If your gate already sags, this isn’t your article. Go read the sagging gate repair first, because the fix for a racked gate is different from the way you keep one from racking.
This is about building a frame that starts square and stays that way. The difference between the two is joinery, and it’s decided in the twenty minutes before you drive a single screw.
Why frames rack, in one paragraph
Four boards screwed into a rectangle at the corners is not a rigid shape. Every corner is effectively a pivot, and the rectangle can fold into a parallelogram without a single joint breaking or a single screw pulling out. Hang that rectangle off one edge and let gravity work on the far corner for a summer and it will fold. Nothing failed. It was never rigid to begin with.
Triangles are rigid. Rectangles are not. Every gate frame is a strategy for turning a rectangle into a pair of triangles, whether by a diagonal brace or by making the corners strong enough that they refuse to pivot.
The joints, ranked
Butt joint with screws. Two boards meeting at 90 degrees, screwed through the face. Screws going into end grain hold about half of what they’d hold in face grain, and end grain in softwood is not much to begin with. This is what most store-bought gates are. This is why most store-bought gates sag.
Butt joint with corner gussets. Add a plywood or board triangle across each corner and the joint gets substantially better, because the gusset itself is a small triangle. Cheap, fast, visible from behind. Fine for a utility gate.
Pocket screws. Angled screws driven through a jig from the inside face. Hidden, quick, considerably stronger than a face-driven butt joint. Use exterior-rated pocket screws, and note that a pocket hole is a small reservoir facing up on the underside of your top rail. Fill them with plugs and seal them or you’re inviting rot.
Half-lap. Remove half the thickness from each mating piece so they interlock flush. This is the sweet spot for a DIY gate: you roughly double the glue surface, you get mechanical resistance to racking because the two pieces physically obstruct each other, and it needs no tools you don’t already have.
Bridle joint. A slot in one piece, a tongue on the other. Stronger than a half-lap, more setup.
Mortise and tenon. The right answer for a hundred-year gate and considerably more work than most people want on a Saturday.
I build half-laps. Below is how.
Cutting a half-lap with a circular saw
A 2×4 is actually 1-1/2 inches thick, so each lap is 3/4 inch deep and as wide as the mating board, which is 3-1/2 inches for a 2×4.
Mark it. Lay the two pieces in their final position, overlapped. Pencil along both edges of the top piece onto the bottom one and vice versa. Now you have the exact width, which is better than measuring it, because your lumber is not perfectly 3-1/2 inches.
Set the saw. Depth to 3/4 inch. Test it on scrap and measure the kerf with a combination square before you touch your real stock. Circular saw depth scales lie, routinely by a sixteenth, and a sixteenth of error across four joints is a gate that won’t sit flat.
Kerf it out. Cut on both marked lines, then make repeated passes about 1/4 inch apart through the waste between them. You’ll end up with a row of thin fingers.
Clear it. Snap the fingers out with a chisel and pare the bottom flat. Hold the chisel bevel down and work from both edges toward the middle so you don’t blow out the far side.
Check it. The mating piece should sit in the lap flush with the surface. If it stands proud, take another whisper off the floor. If it sits low, you cut deep, and the fix is to take the matching amount off the other half rather than shimming.
Assembly, where it’s actually won or lost
Check your work surface first. A frame assembled on a racked surface comes out racked. Put a level across your sawhorses or your garage floor in a few directions. Shim until it’s flat. This step feels excessive and it isn’t.
Dry fit everything. All four joints, no glue, no screws. Confirm the frame lies flat and the laps close fully. Glue is not the moment to discover a joint is proud.
Glue. Titebond III is waterproof, exterior rated, and about twelve dollars a bottle. Cover both faces of every lap. Squeeze-out is correct and means you used enough; wipe it while wet because dried glue rejects stain and you’ll see every drip forever.
Clamp, then measure the diagonals. Tape from corner to corner one way, then the other. Equal diagonals mean the frame is square. This works on any rectangle and it’s more reliable than a framing square, which only checks one corner at a time and can’t see cumulative error.
Adjust by clamping across the long diagonal. If diagonal A is longer than B, put a clamp across A and gently pull it in. A sixteenth of an inch of clamp movement will change the diagonal by more than you expect. Go slowly.
Then measure again. Clamping pressure pulls frames out of square constantly. Every time you tighten anything, re-measure. I’ve been within an eighth, added one more clamp, and lost a quarter inch.
Get both diagonals within 1/8 inch and stop chasing it. Wood moves more than that seasonally.
Fasten while clamped. Two or three exterior screws through each lap, predrilled, offset from each other so they don’t split along one grain line. Screws are holding the joint while the glue cures and adding mechanical strength afterward.
Leave it clamped 24 hours. Titebond III sets in about half an hour but doesn’t reach full strength for a day. Unclamping early and hanging it is how you undo everything above.
The brace still goes in
Half-lapped corners resist racking. They don’t eliminate it, and a gate over about four feet wide should still get a diagonal.
From the bottom of the hinge side, up to the top of the latch side. The wood works in compression, pushing against the drop of the latch corner, and it carries the load down into the hinge post at ground level.
Scribe it rather than calculating the angle. Lay the brace across the frame on the diagonal, mark where it crosses the rails, cut just outside the lines so it has to be tapped home. A brace you can drop in loosely is doing nothing until the gate has already sagged enough to reach it.
Hanging it without undoing your work
Mount the hinges to the post first, at the heights you want, with the leaves swung out of the way.
Set the gate on blocks at your intended bottom clearance, which should be 2 inches minimum. Shim the latch side up until the top of the gate is level, then check the reveal down the hinge side is even top to bottom.
Fasten the top hinge first, then the bottom, then any middle hinge. Top first because that’s the one carrying the load, and if something has to be adjusted you’d rather adjust the ones underneath it.
Use long screws. #10 by 2-1/2 inch exterior structural screws, or 1/4 inch lags with washers. Whatever came in the hinge package is too short.
Pull the blocks. Swing it. It should stay where you put it at any point in the arc, and it should close without you helping.
What to check in twelve months
Diagonals again, on the hung gate, corner to corner. If they’ve moved more than a quarter inch, the frame is racking and you want to know now rather than in year three.
Check the hinge screws with a driver while you’re there. That takes thirty seconds and it’s the single maintenance habit that determines whether any of this lasted.
