A wind fence or a covered shed: two different problems wearing one question
"A wind fence or a shed" is put to me as a choice between two ways of protecting the same pile. It is not. They solve different problems, and mixing them up is why yards end up with a fence that does not keep the material dry, or a shed that fills with dust because the wind still arrives at the door.
A wind fence protects the pile from being moved. A covered shed protects the product from water. If your loss is tonnage blowing off the stockpile, the fence is the cheap answer. If your loss is moisture content, a fence does nothing at all.
Two different problems wearing the same question
The first question is what is being lost.
Bulk material in an open yard loses value in two separate ways. It loses mass — fines lifted off the pile and deposited on the fence line, the road and the neighbouring property, which is a cost, a complaint and often a regulatory problem. And it gains water, which changes the product: a mineral concentrate with a raised moisture content costs more to handle, may degrade in store, and may fall outside what the buyer will accept.
Those two losses respond to completely different interventions. A fence addresses the first. Only a roof addresses the second. A fence with a roof on it is a shed, and a shed without walls is not automatically a fence.
What a wind fence actually does, and what it cannot do
A wind fence works by creating a sheltered wake behind it, and the shape of that wake is decided by porosity and height rather than by how solid it looks.
A solid wall is the worst version of the idea: it produces a strong vortex behind itself and can scour the pile harder than no wall at all. What works is a permeable fence with a controlled open area, positioned at a distance from the pile rather than tight against it, so the wake forms over the material instead of at its edge.
The practical numbers are worth knowing. Fence height sets the protected length — the shelter extends roughly ten to fifteen times the fence height downwind, and the effective reduction in wind speed is greatest in the first few heights behind it. Porosity in the region of forty to fifty per cent is the working range for a fence that reduces speed without generating a damaging recirculation.
That means a fence protecting a pile that is forty metres long needs to be tall enough that the wake reaches the far end, or it needs to be a fence on more than one side. A 4 m (13 ft) fence will not shelter a 60 m (200 ft) pile, no matter how it is built.
Moisture is a product question, not a weather question
Whether a pile needs a roof depends on what happens when it gets wet, and how long it stays wet.
Some materials are effectively indifferent: crushed rock drains and dries and the moisture comes back off in a few days. Others are not. Fines-rich material holds water in the body of the pile and stays wet for weeks, and a concentrate or a product destined for a process with a moisture limit can be out of specification after a single storm.
The useful way to frame it is duration, not rainfall. Ask how many days a year the pile is above its allowable moisture, and what that costs in drying, in rejected loads, or in price. If the answer is a handful of days in the wet season, a roofed area sized for the critical product and an open area for the rest is usually the right answer — and it is much cheaper than roofing the whole yard.
The hybrid that usually wins
In most yards I look at, the sensible configuration is neither of the two options as posed.
It is a smaller covered shed for the product where moisture matters, sitting inside a fenced yard where the bulk of the material is stored open. The fence handles the dust and the mass loss across the whole site. The shed handles the moisture for the part of the product that cannot afford it. The two do different jobs on different areas, and neither has to be built to the full extent of the other.
The configuration also has an operational advantage: a shed with one open long side and a fence in front of that opening gives shelter without trapping dust, which is what a fully enclosed shed on a dry, dusty product tends to do.
Drainage: the part everyone forgets in an open yard
An open yard with a fence around it is a catchment, and its drainage is usually designed after the fact.
Two things go wrong. Water running off the stockpiles carries fines into the yard drainage, and the drainage is sized for rain rather than for rain plus solids, so it silts up in the first season. And the yard falls toward the fence line, which is exactly where the fence posts and their foundations are, so the fence stands in water.
The fix is early and cheap: set the yard levels so water runs to a collection line that is not the fence line, size the channels for solids as well as flow, and detail the fence posts so they are not sitting in a puddle. On a yard that will be paved in phases, decide the final levels before the first phase is poured.
What I want before drawings
- What the material loses: mass, moisture, or both, and roughly how much of each.
- The allowable moisture of the product, and how many days a year it would be exceeded in the open.
- Pile dimensions and the way the yard is worked — dozers pushing from a fixed face, or a loader working around the pile.
- The prevailing wind direction on site, and what is downwind of the yard.
- Whether the yard will be paved, and where water is allowed to leave.
- The wind speed to be used, taken from the applicable code at the project site rather than a round number, plus the fence and shed edge detail — an open-sided shed on a windy site is loaded by internal pressure as much as by external, and the fixings at the open edge are where that shows.
A fence is a wind device. A shed is a water device. Decide which loss you are actually paying for, and the choice stops being a choice.
Written by Johnny Joe, who advises owners and project managers in Africa and South America on steel buildings. If this decision is live on a project, send the layout and the constraint that worries you and I will tell you what it changes.
Related: wind fence porosity and height · pipe storage damage · height set by maintenance access · what drives the cost
