Posts on Uneven or Sloping Ground

Put a long spirit level on almost any British patio and the bubble will sit off centre. That is not bad workmanship. Paving is laid to fall away from the house on purpose, usually between one in eighty and one in sixty, so that rain runs off rather than towards the wall. Across a three-metre projection that deliberate fall amounts to forty or fifty millimetres, and on an older garden that has settled or been relaid the variation across the width of a veranda is often a good deal more. A perfectly level patio is the exception, not the norm.

None of which stops a veranda being square, level and true, because the frame is not built off the ground — it is built off a set line, and the ground is made to meet it. At Roma Verandas this is the part of the job customers see least and worry about most. This guide explains how installers establish a level reference, how post lengths are worked out on a slope, why the roof line has to ignore the patio entirely, how foundations step with falling ground, and why packing under a post is not an acceptable answer.

The Roof Line Cannot Follow the Ground

A veranda roof has a pitch of its own, commonly somewhere around five to nine degrees, falling from the wall out towards the front beam so that water reaches the gutter. That pitch is set by the height of the wall bracket relative to the height of the front beam, and it runs in the projection direction only. Across the width, the front beam is either dead level or carries a small deliberate fall towards an outlet, depending on how the drainage is designed. Either way, the line it takes is decided on a drawing, not by the paving underneath it.

If posts were cut to a single length and stood on sloping ground, the beam would inherit the slope. Two things then go wrong. Water no longer arrives where the outlet is, which in a hidden drainage system means standing water in a gutter. More visibly, the roof line stops agreeing with the house. The eye is unforgiving here, because a beam is read against brick courses, a fascia and a fence line, all of them horizontal; five millimetres in a metre is obvious once noticed and impossible to stop noticing.

Establishing a Datum

The working method is to fix one reference and measure everything from it. The installer sets a datum — most often the line the wall bracket will occupy, chosen to give the pitch and headroom the design calls for under the existing eaves or soffit — and then transfers that horizontal plane out into the garden. A rotary laser on a tripod does this in minutes, throwing a level line that can be read at each proposed post position with a staff or a tape.

What comes back is a set of numbers rather than a single measurement: the distance from that plane down to finished ground at each post position. Those figures are what post lengths are calculated from, along with base plate height and the pitch drop between wall and front beam. The datum also settles headroom, which is worth asking about early. Where the ground falls away, clearance under the front beam is greater at the low end — a beam giving two and a half metres at one post may give two-sixty at the other — and the design has to work at the tight end, particularly where the structure passes over a door or a step.

Cutting Posts to Suit the Ground

On sloping ground, posts are simply different lengths. There is nothing improvised about this: it is normal setting out, and it is why posts are cut on site rather than arriving at a fixed length. The cut is always taken from the foot, never the head, because the top of the post carries a factory-formed connection to the beam and any drainage route through it. A cut foot then sits inside the base plate or shroud where it is never seen.

Base plates on most systems have some threaded adjustment built in — commonly ten to twenty millimetres of levelling travel, sometimes a little more. That range exists to take out the last few millimetres and to bring a frame plumb, not to absorb the fall of a patio. Trying to swallow forty millimetres of slope through levelling feet leaves the thread near the end of its engagement and the plate standing proud on a stalk, which is neither strong nor good to look at. Slope is dealt with by post length and foundation level; the feet finish the job.

Why Packing Under a Base Plate Is Wrong

The tempting shortcut on uneven ground is to stand the post where it lands and slide packers under the low side of the plate. It looks level afterwards, which is the problem. A base plate is designed to bear evenly across its whole face and to spread both the downward load and the tension generated when wind tries to lift the structure. Packed on two corners, it is instead a plate in bending, sitting on point loads, with the anchor bolts working in bending as well as tension.

There are secondary problems too. A gap under a plate holds water and grit against the underside of the aluminium and the head of the fixing, which is exactly where you would not choose to trap moisture, and packers themselves compress, slip or corrode. The correct answer is full bearing: either the pad is cast to the right level so the plate sits flat on concrete, or the remaining discrepancy is bedded with a non-shrink grout so load is carried across the whole footprint. Neither takes long, and both are the difference between a fixing that behaves as designed and one quietly working outside its intended range.

Foundations That Step With the Ground

Where ground falls away, foundations step rather than tilt. Each post gets its own concrete foundation, cast so that its top surface sits at the level that post needs, and dug to a proper depth measured from the ground beside it rather than from the highest point of the garden. That last point catches people out on a slope: a pad that looks deep from the uphill side may have very little cover on the downhill side, and it is the shallow side that governs how the ground behaves in frost and how much lateral restraint the pad actually has.

Proximity to an edge matters for the same reason. A post near the top of a bank, beside a step down to a lawn, or close to a low retaining wall is founded in ground with nothing holding it sideways. The usual approach is to set the pad back from the free face, deepen it so it bears below the retained ground, or tie the two together deliberately — and where an old retaining wall is involved, to look honestly at whether it is sound enough to be part of the arrangement. Raised decking raises a related point: a post must pass through the deck to its own foundation in the ground below, with a neat collar cut around it. Deck joists carry people, not a veranda resisting wind uplift.

The transitional cases are the ones worth flagging at survey. A structure landing with one post on an established patio and another in a lawn has two very different ground conditions to reconcile, and both foundations need to reach the same firm material if the frame is not to twist over time. Terraced gardens raise a design question before an engineering one: whether the structure serves the upper level or the lower, because spanning both means much longer posts on the low side, more wind leverage on them, and a headroom difference that has to be liveable.

Getting It Right on the Day

Almost none of this is difficult. It is measuring, thinking and a certain amount of digging, and a competent installation team does it as routine. What it is not is something to resolve once the frame is out of the van, because post lengths, foundation levels and beam heights all come from the same set of figures and are decided together. A slope nobody recorded at survey becomes a decision made in a hurry by someone standing in a garden with a delivery to unload.

So it is worth mentioning uneven ground at enquiry stage even if it seems minor — a patio that ponds after rain, a step you have stopped noticing, decking of uncertain construction. Photographs taken from the far end of the garden looking back at the house show levels better than any measurement. If the ground behind your house is anything other than flat, that is a normal situation with a normal answer, and the time to work through it is while the drawing is still a drawing.