Concrete Footing for a Pergola: A Detailed Guide to a Strong Base
A beautiful pergola starts with a base that does not move, settle, crack, or lift after the first winter. The visible part of the pergola is wood, metal connectors, fabric, polycarbonate, or decoration, but the part that decides whether the structure stays straight is the footing under the ground.
This guide explains how to make a concrete footing for a pergola in a realistic and practical way. It focuses only on the concrete base: no beams, no brackets, no post bases. The exact dimensions always depend on the soil, wind exposure, pergola size, roof covering, and local frost conditions. For large roofed structures, carports, weak soil, or public/commercial projects, a structural engineer should check the design.
Step 1: Mark, dig, and prepare the hole
Start by marking the exact position of each footing. Use string lines and diagonals to check that the layout is square. A few centimeters of error at the base can become a visible problem when the pergola is assembled.
The easiest way to dig is to cut the outline first with a sharp spade, then remove the soil in layers. If the ground is very dry, water the area lightly the evening before. Do not turn it into mud; the goal is only to soften the top layer. For round holes, an earth auger is fast. For a cleaner square footing, dig with a narrow spade and finish the corners by hand.
Do not leave loose soil at the bottom. The footing should sit on firm, undisturbed ground. If you dig too deep by mistake, do not fill the extra depth with loose soil. Use compacted crushed stone or a lean concrete layer instead.
Add 8–10 cm of crushed stone at the bottom and compact it well. This creates a cleaner base, improves drainage, and helps avoid the concrete sitting directly in wet soil.
Recommended footing sizes
| Situation | Practical footing size |
|---|---|
| Light decorative pergola without roof | 30 × 30 × 60 cm |
| Standard garden pergola with fabric, shade, or light covering | 35 × 35 × 70–80 cm |
| Larger pergola, windy place, polycarbonate, or heavier covering | 40 × 40 × 80–100 cm |
| Soft, filled, or uncertain soil | Wider footing and professional assessment |
For many small pergolas, vertical weight is not the biggest problem. The more serious risks are wind uplift, frost movement, soft soil, poor compaction, and concrete that was mixed with too much water.
Step 2: Formwork and optional reinforcement
If you want a clean visible top above ground level, make simple formwork from boards or OSB. A top height of 5–10 cm above the surrounding terrain is practical because it keeps the future fixing point away from standing water and mud.
For a very light pergola on firm soil, a plain concrete footing can be enough. For a more serious pergola, a roofed pergola, windy locations, or questionable soil, reinforcement is the safer option.
A practical reinforcement cage can be made with four vertical bars, usually Ø8 or Ø10 mm, and stirrups Ø6 mm every 15–20 cm. Keep the steel inside the concrete, not touching the soil or the formwork. Leave roughly 4–5 cm of concrete cover on all sides. Reinforcement that is too close to the edge can rust later and crack the concrete.
Do not place the reinforcement directly on the ground. Lift it on small spacers or stones so concrete can flow under it. The cage does not need to be oversized; it needs to be correctly positioned.
Step 3: Mix, pour, compact, and level the concrete
The best result comes from concrete mixed before pouring: in a bucket, mixing tray, wheelbarrow, or concrete mixer. Add clean water gradually. The mix should be workable, but not watery. Too much water makes pouring easier for a few minutes, but it weakens the final concrete and increases shrinkage.
Can you mix concrete directly in the hole? Only in specific cases. Some fast-setting post concrete products are designed to be poured dry into the hole and hydrated with water. That method should only be used when the manufacturer of the exact product allows it. For a pergola footing that should look professional and reliable, pre-mixing the concrete is usually the better choice because it reduces dry pockets and gives a more consistent result.
Pour the concrete in stages and compact it. Use a rod, piece of rebar, or narrow wooden batten to push through the mix and remove trapped air. Pay attention around the reinforcement cage and corners of the formwork. Poor compaction leaves voids, which reduce strength and make the concrete look bad when the formwork is removed.
Level the top with a trowel. It is better to form a very slight fall away from the center so rainwater does not sit on top of the footing. Cover the fresh concrete if the weather is hot, windy, or rainy. Do not load the footing too early. Concrete gains strength over time; the usual reference for full strength is 28 days, although light work can often continue earlier depending on the mix and conditions.
How much concrete do you need?
Use this simple formula:
Volume = width × length × depth
| Footing size | Concrete volume | Approx. 25 kg bags* |
|---|---|---|
| 30 × 30 × 60 cm | 0.054 m³ / 54 L | 4–5 bags |
| 35 × 35 × 70 cm | 0.086 m³ / 86 L | about 7 bags |
| 40 × 40 × 80 cm | 0.128 m³ / 128 L | 10–11 bags |
| 45 × 45 × 90 cm | 0.182 m³ / 182 L | about 15 bags |
| 50 × 50 × 100 cm | 0.250 m³ / 250 L | about 20 bags |
*The exact yield depends on the concrete product. Many 25 kg dry concrete bags produce roughly 12–13 liters of mixed concrete, but always check the bag.
Simple load check
A basic vertical load check is useful, but it does not replace structural design. Calculate:
Load per footing = total load ÷ number of footings
Soil pressure = load per footing ÷ footing area
Example: a 3 × 4 m pergola has 12 m² of area. If we assume 1.0 kN/m² total load from self-weight, covering, and temporary load, the total is 12 kN. With four footings, each footing carries about 3 kN. A 35 × 35 cm footing has an area of 0.1225 m². The soil pressure is 3 ÷ 0.1225 = 24.5 kN/m². On normal firm soil this vertical pressure is usually not the limiting issue. Wind, uplift, frost depth, and soil quality are often more important.
Common mistakes to avoid
- Making the hole too shallow for the local frost and soil conditions.
- Pouring concrete over loose soil at the bottom.
- Skipping the crushed stone layer and leaving the footing in wet soil.
- Adding too much water to make the concrete easier to pour.
- Using dry concrete in the hole when the product is not designed for that method.
- Putting reinforcement directly on the soil or too close to the concrete edge.
- Not compacting the concrete, leaving air pockets inside.
- Loading or fixing the pergola before the concrete has gained enough strength.
Practical conclusion
For a standard garden pergola, a good starting point is a 35 × 35 cm footing with 70–80 cm depth, compacted crushed stone at the bottom, clean formwork, and optional reinforcement. For larger, roofed, or wind-exposed pergolas, choose 40 × 40 cm and 80–100 cm depth, or ask for structural advice.
The foundation is not the place to save the most money. A slightly larger and better-prepared footing is cheaper than repairing a pergola that has moved, cracked, or leaned after one season.