BALLAST & HOLD-DOWNS
What Keeps It
On The Ground.
A shelter's real enemy isn't weight bearing down — it's uplift.
Here's how we hold them down, and what decides which method your site needs.
WHY IT MATTERS
A Curved Roof Behaves Like A Wing.
When wind passes over a curved fabric shelter, it accelerates across the top and drops the pressure above the structure. The result is lift. On a large span in an exposed location that uplift force can be considerable — and it acts upward, not downward.
Which is why the hold-down design matters as much as the frame. A shelter doesn't fail because the steel wasn't strong enough. It fails because the structure wasn't adequately anchored to what's underneath it.
UpliftThe governing force, not weight
Wind zoneLow through Extra High changes everything
ExposureElevation and terrain category
GroundWhat you can actually anchor into
FIVE WAYS TO HOLD IT DOWN
From Filled Containers To Screw Piles
Which one you need depends on your wind zone, your ground and what you're mounting on. Most sites use one of the first two. Exposed sites and post-mounted shelters need more.
MOST COMMON
Internal Container Ballast
The containers themselves become the hold-down. Loaded with stored goods, aggregate, sand or water ballast, a pair of containers provides substantial resistance to uplift without piles or concrete footings.
- No piling or concrete footings
- Uses what you're storing anyway
- Fully relocatable
- Ballast weight confirmed by engineering
FIXED WEIGHT
External Concrete Ballast
Precast concrete blocks placed against or inside the container footprint, tied back to the structure. Useful where the containers won't be consistently loaded, or where you need certainty about the hold-down weight rather than relying on stored contents.
- Known, fixed ballast weight
- Doesn't depend on what's stored
- No piling required
- Blocks can be relocated
POURED IN
Concrete Poured Into Containers
Where a shelter is permanent and the containers won't be moved, concrete can be poured directly into the container base to create a fixed, engineered mass. The most certain form of ballast short of piling.
- Highest ballast certainty
- Suits permanent installations
- No separate footings needed
- Containers no longer relocatable
FIXED DOWN
Base Plate & Bolt
Where there's existing concrete — a hardstand, a slab, a pad — the structure can be bolted directly to it through engineered base plates. Clean, low profile, and no ballast mass required.
- Uses existing concrete
- Low profile, nothing in the way
- Requires adequate slab thickness
- Slab capacity confirmed at design
EXPOSED SITES
Screw Piles
Helical steel piles wound into the ground by machine, giving genuine tension capacity rather than relying on dead weight. The answer for high wind zones, and sites where some of the other options won't suit.
They also go straight into undisturbed ground. No pad to build first, almost no spoil, and they can be load tested as they are installed. If earthworks are difficult, expensive or simply not wanted, this is usually the way around them.
Screw piles can be unwound and removed if the shelter is relocated, which conventional concrete footings can't.
- Real uplift resistance, not just mass
- No prepared pad needed for the piles
- Installed in hours, no concrete cure time
- Removable if the shelter moves
- Suits high wind zones and post mounting
Most methods need a prepared pad. Screw piles don't. Ballast, block walls and base plates all sit on prepared ground — topsoil stripped and compacted hardfill laid to the bearing capacity the engineering calls for. Ballast replaces piles and footings, not groundwork. Screw piles are the exception: they wind straight into undisturbed ground, which is part of why they suit sites where earthworks are difficult.
HOW IT'S DECIDED
Five Things Set Your Hold-Down
We don't guess at this. Every shelter is designed against the same set of inputs, and the hold-down falls out of that calculation.
01
Wind Zone
Sites are classified from Low through Medium, High, Very High and Extra High. That classification comes from the wind region your site sits in (NZ1 to NZ4), combined with terrain and topography — and it is the single biggest driver of the hold-down design.
02
Terrain & Exposure
Our standard designs assume Terrain Category 2. A shelter on an exposed ridge, near open water or on elevated ground sees higher loads than one tucked behind existing buildings, and the design changes accordingly.
