Gravel-and-Compacted-Soil or Concrete Footings for Structure Posts
Property owner deciding what depth and method of foundation to use for ground-level pergola or canopy posts.
When a pergola, canopy or veranda sits on the ground rather than being wall-attached, its posts need stable bases. You can use compacted gravel in prepared holes, or you can dig deeper and set the posts in concrete. The cheaper, quicker option doesn't always stay stable long-term. This page explains the reality of each approach.
Gravel Base: Quick Installation, Settling Risk
Gravel-and-compacted-soil bases are quick. The installer digs a hole (typically 300–450mm deep), adds gravel in layers (100mm base, compacted; then another layer; repeat), and sets the post base or shoe on the compacted surface. The work takes a few hours per post. Cost is low—just gravel and labour. The risk is settling. Soil underneath the gravel can compress over years, especially if the ground is clay or there's underground water movement. As the base settles, the post shifts slightly, which shifts the whole structure. Bolts loosen, panels can rack (go out of square), and in winter, frost heave can lift a post, then thaw can settle it lower than it started. The lifespan of a gravel base is typically 5–8 years before settling becomes noticeable; then you're adjusting bolts and checking level regularly.
Concrete Base: Deeper, Permanent, Initial Disturbance
A concrete base means digging deeper (typically 600–900mm depending on soil conditions and structure weight), removing poor soil, adding a gravel base, and then pouring concrete around the post shoe or into a footing. The concrete sets hard and doesn't settle (assuming correct depth and good concrete). The post stays in exactly the same spot for 20+ years. The upfront work is more: deeper digging, concrete mixing or ordering a truck, waiting for the concrete to cure (usually 48 hours before you can build on it). Cost is higher—concrete, labour, and time lost while it cures. But the long-term stability is genuine.
Soil Type and Ground Conditions Determine Which Is Adequate
On stable ground—clay with good drainage, or chalk—a 450mm compacted gravel base might stay put for 10+ years. On soft ground (peat, silt, made-up ground), it settles faster. On sloping ground, water movement under the base accelerates settling. In an exposed location where the structure moves in wind, post movement is amplified. The installer should assess your soil and advise: if the ground is poor quality or the structure is large and exposed, concrete is essential. If the ground is stable and the structure is modest, gravel might be adequate, though concrete is always the safer choice.
- Gravel: 300–450mm deep, quick, settles over 5–8 years
- Concrete: 600–900mm deep, permanent, needs curing time
- Gravel: suitable for stable soil, modest structures
- Concrete: essential for poor soil, large structures, exposed sites
Frost Heave and Winter Movement
In winter, if groundwater near a gravel base freezes, it expands (frost heave), pushing the post up. When it thaws, the post settles back—usually slightly lower than it started. Over winters, this creep adds up. Concrete bases, being deeper and below the typical frost line in the UK (usually 600mm+), avoid frost heave almost entirely. Another reason concrete is more stable long-term.
Maintenance and Adjustment Over Time
A gravel-based pergola needs annual checking: look for tilting posts, check that bolts are still tight (they work loose as the base settles), and be ready to shim or re-tighten. A concrete-based structure rarely needs that attention. After ten years, a gravel base usually needs reworking—digging down, removing settled gravel, recompacting, and possibly adding concrete if you're tired of the maintenance. By that point, you've invested time and money in upkeep; concrete from the start would have been simpler.
Cost Comparison Over the Structure's Life
A gravel base is cheaper to install but needs periodic maintenance and reworking every few years, which adds up over time. A concrete base costs more upfront but needs almost no ongoing spend afterwards. Over 15 years, both might end up broadly similar in total cost, but concrete wins on convenience and reliability. Many installers now favour concrete even for modest structures, because it reduces future complaints and call-outs.
Common questions
How deep do concrete footings need to be to avoid frost heave?
In the UK, 600mm is generally safe; 750–900mm is better if the soil is very wet or the location is exposed. Your installer should advise based on soil survey and local conditions.
Can I upgrade a gravel base to concrete after the structure is installed?
Yes, but it's disruptive. You'd need to support or remove the structure, dig out the gravel, pour concrete, and reinstall. Easier to get it right the first time.
Is concrete better for a coastal location where soil might move with salt water?
Yes. Concrete is more stable in saline environments because it doesn't soften or compress like soil. Coastal sites almost always benefit from concrete bases.
How long does concrete need to cure before you can attach the structure?
Usually 48 hours for initial strength, but 7 days is better if you can wait. This is a real delay in the installation schedule.
Do gravel bases need drainage, or does water just soak into the ground?
Gravel allows drainage, but if the base is in a low spot and water pools, you get settling and frost heave risk. Slope and drainage are important for both gravel and concrete.
In short
Gravel bases are fast and cheap but settle over 5–8 years, requiring ongoing maintenance. Concrete bases cost more upfront but stay stable for 20+ years with virtually no attention.
What to do next
Unsure whether to go with a quick gravel base or invest in concrete footings for long-term stability? We can assess your site, soil conditions and structure size, and recommend the foundation that'll keep your structure solid for years.
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