Metal Garden Storage: Galvanised vs Aluminium, and Damp

Metal garden storage is the right choice when security and a long life matter more than looks: a steel box or locker will not rot, cannot be cut open with a knife like plastic, and the better ones come with long guarantees against rusting through. The catch is condensation. Metal is cold at night, so moisture forms on the inside of the lid and walls, and that decides what you can safely keep inside. This page covers the types of metal storage, what the coatings mean, how to stop damp, how to secure a unit, and what you should never store in one.

If you want a full walk-in building rather than a box or locker, our page on metal garden sheds covers bases, assembly and planning for those.

Types of metal garden storage

Storage boxes and chests. Low, lift-lid units for cushions, toys, hoses and garden tools. Easy to fit on a patio and quick to assemble.
Tall lockers and cabinets. Upright units with doors and shelves, for tools, pots and the things you would otherwise keep in a shed. They take up very little floor space against a wall.
Bike stores. Lockable steel units sized for one to four bikes, with or without a ramp. These are where security ratings matter most.
Bin stores. Open-fronted or lidded frames that hide wheelie bins. Our page on garden bin storage covers sizes for council bins.
Mower and tool stores. Low units with a wide front or side door so a mower can be wheeled straight in.

Pick by what you will store, then by size. Measure the largest single item, such as a mower with the handle folded or a bike with the bars turned, and check the internal dimensions, not the external ones. The frame, door frame and lid all take space.

Galvanised, coated or aluminium: what the finish means

Almost all metal garden storage is made of one of three things.

Galvanised steel

Galvanised steel has a coating of zinc that protects the steel underneath. The zinc corrodes slowly in place of the steel, so a small scratch does not turn into a rust patch straight away. Bare galvanised steel has one weakness: if water sits on it for long periods, such as condensation that cannot dry out, it forms a white powdery deposit known as white rust. Rainclear’s storage guidance for galvanised steel puts this down to rapid temperature changes and recommends raising the steel off the ground on a slight slope so water runs off.

Galvanised and coated steel

Most boxes and lockers sold for gardens are galvanised and then painted, powder coated or polymer coated. That gives two barriers: the coating keeps water off the zinc, and the zinc protects the steel where the coating gets scratched. It is also how you get colours like anthracite grey or green. As an example, Biohort builds its StoreMax boxes from hot-dipped galvanised steel with a polyamide coating and backs them with a 20-year guarantee. For most gardens, galvanised and coated steel is the best balance of strength, life and looks.

Aluminium

Aluminium does not rust, and it is much lighter than steel, which makes it easier to move and assemble. It is softer, so it dents more easily, and a light unit needs fixing down more firmly. It suits coastal gardens, where salt air is hard on steel, and places where you want something you can lift and move.

How to stop condensation and damp inside

Condensation forms when warm, moist air meets a cold surface. A metal lid cools fast on a clear night, so water forms underneath it and drips onto whatever is stored below. It is worst in spring and autumn, when days are mild and nights are cold. You cannot stop it entirely, but you can keep it off your things:

Buy a unit with vents. Air moving through the unit carries moisture out. Several makers build vents into the walls or under the lid; check the listing.
Raise it off the ground. A box sitting directly on soil or grass draws moisture up. Put it on paving slabs, a concrete pad or a frame, ideally with a very slight fall so rain runs off the lid and away from the base.
Keep things off the floor and walls. Use a slatted shelf, pallet or plastic crates inside so drips fall past your things, not onto them.
Line the lid. Anti-condensation lining or a sheet of closed-cell foam under the lid reduces drips. Some makers sell their own.
Bag anything that must stay dry. Cushions go in breathable covers, not sealed plastic, so trapped damp can still escape.
Open it on dry days. Ten minutes with the lid up on a sunny afternoon lets the inside dry out.

Our page on waterproof garden storage boxes explains why “waterproof” on a listing rarely means sealed.

