Steel or timber on a playground: durability and the cost of upkeep

Councillors look at two visualisations. One shows a timber castle, the other a steel carousel. The castle wins because it looks better. Five years later somebody else is looking for money to replace the posts, which have started to rot at ground level.

Side by side

Where timber actually fails

Not on the surface, but at the ground. The zone where a post meets the soil stays damp, and that is where decay begins, usually invisible from outside for the first few years. The element looks sound and is already soft underneath.

This is what the use classes in EN 335 are about. Timber in permanent ground contact sits in the most demanding class there is, and it is the only part of a structure exposed to both moisture and soil organisms at the same time. Good design keeps the timber out of the ground entirely, on galvanised steel shoes set into the concrete, so that the post ends above grade and can dry. Where posts are concreted in directly, the service life of the whole unit is decided by its worst twenty centimetres.

Which is why inspecting timber equipment cannot be a matter of looking at it. The hardness of the timber is tested in the ground zone, for instance by pushing a blade in at the base of a post. It is one of the checks most often skipped, and it belongs in the annual main inspection described in the piece on playground inspections.

The second typical problem is shrinkage cracking. Timber moves with the humidity of the air, and the splits that open up can catch fingers and hold water. Water sitting in a split then freezes, which widens it, and the cycle repeats every winter.

Where steel fails

In one place: wherever the coating has been broken. A chip in the paint exposes the steel and corrosion starts from there.

The order of the protective layers is what decides how serious that is. A structure that is galvanised first and powder coated afterwards has two layers of protection, and damage to the paint exposes zinc rather than steel. An element painted straight onto steel loses its protection with the first deep scratch, and the rust then spreads under the coating and can weaken the section within a few seasons. The mechanism behind that is set out in the piece on powder coating and galvanising.

Sealing a chip with touch up paint takes a few minutes and closes the matter for years. A neglected chip on a structure with no zinc under the paint costs a replacement part.

Fixings are the part nobody compares

A timber structure is held together by steel: bolts, plates, brackets, post shoes. Those fixings work in wet timber, which is an aggressive environment for them, and a corroded bolt in a sound beam is as much of a defect as a rotten beam with a sound bolt. Galvanised fixings are the minimum here, and they are also the first thing to check when a timber unit starts to feel loose. On a steel structure the fixings and the frame are the same material with the same coating, so they age at the same rate.

The cost over ten years

The purchase price of the two options is often similar. What diverges is the cost of keeping them, and that is the number nobody works out when the decision is being made.

For a timber unit, assume the coatings are renewed every one to three years, whether in labour or in materials, plus replacement of the elements that did not survive. For a galvanised steel unit what remains is tightening the connections during the operational inspection and spot repairs to the paint. Moving parts, meaning the bearing of a carousel or the suspension of a swing, wear out regardless of what the frame is made of.

Surface temperature

This argument comes up often and deserves an honest answer: metal in full sun gets hotter than timber. The difference concerns dark, smooth, large surfaces above all, and those are exactly the elements a child sits or stands on.

Which is why platforms are made from perforated or treadplate sheet rather than a smooth panel, and the darkest shades are kept off seats. On a carousel that has a second benefit: the perforation drains water, so the platform dries faster after rain and nothing is left standing to freeze in a hollow over winter.

Safety does not depend on the material

Whether a unit is safe is decided by conformity with EN 1176: no entrapment gaps, no sharp edges, correct heights and safety zones. An uncertified timber carousel is exactly as unsafe as an uncertified steel one.

In practice, timber left without maintenance creates one risk that a steel structure does not: splinters and flaking at the surfaces children put their hands on.

When to choose timber

• When the site is in a park, next to a listed building, or anywhere visual coherence matters
• When there is somebody to look after it, for example a municipal works team with a crew and a schedule
• When the unit is large and multi functional and does not stand somewhere exposed to vandalism

When to choose steel

• When nobody will be carrying out annual maintenance
• When the site is open and unsupervised
• When the unit has moving parts, because there stiffness and repeatable manufacture are what count
• When the budget is a one off and makes no provision for spending in later years

That last point covers most public purchases. The money is there for the purchase; for upkeep there is usually none at all, and a unit that needs annual work then goes without it. A structure that tolerates being ignored is worth more on such a site than one that would look better if only somebody maintained it.