03
Span & Height
A wider span with a higher apex presents more area to the wind and generates more uplift. A 24m truss on double-stacked Hi-Cubes is a very different proposition to a 6m shelter on single containers.
04
End Walls
How the ends are closed changes the loads. A shelter open at both ends lets wind pass straight through. Close one end and the wind has somewhere to push against and nowhere to go, which increases the load the hold-down has to carry.
05
Ground Conditions
What you can anchor into, and what the ground beneath will carry. Existing concrete allows base plates. Soft or variable ground may favour screw piles, which also avoid needing a prepared pad at all.
Worth knowing before you decide on end walls: adding one is not just a cover question, it is a structural one. If you are weighing up whether to close an end, tell us at design stage rather than after — retro-fitting a wall to a shelter that was engineered open can mean revisiting the hold-down.
On specific figures: ballast weights, pile depths, bolt patterns and required bearing capacity are all site-specific and come out of the engineering for your particular shelter, location and mounting method. We'd rather give you the right number for your site than a general one that might not hold.
SIDE BY SIDE
Hold-Down Options Compared
Most container-mounted shelters on typical sites use container ballast. Post-mounted shelters, exposed sites and high wind zones usually need piles or a bolted connection — and screw piles are the one option that doesn't need the ground prepared first.
BALLAST & HOLD-DOWN FAQS
Common Questions
What engineers, site managers and property owners ask us most.
Do I really need foundations?
Not in the conventional sense, for most container-mounted shelters. Where the containers carry sufficient ballast there are no piles and no concrete footings, which is the main cost and time advantage over a conventional build.
What you do still need is a prepared site. Expect the topsoil to be stripped and compacted hardfill laid to the bearing capacity the engineering specifies. Ballast replaces footings, not groundwork. Screw piles are the exception — they go straight into undisturbed ground.
What site preparation is involved?
For ballast, block walls and base plates: stripping the topsoil and laying compacted metal hardfill across the footprint, built up and compacted to the bearing capacity your shelter's engineering calls for. On an established yard with existing hardstand or concrete, much of that may already be done.
Screw piles don't need it. They wind straight into undisturbed ground, which is often the deciding factor on sites where earthworks are difficult, expensive or simply not wanted.
Do end walls change the hold-down?
Yes, and more than people expect. A shelter open at both ends lets wind pass straight through. Close one end and the wind has something to push against and nowhere to go, which increases the load the structure and the hold-down have to carry.
So it's worth telling us at design stage whether you want ends closed, rather than adding one later. Retro-fitting an end wall to a shelter engineered as open can mean revisiting the hold-down design.
How much ballast do the containers need?
It depends on your span, height, wind zone, terrain category and how the ends are closed, so there isn't a single figure that applies everywhere. It's calculated as part of the engineering for your shelter and we'll give you the number for your site.
What if my containers are empty?
Then they aren't doing the job. If the containers won't be reliably loaded, we'd generally recommend external concrete ballast or a bolted or piled connection instead — something with a known, fixed capacity rather than one that depends on what happens to be stored that week.
Can I bolt to my existing concrete?
Often yes. It comes down to slab thickness, reinforcement and condition. We'll confirm whether your slab can take the loads as part of the design — and if it can't, screw piles alongside the slab are usually the alternative.
How deep do screw piles go?
That varies with ground conditions and the loads being carried. Screw piles are wound in until they reach the required torque, which correlates to capacity, so the depth is determined on site rather than fixed in advance. They can be load tested during installation.
Can hold-downs be removed if we relocate?
Ballast and bolted connections, yes, straightforwardly. Screw piles can be unwound and taken out, which is a real advantage over conventional concrete footings that have to be broken out and disposed of. Poured concrete inside containers is the one option that isn't practically reversible.
Will this satisfy a building consent application?
Where consent is required, the hold-down design forms part of the engineering documentation we provide. Requirements vary by council and by project, so talk to us early and we'll tell you what's involved for your site.
GROUND CONDITIONS?
Tell us what you're building on.
Concrete, hardfill, soft ground or nothing at all — send us the site details and we'll confirm the hold-down design along with your quote.
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