Short answer: run the diagonal brace from the bottom of the hinge side up to the top of the latch side, so it is pushed (in compression) by the gate's weight. On wide gates, add a tension cable or rod on the opposite diagonal, from the top of the hinge side down to the bottom of the latch end, with a turnbuckle to take up slack. Set the hinge post heavier and deeper than your line posts.
Why gates sag
A gate is a rectangle hanging off one side. Its weight tries to fold it into a parallelogram, with the latch end dropping. Every sag-free design stops that rectangle from changing shape, using a brace that resists being pushed, a cable that resists being pulled, or both.
The brace direction, and the most common mistake
A wooden brace is strong when pushed end to end and weak when pulled, because a pulled brace relies only on its nails. So it must run bottom-hinge → top-latch. Built the other way, from top hinge to bottom latch, the brace hangs off its fasteners and the gate still drops. As the University of Maryland Extension gate sheet behind our Five Field Designs plan puts it, the diagonal has to carry the load in compression.
Anti-sag cables for 12 to 16 ft gates
On long field gates a single brace gets long and springy. Add a galvanized cable and turnbuckle on the other diagonal, or a rod from the top of a tall hinge post out to the far top corner as the original sheets show. Re-tension it each spring as the wood dries and shrinks. Our Classic 16-ft Board Farm Gate keeps the original 1×4 head brace up to its tall hinge stile, with an optional cable and turnbuckle behind it for re-tensioning each spring.
Board layout that stays square
- Use 1×6 boards for the rails. Six boards at 5½ in give a 4 ft gate.
- Sandwich the rails between paired vertical battens and through-bolt every crossing with carriage bolts. There's no joinery to cut, and bolts don't loosen the way nails do.
- Close the gaps toward the bottom (2 to 2½ in) and open them toward the top, so small stock can't push through low down.
The hinge post does half the work
A perfect gate on a leaning post still sags. Use a heavier post on the hinge side than on the latch side. Set roughly a third of its length in the ground, in concrete or well-tamped gravel. Check it for plumb before you hang the gate.
Frequently asked
Should a gate brace go up or down from the hinge?
Up: from the bottom hinge corner to the top latch corner.
Can I fix an existing sagging gate?
Usually. Re-plumb the hinge post first, then lift the latch end to square and fit a cable-and-turnbuckle kit from the top hinge corner to the bottom latch corner.
Paint or stain?
Penetrating exterior stain. It doesn't trap water the way paint film does, and it can be re-coated without scraping. Seal cut ends before assembly.