The Gap Under the Fence: Where Guidance Meets the Ground

A wildlife friendly fence is usually sold as a type: plain wire, no barbs, low enough to jump. That is a shopping list, not a specification. What decides whether an animal gets through is the opening at the bottom, measured against the ground the fence actually stands on. Fix that opening with a single number and a line that passes inspection on flat pasture becomes a taller obstacle at a slope of one in ten, and a wall on the fall behind it.

Executive Summary

Every published standard for wildlife friendly fencing reduces to five decisions: the height of the top wire, the spacing of the top two wires, the clearance under the lowest wire, whether that wire is smooth or barbed, and how visible the line is. The numbers vary by jurisdiction; the shape of the answer does not. Every height in that list is measured on level ground, and level ground is the one thing a fence rarely gets.

Ground is where they part company. A 42 inch fence a pronghorn clears on the flat is a 75 inch obstacle where the ground climbs one in two, and the guidance says so itself. Add mesh and a second governor appears, because a panel is only as permeable as its aperture: 102 mm passes a rabbit, 20 mm passes nothing wider than 20 mm. The smallest opening on the line decides, not the number printed in the middle of the drawing.

The reference specification is the animal fence mesh product page.

Send the line length and the species that has to pass

Quick Answer:

How big a gap under a fence should be is 16 inches minimum and 18 inches preferred between the lowest wire and the ground, on ground that does not climb. That is 406 mm to 457 mm. The better answer is that clearance has to be read at the tightest point along the line, and it only governs if nothing above it is smaller. On a mesh panel it never is: the aperture is four times narrower than a compliant gap, so the mesh decides and the gap is decoration.

Key Takeaways

• The guidance assumes level ground; the same fence can present a barrier nearly twice as high on a fall.
• The governing opening is the narrowest one anywhere on the line, gap or aperture, whichever is smaller.
• A panel that is 94 percent hole can still stop an animal, because permeability is set by the widest single opening.
• Small apertures are the least friendly geometry in the catalog and the most useful where exclusion is the requirement.

What the Guidance Actually Writes Down

The published figures are consistent enough to treat as a consensus. A top wire at 40 inches, 42 at the outside. Twelve inches between it and the wire below, because animals tangle back legs when that gap closes. A lowest wire at 16 inches minimum, 18 preferred. Smooth wire top and bottom. Posts at 16.5 foot intervals. Visibility marked, usually with a sleeve of pipe over the top strand so a running animal can judge height before it commits.

Read that list again and notice what it measures. Every figure is taken at a post. None describes the span between the posts, and none describes the ground beneath it. A gap under the fence is therefore not one dimension but a range that runs the length of the line, and the specification quietly assumes the range away. A fence that satisfies every line of the list can still fail the animal, because the animal never meets the list; it meets the ground. What the fence is for in the first place is set out in the case for this kind of line.

The Ground Returns a Different Number

The clearest treatment comes from Montana Fish, Wildlife & Parks, whose second edition landowner guide remains a standard reference: A Landowner’s Guide to Wildlife Friendly Fences. It states the mechanism plainly. “As ground slope increases, the height an animal must jump to clear the fence increases considerably.” Then it puts numbers on it. A 42 inch fence passable on the level becomes 48.6 inches of effective barrier at a 10 percent slope, 62 inches at 30 percent, and 75 inches at 50 percent. “Fences on steep slopes become nearly impossible for animals to jump over without injury,” the guide concludes, which is a polite way of saying the fence has stopped being a fence.

Those figures are worth pushing on, because they hide an assumption. Take the increase at each slope and divide by the gradient: 6.6 inches at 10 percent, 20 at 30, 33 at 50. Each implies a horizontal run of 66 inches, five and a half feet. The guide is pricing a landing that far beyond the wire. So the height an animal must clear is not the post height. It is the post height plus the rise over the next five and a half feet, and that rise depends entirely on the slope the fence stands on.

The consequence is that a steep line needs a different design, not a taller one. A drop-down fence section, lowered where the animals already cross, converts a 75 inch obstacle back into a passable one at the one place that matters. A wildlife corridor through a fenced valley is a series of those decisions, and a fall line without a drop-down is a wall with a drawing that says otherwise. The height rules that follow are worth reading beside the slope figures, since a height quoted without a gradient is half a specification.

The Smallest Opening Is the One That Governs

Put mesh on the line and the arithmetic changes character. Our apertures run from 20 mm to 102 mm, which is 0.79 inch to four inches, with wire diameter from 1.2 mm to 3.2 mm. Set any of them beside a compliant 406 mm ground clearance and that clearance is four times wider than the largest hole in the panel. Nothing that fits through the mesh needs the gap.

