Metal or timber frame for a terrace: which to choose

How each material copes with ground movement and frost

Estonian ground freezes every winter, and clay or poorly draining soil can heave a few centimetres as it does. Any terrace substructure that sits directly on the ground, rather than on posts driven below frost depth, will move a little with it. The question is how the frame tolerates that movement without the deck surface following it visibly.

A timber frame on concrete post footings or screw piles moves with the posts, and timber itself has a little give at the joints, which absorbs small shifts. The risk is at ground contact: timber that stays damp against soil or snow cover rots from the inside before it looks bad from outside, even when pressure-treated.

A welded steel frame does not rot, but it is rigid. If footings are not set below frost depth or move unevenly, a steel frame transmits that movement to every joint at once rather than flexing locally, and a weld can crack rather than creak. Get the footings right and this is not a practical difference; get them wrong and steel is less forgiving.

What it costs to build, and what gets added later

Timber substructure is usually the cheaper starting point: the material itself costs less than steel, and it is straightforward to cut and adjust on site as the ground reveals itself. It suits terraces where the budget is set early and the ground is reasonably firm and well-drained.

A welded steel frame costs more to fabricate and fit, since it involves cutting, welding and treating steel sections rather than bolting timber together. We price steel structures from 250 € per job and welding work from 55 € per hour, and a fixed quote follows once we know the span and the ground conditions.

The costs that come later differ too. A timber frame usually needs sections replaced within its working life, priced as ordinary carpentry. A steel frame's later costs are mostly about the coating, not the structure: a re-treatment of exposed steel is a smaller and less frequent job than replacing rotten joists.

The maintenance each one actually needs

Timber substructure needs the same attention as any outdoor wood: keeping it clear of leaves and soil contact, checking for soft spots where water collects, particularly near the ground and around post connections, and re-treating cut ends where the original preservative did not reach.

Steel needs almost no attention if it left our workshop hot-dip galvanised or properly primed and painted, but it is not maintenance-free forever. Scratches through the coating, especially where decking screws or clips bite into the frame, are where rust starts, so an occasional look underneath the deck boards is worth doing.

Neither material removes the need to check the decking boards themselves, which weather regardless of what sits beneath them. The frame choice affects how often you think about the substructure, not whether you think about the terrace at all.

Which suits a typical Tallinn house or flat's terrace

For a ground-level terrace off a house in Nõmme or further out in Harju county, on reasonably stable ground and a modest budget, a well-built timber frame on proper footings is a sound and familiar choice, and it is what most terraces here are built with.

A steel frame earns its cost where the terrace is raised well above the ground, spans further between supports than timber comfortably allows, or sits on ground that is soft, uneven or likely to settle unevenly, such as reclaimed or filled plots common on newer developments.

It also suits terraces attached to an apartment building, where a korteriühistu wants a structure that will not need structural repair within a normal maintenance cycle, even if the decking on top is replaced sooner.

What to settle before you decide

The ground matters more than either material's general reputation. A soil check or at minimum a look at how neighbouring structures have settled tells you more about frost risk than any comparison of steel against timber in the abstract.

Decide the terrace's height, span and load before the material, since a low terrace close to the ground rarely needs steel's extra strength, while a raised or long-span deck often does regardless of preference.

We build both, quote each on its own footings and span rather than by default, and our decking and terrace work starts from 90 € per job once the substructure is agreed.

• Check how wet or clay-heavy the ground is where the terrace will sit
• Decide the terrace's height above ground and the span between supports
• Ask whether existing structures nearby have settled or heaved
• Set a maintenance expectation: periodic re-treatment versus occasional inspection
• Get a fixed quote for the frame material you are leaning towards before comparing costs