Security: locks, fixings and ratings

A metal unit is only as secure as its weakest point, which is usually the lock or the way it is fixed down. Things to look for:

A proper lock or a hasp for a padlock. A cylinder lock or a shrouded hasp that hides the padlock shackle is much harder to attack with bolt croppers than an exposed hasp.
Hidden hinges and fixings. If the hinge pins or panel bolts can be undone from outside, the lock does not matter.
Bolt-down points. Fix the unit to a concrete base or solid paving with expanding anchor bolts from the inside, so they cannot be reached when it is locked.
Certification for bike stores. A few UK makers have their stores independently tested. Asgard, a UK maker of steel storage, says many of its bike stores are certified by the Loss Prevention Certification Board to level 1 and by the police-backed Secured by Design scheme. For a bike store, an accreditation like this is worth more than any marketing claim.
A ground anchor for bikes. Even inside a locked store, lock bikes to a ground anchor. Sold Secure grades anchors and chains from bronze to diamond; insurers often ask for a minimum grade.

For more on bike storage sizes and anchors, see our page on bike sheds for the garden.

What to keep in metal garden storage, and what not to

Good things to keep in a metal unit:

Hand tools, spades, forks and hoses.
Mowers, strimmers and hedge trimmers (empty the fuel from petrol tools for winter, as the maker advises).
Pots, compost bags and garden sundries.
Bikes, scooters and outdoor toys.
Cushions, if bagged and kept off the floor.

Things to keep out:

Anything that hates damp: paper, cardboard boxes, electronics, power tool batteries and chargers. Bring battery packs indoors in winter; cold and damp shorten their life.
Paint and water-based products in winter. They can freeze and spoil in an unheated box.
Large amounts of petrol. The HSE says you can store up to 30 litres of petrol at home without telling your local Petroleum Enforcement Authority, in plastic containers of up to 10 litres or metal containers of up to 20 litres. Keep it in proper fuel cans, away from the house, out of direct sun and away from anything that could spark.
Chemicals in leaking containers. Weedkillers and fertilisers should be sealed and kept on a shelf, away from children.

A quick buying checklist

Measure the largest item and check the internal size.
Choose galvanised and coated steel for most gardens, aluminium for coastal or movable use.
Check for vents, a lockable hasp or cylinder lock, and bolt-down points.
Check the guarantee: what it covers (rusting through, panels, paint) and for how long.
Plan the base: level paving or concrete, with a slight fall.
For bikes, look for an independent security accreditation and add a ground anchor.

If you are still choosing between materials, our guides to wooden garden storage and garden storage cabinets set out the alternatives, and the Best Garden Room homepage has the rest of our garden building advice.

Frequently asked questions

Does metal garden storage rust?

Galvanised steel resists rust because the zinc coating corrodes in place of the steel, and a paint or polymer coat on top adds a second barrier. Deep scratches and long-standing condensation can still cause corrosion over time. Aluminium does not rust.

Is metal garden storage waterproof?

Most units keep rain out, but very few are sealed, and condensation can form inside whatever the listing says. Raise the unit off the ground, choose one with vents, and keep stored items off the floor.

Is metal or plastic garden storage better?

Metal is stronger and harder to break into, and coated steel does not go brittle in sunlight the way cheaper plastics can. Plastic suffers less from condensation and does not dent. For security, especially bikes and tools, metal is the better choice.

Do metal storage boxes need a base?

Yes. A level, solid base of paving slabs or concrete stops the frame twisting, keeps the bottom out of standing water and gives you something to bolt it to.

Can you store petrol in a metal garden storage box?

Small amounts in approved fuel cans, yes. The HSE allows up to 30 litres at home without notifying the authority, in plastic cans of up to 10 litres or metal cans of up to 20 litres. Keep the box away from the house and out of direct sun.

How do I stop my metal storage box being blown over?

Bolt it down. Most boxes and lockers have holes in the floor for anchor bolts into concrete or paving. A light aluminium unit in an exposed garden should always be fixed.