Thursday, October 13, 2011

Building a Gate That Won't Sag

Here's a plan for a simple gate.  I've built two of these for my own yard, and one larger one for a neighbor. Despite his kids occasionally climbing on it, it has held up very well.  No special hardware or fussy joinery.  Just a couple of miter cuts and a little analysis of the physics of gates.

Cedar Gate in my yard

If you just attached two pieces of wood at a right angle, one vertical (a stile) and one horizontal (a rail), and put hinges on the stile, the rail can swing back and forth as a very simple gate.  It probably won't last too long, because all the weight of the rail is trying to twist the joint apart.

By installing a support at an angle between the base of the stile and the far end of the rail, you form a triangle, which is a very stable shape.  The weight of the rail is now supported by the angled brace and the top of the stile, and there are no forces acting to twist the rail-stile joint and cause the gate to sag.

You could theoretically get the same behavior from an inverted version, where the rail is low and the diagonal brace runs from the top of the stile down to the end of the rail, but now the ends of the brace are under tension.  Without special joinery (or welded metal fabrication) the parts will tend to separate.

So take that triangle and hang whatever kind of decorative stuff you'd like to on it.  In my version, I start with two rails, about 1/2" narrower than the opening you're putting the gate into.  Put them far enough apart that the brace runs at a 45˚ angle or higher with respect to the ground.  A short, wide gate is under much more strain than a tall narrow gate.  If the opening is very wide, you might consider a double gate, with one on each post and the latch in the center, but those have their own problems.
You can see how the wooden components form the rail, stile, and support from the diagram above.  When I build mine, I just put screws through the decorative pickets into the rails and support.  I don't actually join the support to the rail, and there is no framing member to form the stile, since the pickets serve that function.