Mounting Onto Anything But a Concrete Base

The textbook installation has four posts on four concrete pads on flat, uniform ground. A good many real gardens do not offer that. There is a retaining wall along one edge, a step halfway across, an old concrete base at one end and soft border at the other, or the only sensible position puts two posts on a terrace and two on the level below it.

All of these are workable. What they are not is improvisable on the day, because each one changes how load reaches the ground and what the structure needs to stay square. This is what to think about when the posts are not all landing on the same thing.

Mixed Materials Under One Structure

The most common awkward case, and the one people most often walk into without noticing: two posts on a solid old slab, two on a border that used to be a lawn.

The problem is not that either support is inadequate. It is that they behave differently. Concrete over a compacted sub-base does not move. Soil does — it swells and shrinks with moisture, it heaves in frost, and it consolidates under sustained load. Over a decade you get differential movement, which is the technical term for one end of your structure going down while the other stays where it was.

Differential movement is worse than uniform settlement, because a structure that drops evenly is merely lower, while one that drops at one corner goes out of square. Glazing that was cut for a square opening does not fit a parallelogram, and sliding doors that were smooth become stiff.

The answer is to make every post's support the same, whatever is around it: a properly founded pad taken down to consistent bearing at every position, rather than using the good ground where it happens to exist and doing something lighter where it does not.

Posts on or Against a Wall

Sometimes the only place a post can go is on top of a low wall, against a retaining wall, or hard against a boundary.

On top of a wall is the case to be most careful with. Garden walls are built to carry their own weight and resist a bit of wind. They are not designed for a concentrated point load applied at the top, and a single-skin wall in particular has very little capacity for it. The load also arrives with a horizontal component from wind, which is exactly the direction a free-standing wall is weakest in. A post on a wall needs somebody to look at the wall, and often needs the load taken past it into a pad beside or behind it.

Against a retaining wall is a different question, because a retaining structure is already loaded by the ground behind it and adding to that is not neutral. The safe approach is to keep foundations clear of the zone that loads the wall, which usually means bringing the post line forward — and that has a design silver lining, since setting posts back from the edge produces the overhanging terrace-roof look people pay extra for elsewhere.

Hard against a wall has a purely practical catch worth knowing: where concrete bases are used, a post cannot sit tight to a wall because the block is wider than the post. The manufacturers' remedy is to saw the flange off one side of the post and drill two new holes so it can still be fixed in four places.

Multi-Level Sites

A structure spanning a step or a change of level is entirely normal, and the geometry is simpler than it looks — the posts are just different lengths.

What matters is that the gutter stays level along its length. The roof falls across the structure at its fixed pitch, not along it, so a gutter that follows the ground is a gutter that will not drain to its outlet. Every post is therefore measured individually from its own base up to a common datum, established with a laser or a straightedge before anything is cut.

The other consideration is that a taller post is a longer lever. Wind pressure on the roof applies a moment at the base of every post, and that moment grows with height, so the posts on the lower level are working harder than their neighbours and their foundations should reflect it. The manuals make the same point in a different way: the published load figures assume a maximum height from floor to top of gutter, and different figures apply to taller posts.

Where the level change is substantial, it is often better to build across it with a deeper pad on the low side than to build a retaining structure specifically to level the ground.

Raised Terraces and Structures

Building on top of something else — a raised terrace, a flat roof, a garage, a suspended deck — is the case where the answer genuinely may be no, and it is worth establishing that early.

The question is whether the structure underneath was designed to take a new permanent point load plus wind uplift, and the honest answer is that unless somebody designed it for that, nobody knows. Existing terraces and flat roofs were designed for a distributed load, not four concentrated ones, and uplift is a load case they were almost certainly never assessed for.

It also introduces waterproofing. Any fixing through a flat roof or a terrace deck is a penetration in a waterproof layer, and the detail around it is the thing that will or will not leak in ten years.

This is the one situation in this whole subject where the right move is a structural engineer rather than a good installer, and it is a modest cost against finding out the hard way.

Bracing Matters More Here

One thread runs through all of these situations. The more unusual the support conditions, the more the structure depends on being stiff in itself rather than being propped by circumstance.

A wall-mounted structure is braced along one edge by the building. A freestanding one is not, and everything that stops it racking comes from the connection detail and from the posts being genuinely fixed at their bases. On a mixed or multi-level site, the case for proper bolted foundations at every post stops being about settlement and starts being about the frame staying square.

How to Bring It to a Supplier

Photographs from several angles, and a plain description of what is under each post position: this is a slab, this is border, this is on top of a wall, this end is two courses lower. Add anything you know about what is buried — a soakaway, a service run, an old base.

Find Out What Is Already Buried

Before any digging on an awkward site, establish what is down there. It is the step people skip and the one that turns a straightforward day into an expensive one.

Drain runs and inspection chambers are usually visible if you look for covers and follow the logic from the soil stack. Water and gas supplies come in from the boundary towards the meter, often in a straight line you can guess at. Electricity to a garage or outbuilding is the one most likely to be shallow and unmarked. Where a service crosses a proposed post position, the post moves — which changes the bay widths, which changes the whole layout, and that is exactly the sort of thing you want to discover before a drawing is signed off rather than with a spade in your hand.

That is the information from which a layout gets derived, and it is worth giving before a drawing exists rather than after. A site that has been described honestly can nearly always be built on. A site that gets described as flat and turns out not to be is where installations go wrong.