A snow fence slows the wind enough that its snow load drops out upwind, in a drift you control. Porosity near 50% is the design standard, and the setback is set by the fence height.
Snow fence: porosity, height, and how far back it goes
window SNOW SEASONchecked 2026-09-16responsible Windrow Field Manual (a named masthead, not a person)
[['1.2 m', 'About 15–20 m', 'A modest drift, a short fetch'], ['2.4 m', 'About 30–40 m', 'The common highway case'], ['3.6 m', 'About 45–60 m', 'Large capacity, needs the land']]
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Why 50% porous and not solid
A solid fence stops the wind. The snow that was carried in the air drops in the lee, immediately behind the fence, in a drift that is both large and close to whatever you were protecting. A porous fence lets some air through, so the wind speed on the lee side falls gradually and the snow deposits over a long, shallow drift that starts upwind and lands where the designer put it.
| Fence porosity | Where the drift lands | Effect on the road |
|---|---|---|
| Solid, 0% | Immediately downwind, tall and short | Worse than no fence |
| About 50% | Long, shallow drift in the design zone | The standard case |
| Very open, above 70% | Little deposition at all | Not enough capacity to matter |
Height sets the setback, and the setback is the design
The drift length scales with fence height, and the fence must be far enough upwind that the drift finishes before the road. The working rule in the transport literature is that the setback is several times the fence height, and the number of heights depends on whether the fence is a structural one or a living one.
Porosity is the fraction of the fence's face that is open. A slatted fence with 50% open area and a living fence of the same effective porosity behave similarly, which is why the two can be compared at all.
Boundary: a snow fence is not a snow barrier and not a windbreak
A barrier is solid by definition and is used to stop snow, not to place it. A windbreak is usually a living structure placed for a different purpose, such as sheltering stock or reducing soil erosion, and its snow behaviour is a side effect. Near kin, different design intent, different numbers.
The misreading that recurs: reading a windbreak spacing figure and applying it as a snow-fence setback. Windbreak spacings are chosen for wind reduction over a field, not for where a drift lands relative to a carriageway.
A sentence to defend
The manual holds that a 50% porous fence set at the textbook distance is worse than a 40% porous fence set too close, because the second one at least puts the drift in the wrong place predictably, while the first creates a false confidence that the setback arithmetic was done. Design by drift length, not by porosity number alone.
Where the numbers come from
- S1Design, Living Snow Fences, Minnesota Department of Transportationsupports the design relationship between fence height, setback and drift
- S2How Do Snow Fences Work?, Road Weather Management Technical Brief, Federal Highway Administrationsupports the porosity behaviour that lets a porous fence place the drift
- S3Snow Storage Utility Strip Design Guidance (RDI 17-03), New York State Department of Transportationsupports the placement guidance that keeps a snow storage area off the carriageway
- S4Cost Benefit, Living Snow Fences, Minnesota Department of Transportationsupports the cost-benefit framing used when a fence is being justified
One link, one document. No home pages, no search results, no bare domains.
Put this page down Put this page down when you know your fence height and have set the setback from the drift length, not from habit.