Foundations for Timber Structures – How to Plan Pad Footings, Slabs & Ground Screws

Whether it's a pergola, carport or garden shed: the lifespan of your timber structure is decided down at the foundations. This guide explains which foundation type suits which project, why frost depth matters so much, and how to build a concrete pad foundation yourself, step by step.

Why the foundation decides how long your structure lasts

Timber is a forgiving building material – as long as it stays dry. A post set directly in the ground will rot through in 5 to 15 years, depending on the species. A foundation therefore has two jobs: it transfers the loads of your structure safely into the ground, and it keeps the timber permanently clear of damp soil. Then there is anchorage against wind: a carport roof with 30 m² of area acts like a sail in a storm – without a solid connection to the foundation, wind uplift can lift the whole structure. Cut corners here and you will pay later with leaning posts, sticking doors and costly repairs.

The four foundation types compared

For a pergola, carport, garden shed or patio roof, essentially four foundation types come into question. Which one fits depends on the load, the ground conditions and your budget.

Pad foundation (concrete post footing)

The classic for post-built structures: a single concrete pad sits under each post, typically 30 × 30 cm to 40 × 40 cm in cross-section and around 80 cm deep. Pad foundations are cheap, can be dug by hand or with an earth auger, and are almost always the first choice for pergolas, carports and light canopies.

Strip foundation

A continuous concrete strip under load-bearing walls, as a rule of thumb 30 cm wide and 80 cm deep. Worthwhile for garden buildings with load-bearing perimeter walls, or as a frost-free bearing for a thinner slab. The effort for excavation, formwork and concrete is considerably higher than for pad foundations.

Concrete slab

A continuous reinforced concrete slab, usually 15 to 25 cm thick, laid on a compacted gravel bed with a membrane as a damp-proof barrier. Ideal for garden rooms, tool sheds and workshops, because the slab doubles as the finished floor. Around the perimeter it is often combined with a thickened frost skirt taken down to frost-free depth.

Ground screws (screw piles)

Galvanised steel screws 60 to 100 cm long are driven into the ground by machine or with a turning bar. No concrete, hardly any digging, immediately loadable and completely removable without a trace – ideal for pergolas, fences, raised decks and light lean-to extensions. Their limits: very stony ground, loose fill and high point loads.

All figures are rough guide prices for buying materials. Regional prices, soil class and disposal costs for the excavated spoil can shift the budget considerably.

Frost depth: why 80 cm is the critical mark

Water expands by around 9 per cent when it freezes. If the ground beneath a foundation freezes through, frost heave lifts the foundation – in spring it settles back down, but rarely to exactly its old position. After a few winters the structure leans, screwed connections pull out and the roof twists. That is why the rule of thumb across much of Central Europe is: take foundations down at least 80 cm. In cold regions and in the mountains, 100 to 150 cm may be necessary – local requirements always take precedence, so check with your local building authority.

For very light structures such as a small pergola, builders sometimes found shallower in practice and instead install a thick, capillary-breaking gravel layer in which no standing water can collect. That only works in free-draining ground – when in doubt, dig the full 80 cm.

Post anchors and post bases: protecting timber by design

The golden rule: timber never goes directly into concrete and never sits directly on the ground. Galvanised steel post supports create clearance – as a guide, at least 15 cm between the underside of the post and the ground, so that splash water cannot keep soaking the end grain.

Cast-in H-shaped post anchors

The H anchor is set directly into the fresh concrete pad and is the most rigid standard solution for pergolas and carports, because it also resists bending forces. The anchor width must match the post, for example 121 mm for 12 × 12 cm posts, with a steel thickness of usually 5 to 6 mm. The drawback: the anchor has to sit perfectly plumb and in line while you are pouring – there is very little scope for correcting it afterwards.

Bolt-down post bases

Bolt-down post supports in U form or with a concealed pin are fixed with heavy-duty anchor bolts only after the concrete has cured. The big advantage: you can still correct the position by a few millimetres, and height-adjustable models additionally take up 2 to 10 cm of level difference between the footings. On tall structures with high wind loads, however, they are somewhat less rigid in bending than a cast-in H anchor.

Step by step: building a pad foundation yourself

Here is how to build a classic 40 × 40 × 80 cm pad foundation for a 12 × 12 cm post:

1. Set out the positions: transfer the post grid using string lines on profile boards and a tape measure. Check the diagonals – on a rectangular layout, both must be exactly the same length.
2. Dig the hole: 40 × 40 cm footprint, 80 cm deep – with a spade and post-hole digger, or with an earth auger (Ø 20–30 cm, widened out at the bottom).
3. Lay a gravel bed: fill in and compact 10 cm of gravel or crushed stone. This breaks rising damp and creates a level, load-bearing base.
4. Mix the concrete: a footing this size needs around 0.13 m³ of concrete, which is roughly 10 to 11 bags of 25 kg ready-mix concrete (guide value, strength class C20/25).
5. Fill in layers: place the concrete in lifts of 20 to 30 cm and rod each layer with a stick so that air bubbles can escape.
6. Set the H anchor: push the anchor into the fresh concrete, plumb it up with a spirit level and check line and height against the string. All anchors should end up at the same height – small differences can be taken up later in the post lengths.
7. Let it cure: after 2 to 3 days you can carefully carry on building; concrete only reaches its full strength after around 28 days. Keep the surface damp in hot weather – and never pour concrete in freezing conditions.

Common mistakes – and how to avoid them

• Concreting posts in: capillary action draws water into the timber and the post rots invisibly inside the concrete. Always use a post anchor.
• Ignoring frost depth: budget footings only 40 cm deep heave in the first hard winter and stand crooked afterwards.
• Setting anchors by eye: without string lines, small deviations add up. Even a 2 cm offset becomes painfully obvious when you lay the purlins.
• No gravel bed: standing water under the footing encourages frost heave and settlement.
• Loading too early: putting heavy beams on the anchors after just one day risks cracks in the green concrete.

Permits, structural design and planning

How deep, how big, and whether you need permission at all – that is governed by local planning rules and building regulations, which can vary considerably from one area to the next. For larger roof areas, high snow-load zones or sloping sites there is no way around a structural engineer. All dimensions and prices in this article are guide values for early planning; the local regulations always apply – check with your local building authority and, when in doubt, have the numbers verified by a qualified professional.

For the build itself, a clean foundation plan with all the axis dimensions pays off. Tip: in a 3D planner such as HolzBau 3D you can read off the post positions of your structure to the millimetre and transfer the axis dimensions straight to your string lines – so every anchor sits exactly where its post will stand.