Open area misleads here. Across the range the panels are 89 to 94 percent hole, so the finest and the coarsest differ by a few points of air. What passes under a fence differs by a factor of five, because a body is a linear thing and an aperture is a linear dimension. A panel of mesh with small holes can be ninety percent air and still be a wall to anything wider than a finger.

So the narrowest opening on the fence decides, and it is rarely the one being discussed. It may be the aperture, a four inch gap under a gate, or a brace that pulls the wire eight inches below the drawing. Guidance built around wire gets this right by instinct, which is why it warns against ringlock and woven deer wire: no ground clearance rescues a small aperture. The same logic runs in reverse for aperture size. If the job is passage, the aperture is the wrong dimension to specify tight. If the job is exclusion, it is the only one worth specifying at all, and the ground gap becomes the detail that stops an animal going under instead of through. Where a panel has to do that work, the mesh is bought to a specification rather than to a price per square meter.

Why the Bottom Edge Is a Build Dimension

The bottom of a fence is where wire and panel part ways, and the difference is not small. On a stock fence the lowest wire is fixed at the posts and hangs between them. Tension decides whether it sits at 18 inches or sags to twelve, and tension is a maintenance condition rather than a design decision.

A mesh panel does not sag in the same sense. Its bottom edge is a line of locked nodes, and it sits where the frame puts it. That turns clearance into something you can hold to a tolerance, and it makes the ground the variable instead of the wire. On undulating ground a rigid panel bridges the dip, so clearance grows at the low point and shrinks at the crown. Set a panel at ground level on a rise and it buries; set it at a fixed clearance and it leaves daylight on the fall. Neither is wrong, and both need to be read off a survey rather than a detail drawing. Panels that keep their clearance under load are the only version where the number survives installation.

That is why a lay-down fence and a fixed panel answer different questions. The lay-down section is a gate in disguise, open for a migration window and shut for the rest of the year. A panel set to a clearance is a permanent statement about which animals may pass, and it should be chosen on that basis rather than on the price of a roll.

What a Rigid Panel Does at Ground Level

The argument for stainless steel rope mesh is not that it is friendlier than wire. It is that its geometry is fixed, which makes every one of those dimensions a number you can hold. Each intersection is a node that cannot slip, so a load applied at one opening stays at that opening and the aperture you specified is the aperture that arrives on site. A handwoven mesh panel of 9 m by 18 m, 162 square meters, can be woven to the frame opening instead of cut from a roll and stretched to fit, which removes the slack edge where a clearance usually goes wrong.

Wire diameter does work here that has nothing to do with strength. Across 1.2 mm to 3.2 mm the strand is smooth and continuous, so there is no barb to catch a wing and no thin filament to cut. The heavier strand also holds the aperture square under wind load, so the opening the animal met on the way in is the opening available on the way out. A panel does not negotiate: it is the same hole in a gale as in still air.

None of which makes it a passage product. A 20 mm panel is a wall, correctly and permanently, and a 102 mm panel is a wall to anything wider than four inches. What a rigid panel offers is a definite answer where exclusion is the requirement, and no false promise where it is not. Panels cut to the opening rather than stretched over it survive the second winter.

The Top Half Decides the Other Half

Injury on a wildlife line concentrates at the top of the fence, and the mechanism is the barbs. A smooth strand slides; a barbed one grips, and it grips where the animal has the most leverage and the least control, which is the top of a jump. Replacing the barbs with plain wire removes the grip rather than the fence, which is why the guidance wants smooth wire at the top and the bottom and tolerates barbs only in the middle strands a jumping animal never reaches.

Visibility is the other half of the same problem, and it is routinely underbuilt. A stock fence in a gray paddock, seen by an animal moving at speed in low light, is close to invisible. A sight wire, a sleeve of pipe over the top strand, a strip of tape, all work because they give the eye a horizontal edge to judge height against. The rule of thumb is that the top two wires should read as one line from twenty paces, and where they do not, the fence is not visible enough for the animals using it.

Measure the Line, Not the Drawing

The last step is the one most often skipped and it costs nothing. Walk the finished line and measure the smallest opening at ten points, choosing the worst ground rather than the average: the crown of a rise, the lip of a ditch, wherever a brace pulls the wire down. Record the lowest number on the drawing as a comparison, then write down the lowest number on the ground. That second figure is the clearance the fence actually offers. Anyone working through which fence suits which animal will find the same logic applies above ground.

A fence check walk also finds the failures nobody drew. A wombat gate cut into one bay, 20 cm high and 30 to 40 cm wide, is cheaper than a hundred meters of specification and it solves what the specification cannot: a low, heavy animal that will not use a gap it cannot see through. Mark it, keep the grass off it, and check it after the first wet season, because that is when a clearance closes.

Send the survey and we will size the aperture and the clearance together

FAQ