Setting metal posts for a terrace roof or pergola: a DIY guide

How deep and how wide to dig the hole

In most of Harju county the frost line sits around 1–1.2 metres, and a post that stands proud through winter on a shallower footing will heave as the ground freezes and thaws. For a small terrace roof or pergola, dig to at least 80–100 centimetres, more if your soil is heavy clay that holds water. Sandy, free-draining ground is more forgiving.

The hole diameter matters as much as the depth. As a rule of thumb, make it three times the post's own width, so a 100 mm steel post wants a hole around 300 mm across. Too narrow and the concrete has no real body around the steel; too wide just wastes concrete for no gain.

Use a post-hole auger or a narrow spade rather than a general excavation. A hand or petrol auger keeps the sides straight, which matters when you come to pour, and disturbs far less of the surrounding lawn or gravel than digging by hand.

Which concrete mix to use and how to pour it

A standard post-mix concrete, or a site-mixed C20/C25 with a fairly stiff, low-slump consistency, both work well for this. Buy ready-mixed post-fix bags if you're only setting a handful of posts; it's more expensive per kilogram but there's no batching to get wrong and no wasted material.

Put 5–10 centimetres of compacted gravel or crushed stone in the bottom of the hole before the concrete goes in. This gives the post something free-draining to stand on rather than sitting in a puddle at the base, which is where rust starts on steel posts far more often than anywhere further up.

Pour in stages if the hole is deep, tamping as you go to knock out air pockets, and finish the concrete in a slight dome above ground level so rainwater runs away from the post rather than pooling around it.

Getting the post plumb and giving it time to cure

Check plumb on two faces at 90 degrees to each other with a spirit level, not just one side, and brace the post with two timber battens screwed to stakes driven into the ground either side. Recheck after the first pour stage, since concrete can shift a post slightly as it settles.

Leave the braces in place and leave the post alone for at least 48 hours before you put any load on it, and a full week before you fix rafters or a roof structure to it. Concrete gains most of its working strength in the first days but keeps hardening for weeks, so treat early loading as the main way this job goes wrong.

Keeping water away from the base of the post

Steel posts fail at ground level long before they fail anywhere else, because that's where water collects and where the surface is hardest to inspect once the post is finished. The concrete collar sloping away from the post, mentioned above, is the first line of defence.

Beyond that, a coat of bitumen paint or a proper anti-corrosion primer on the buried section of the post before it goes in the ground adds real protection, and is worth the extra hour regardless of what mix you use.

Avoid leaving a bare gap between post and concrete at the surface where water can sit and freeze; either finish the concrete tight against the steel or use a proper post-base bracket with a standoff, which also lets you inspect and replace a post later without breaking concrete.

• Gravel bed at the bottom of the hole, not bare soil
• Concrete finished as a dome, sloping water away from the post
• Anti-corrosion coating on the buried section before pouring
• No bare gap at ground level where water can collect and freeze
• Post-base brackets as an alternative where future replacement matters

When the structure is too big to set posts yourself

A small terrace roof or pergola over a few square metres, on four or six posts, sits comfortably within DIY territory if you follow the steps above. Once the roof gets larger, carries a heavier covering such as tiles or a green roof, or the posts need to resist real wind load because the structure is enclosed on one or more sides, the calculations behind post depth, diameter and spacing stop being rules of thumb and become genuine structural sizing.

That is also the point where welded steel connections, rather than bolted brackets, start to matter for a rigid, long-lasting frame, and welding this scale of steelwork well is a specific skill rather than a DIY afternoon.

For anything past a small pergola, we quote steel structures from 250 € per job as a fixed price after seeing photos or a sketch, and welding work separately from 55 € per hour where an existing frame needs joining or repair. If the terrace decking itself is part of the project, that's quoted from 90 € per job.