate. Spread the hinges as far apart as the frame allows, and fix them into something that will not let go under a sustained outward pull. Screws driven into the end grain of a post withdraw over a season of that load. Through-bolts do not.
Which way the brace goes, and why the wrong way is invisible
The frame is a rectangle of four pieces with a pinned corner at each junction, which is a mechanism rather than a structure: push the top latch corner sideways and it folds into a parallelogram. The diagonal is what stops the folding, and it can stop it in one of two ways depending on which diagonal you use.
A timber brace works in compression. As the latch corner tries to drop, the diagonal from the bottom hinge corner up to the top latch corner is squeezed, and wood in compression is stiff and does not care about its fasteners. That is the brace to fit, and it runs uphill from the hinge side.
A cable, wire or turnbuckle works in tension. It has to be on the other diagonal, from the top hinge corner down to the bottom latch corner, so that the sagging gate stretches it. Tighten it and the latch corner lifts.
Fit either one on the opposite diagonal and it does nothing. The timber brace, on the tension diagonal, is simply asked to pull, so all the load goes into the few screws at its ends, which loosen, and the gate sags exactly as if there were no brace at all. The cable, on the compression diagonal, goes slack the moment the gate settles and the turnbuckle can be wound until it deforms the frame without ever picking up load. Both failures look identical from the outside: a gate with visible anti-sag hardware in it, sagging.
| Device | Works in | Runs from | To |
|---|---|---|---|
| Timber or steel brace | Compression, it pushes | Bottom corner, hinge side | Top corner, latch side |
| Cable, rod or turnbuckle kit | Tension, it pulls | Top corner, hinge side | Bottom corner, latch side |
| Plywood or sheet gusset | Shear, either diagonal | Any corner, but the fasteners around the whole perimeter carry it | |
Angle, and the gate that is too wide to brace
A diagonal is most useful somewhere near 45 degrees. Steeper than about 60 and it is nearly vertical, doing little against a horizontal fold. Shallower than about 30 and it is nearly horizontal, which is the case for a wide, low gate: a four foot high gate in a six foot opening gives a diagonal at roughly 27 degrees, and no amount of timber in that position makes the frame behave.
The fix for a wide gate is not a bigger brace. It is two leaves. Splitting a six foot opening into two three foot leaves halves the lever arm on each set of hinges, halves the weight each carries, and steepens both diagonals into the useful range. A drop bolt into the ground at the meeting point takes over the job the missing latch post would have done.
Gaps, clearance and the wood moving
The gaps at hinge and latch are not slop, they are working dimensions. The hinge side needs enough to clear the knuckle of the hinge through the full swing. The latch side needs enough that a leaf which has taken on moisture still closes; a timber gate can grow a noticeable fraction of an inch across its width between a dry autumn and a wet spring, and a gate fitted with no latch gap in August binds in March. Ground clearance is measured at the highest point of the ground the gate passes over, not at the point where it hangs closed, which is where people discover that a gate opening downhill clears beautifully and opening uphill does not open at all.
The privacy fence board calculator covers the fence either side of this opening and removes the gate width from the run. Hinge post depth and the concrete around it belong to the fence post calculator, and it is worth over-sizing that one hole. Where the ground falls across the opening, the fence on a slope calculator shows the clearance the gate has to keep, and for a gate in wire fencing the livestock fence calculator handles the braced assembly that a gate post needs at the end of a strained run.
Questions people ask
Which way does the diagonal brace go on a wooden gate?
From the bottom corner on the hinge side, up to the top corner on the latch side. It works by being pushed, and wood pushes well. The opposite diagonal, which people fit at least as often, asks the same piece of wood to pull, and then the entire load sits on the handful of screws at each end. Those loosen, the joint opens, and the gate sags with a brace in it. The test that settles it without any theory: sight along the brace and ask whether the gate dropping would squeeze it shorter or stretch it longer. Squeeze is correct.
Then why do anti-sag kits run the other way?
Because a cable or a threaded rod is the opposite kind of member. It has no compressive strength at all, it can only pull, so it must sit on the diagonal that gets longer as the gate sags: top hinge corner down to bottom latch corner. Tightening the turnbuckle shortens that diagonal and lifts the latch corner. The two devices are not alternatives to be fitted the same way, they are opposites, and the single most common gate repair error is buying a cable kit and installing it along the line where the old timber brace was.
How many hinges does a gate need?
Two carries most light gates up to about five feet high. Three is the usual answer for a six foot privacy gate, and the reason is less about the total weight than about the outward pull on the top hinge and the way it concentrates in one fixing. A third hinge in the middle also stops the hinge stile itself bowing, which happens on tall gates long before the frame goes out of square. Above about seven feet or on a sheeted gate that catches wind, four is not excessive. Whatever the number, the top and bottom hinges want to be as close to the ends of the stile as the frame permits, because the spacing between them is what divides the pull.
The gate was square when I built it and it sags now. Is the frame the problem?
Check the post first, because it usually is. A hinge post takes a permanent outward pull at the top and a permanent downward load, and if it moves even a degree the gate drops at the latch by the leaf width times the tangent of that angle. On a four foot leaf, one degree of lean drops the latch edge by about three quarters of an inch, and the gate itself is still perfectly square. Put a level on the hinge post before you touch the gate. If the post has moved, bracing or shimming the gate corrects the symptom for a season and then it moves again.
Does the frame need to be inside the boards or behind them?
Either works structurally, and the choice is mostly about what shows. A frame behind the boarding gives a gate whose face matches the fence, which is what most people want, but it means the boards are carrying no load and the whole gate depends on the frame and its diagonal. A frame that is visible on one face, with the boards inside it, lets you fix the boards into both stiles and both rails, and the boarding then contributes real stiffness in shear. If you are building a wide gate and want it to survive, the second arrangement is the stronger one even though the first is more commonly built.