Levelling a Sloping Patio for Sliding Doors

Put a spirit level on almost any British patio and the bubble sits off centre. That is not poor workmanship; it is the entire point. Paving is laid to fall away from the house so rain runs off instead of standing against the wall, and the conventional figures are a fall of one in sixty to one in eighty — twelve to seventeen millimetres for every metre. Across a four metre run that comes to fifty or sixty-five millimetres of drop. Nobody notices it walking on it. A sliding glass panel notices it within about half a metre of travel.

At Roma Verandas this is settled at survey rather than left to installation day, because the answer changes the height the panels are made to and glass cannot be trimmed on site. This guide explains why a sliding run needs a level track, what tolerance it needs, the practical ways of achieving one on paving that falls, and how water is then kept moving at the threshold.

Why a Sliding Run Cares and a Patio Doesn't

Two problems arise. The first is gravity. A glazed sliding panel weighs forty kilograms or more and runs on low-friction rollers, which is what you want when you push it and not what you want when you let go. On a track falling sixty millimetres over four metres — a slope under one degree — a panel released halfway will drift, and a heavy one closes itself with more enthusiasm than anybody intended. Doors that will not stay where you put them are the commonest complaint from a run that was never level.

The second is geometric and less obvious. The head track hangs from the veranda front beam, and that beam is level because the frame is built square. If the sill follows the fall of the paving while the head stays level, the opening is not a rectangle but a shallow trapezium, and the panels, being rectangles, cannot fill it. At one end the gap at the top closes up and at the other it opens; brush seals lose contact, interlocks stop engaging along their full height, and a lock set to line up in one position no longer does. This is also why letting the track follow the ground is not an available answer.

What "Level Enough" Actually Means

Most systems ask for the sill to be level within a very small margin over the whole opening — commonly a couple of millimetres across three or four metres, and free of twist across the width of the track. That is a joinery tolerance being asked of a garden surface, which is the heart of the difficulty. It is achievable, but not by adjusting paving.

Flatness matters as much as level and is more often missed. A track correct at both ends but dipping two millimetres in the middle is still wrong, because the rollers ride whatever is under them at that moment rather than an average. Continuous support matters for the same reason: a track bedded only at intervals deflects as a panel passes over an unsupported stretch, and that deflection produces the binding a level survey said could not happen. The track wants a solid, flat, continuous bearing along its whole length, on a surface that will not move seasonally.

The Practical Ways of Getting a Level Track

The most robust solution is a levelled plinth or kerb under the run — engineering brick on edge, a cast concrete upstand, or matching paving on a full mortar bed, worked dead level with the patio falling away. Because the plinth top is level and the ground is not, its exposed height varies: near flush at the high end, perhaps fifty or sixty millimetres proud at the low end. That reads as a deliberate kerb provided it runs the full length of the elevation, and it lifts the track clear of surface water. The cost is a threshold step, worth thinking about if anyone in the household is unsteady on their feet.

The neater answer for smaller discrepancies is a level sub-frame. The side element is made as its own aluminium frame — cill, jambs and head — set level and packed off the paving, with the tapering gap beneath closed by a trim. It is quick, needs no masonry, and looks intentional while the gap stays modest. Past thirty or forty millimetres it stops looking like a frame and starts looking like a gap with a cover over it, and the void behind the trim collects leaves and grit. That is usually where masonry becomes the better answer rather than the more expensive one.

Bedding the track on a screed is the third option and the most limited. Feathering a mortar bed up to the shortfall works over small differences, perhaps twenty millimetres, but a thin feathered edge is fragile in frost and a thick one needs to be a compacted bed rather than a fillet pushed under with a trowel. The fourth is to lift and relay the strip of paving beneath the doors so that it is level, picking the fall back up beyond it. It gives much the best appearance, is the most disruptive and expensive, and leaves a flat apron outside the doors needing drainage of its own.

Which is right depends less on preference than on the size of the shortfall and whether the paving is staying. Sliding panels are also the forgiving case. Any bifold arrangement is more demanding, because each panel and hinge point adds its own small error, the panels must stay parallel throughout the folding movement, and a run that binds only when three-quarters folded is an awkward fault to chase afterwards.

Keeping Water Moving at the Threshold

The moment a level line is drawn across a falling patio, a small dam has been built across the path the water was taking. Rain blown under the roof, run-off from the paving above and any gutter overflow now arrive at a track that no longer sheds sideways. The usual answers are a channel drain immediately outside the run discharging to a gully or soakaway, a sharper fall on the paving just beyond, or the plinth approach above, which lifts the track out of the wash altogether.

Two things are worth knowing about the track itself. Aluminium tracks generally have weep slots to let water that gets in find its way out, so the outer face should not be sealed to the paving; a bead of mastic along the outside is a reliable way of turning a draining track into a reservoir. And it is honest to say that sliding glass doors on a veranda are weather-resisting rather than weathertight. They have no thermal break and no multi-point compression seal, because they enclose a covered outdoor space rather than a heated room. In driving rain a little water passes the brushes, so the threshold detail exists to drain it away quickly rather than to keep every drop out.

Why This Is Settled Before Anything Is Made

Glazed panels are cut to the opening. Add a sixty millimetre plinth and every panel becomes sixty millimetres shorter, which means different glass, not a shorter frame. There is no trimming and no packing that recovers it, so the levelling method has to be decided before the order is placed, not discovered when the installers arrive.

The height it takes also comes out of the opening. If the front beam is already at two metres because of a low eaves line, a plinth raising the sill by another sixty millimetres eats into the clear height people walk through. There is a sequencing point too: the surveyor measures the levels that will exist on installation day. If the patio is to be relaid, that work should be agreed and ideally completed first — a survey taken over paving about to change is a survey of something that will not be there.

An Honest Word About Old Patios

Sometimes the right advice is to deal with the paving rather than work around it. If a patio is spot-bedded on dabs of mortar and rocks underfoot, no amount of careful packing under a track will keep a sliding run sweet for ten years, because the surface it is referenced to is still moving. Levelling ground that is not stable buys a good first season and a slow decline after it.

It is also worth saying that a fair number of customers, once the options and their costs are laid out, decide the side in question does not need sliding doors at all. A fixed glass panel or a run of louvres has no running gear, cares far less about a millimetre or two, and costs less. That is often the better answer rather than a lesser one. If you have a patio with a pronounced fall and are unsure how much of a problem it really is, it is exactly the thing to measure and talk through before anything is ordered. Usually the shortfall is smaller than people fear.