Plumbing Steel Posts for a 4-Foot Snow Fence Upwind of a Mountain Driveway With a Magnetic Post Level: 8-Foot Centers, a 6-Inch Bottom Gap, and 160 Feet of Setback

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A long driveway on an open mountain slope can drift shut overnight while the rest of the property blows bare. A short snow fence set in the right place stores that snow before it reaches the gravel.

For a 4-foot snow fence, drive steel posts on 8-foot centers, plumb on two faces, and leave a 6-inch gap under the fencing. Set the fence at least 160 feet upwind of what you want clear, square to the prevailing wind, and run it well past both ends of the problem.

Where the numbers come from

The national Strategic Highway Research Program's Snow Fence Guide has a short section on 4-foot fences for places where a taller fence will not fit. It calls for a 6-inch bottom gap, steel posts on 8-foot centers, and a fence placed at least 160 feet from the road shoulder.

The same guide puts the best porosity at 40 to 50 percent, so you want fencing that is about half open. It also says fences should run perpendicular to the prevailing wind, though they can be off by up to 25 degrees without hurting performance.

Reading the wind before you drive a post

I do the layout in winter, after a real blow, because the drifts tell you the wind direction better than any forecast. The guide suggests reading the streamlined drifts behind shrubs and rocks with a hand compass.

1. Walk the drifted stretch of driveway and flag where the drift starts and ends.
2. Stand in the lee of a few bushes and take a compass bearing along the long axis of their drifts.
3. Lay the fence line square to that bearing, at least 160 feet upwind of the driveway edge.
4. Extend the line past each end of the drifted stretch. The guide's general rule is 20 times the fence height on either side, about 90 feet each way for a 4 1/2-foot fence.

The mistake that costs most first-timers is setting the fence close to the driveway because it feels like it should catch more. The guide is blunt about it: snow fences too close to the road can increase the amount of snow on the road, because the drift behind the fence lands right on the surface you were trying to protect.

Plumbing the posts

On a 4-foot fence the posts are doing all the work against wind load, and a leaning post loads the fabric unevenly. I strap a magnetic post level to each steel post before the first hit, so I can watch both faces while driving.

1. Stretch a string line along the flagged fence line at about knee height.
2. Set the first post, clip the level on, and drive it a few inches. Check both bubbles and correct with a push before the post is buried deep enough to resist.
3. Drive the rest of the way, stopping every few hits to re-read the level.
4. Measure 8 feet center to center to the next post, and keep going down the line.
5. Keep each end post no more than 6 feet from its neighbor, which the guide says makes the end easier to brace.

In rocky mountain soil a post will sometimes skate sideways off a buried stone and lean. If a few hits will not correct it, pull it and shift the hole a few inches along the string rather than leave it crooked.

Bracing and hanging the fencing

The guide braces each end post with a second steel post driven at an angle, running from near the top of the end post to the ground line of the next post, wired in place. It warns against side braces and guy wires on post-supported fences, since buried braces take big loads from settling snow.

1. Measure the 6-inch bottom gap from the top of the winter vegetation, not from bare soil.
2. Pull the fencing taut. The guide gives at least 250 pounds of pull for 4-foot picket fencing, and 500 to 1,000 pounds for most synthetic fencing.
3. On plastic fencing, sandwich it between two 2x2 boards wired tight to each post at the center and 6 inches from each edge.

Minnesota's transportation department notes on its snow fence page that structural fences are left standing year-round, with posts typically 10 to 15 feet apart on the taller permanent versions. A 4-foot fence on 8-foot centers is the small, closer-spaced cousin of that design.

Picking up materials

Tractor Supply carries steel T-posts, wire, and orange plastic snow fence most of the fall. Home Depot stocks 2x2 furring strips if you go the sandwich route.

For driving posts in rocky ground without bending them, see this walkthrough on driving 6-foot and 8-foot T-posts in clay and rocky soil.