7 Millimetre Fall per Metre Set out with a Stabila Level across a 10-Metre Patio Run

70mm Total Drop, Then the Pegs Go In

Start at the house wall and work out. A 10-metre run at 7mm per metre gives 70mm total fall away from the building, and that number gets marked on a datum peg before a single wheelbarrow of mortar arrives. Knock a peg in tight against the wall so its top sits 150mm below the damp proof course, which is two brick courses on most builds. That is the high datum. Then a second peg 10 metres out at the intended edge, and the top of that one drops 70mm lower.

Run a taut string line between the two peg tops. A Stabila 196-2 spirit level laid on a straightedge along that line confirms the bubble reads consistently downhill, not a see-saw of high spots. On a 10-metre pull the string will sag in the middle by a few millimetres of its own weight, so tension it hard and check the mid-point against a third peg. Most people who skip that third peg end up with a belly in the middle of the patio that ponds after every shower. The pegs cost minutes. Ripping out ponding flags costs a day.

Reading the Bubble on a Stabila Without Fooling Yourself

A 600mm level bubble does not directly show 7mm per metre. It shows level, or a rough tilt, and that is the trap. To read fall properly, set a packer of known thickness under the low end of the level. A 4.2mm packer under a 600mm Stabila gives you exactly 7mm per metre when the bubble centres, because 4.2 divided by 0.6 lands on 7. Cut a small hardwood block to that thickness and keep it in the tool bag. Every time the bubble sits central with the block under the downhill end, that stretch is running true fall.

The longer levels lie less. A 1800mm Stabila with a 12.6mm packer reads the same 7mm per metre and averages out the little undulations a 600mm skips over. Two vials also drift over years of being dropped in a van, so check the level against itself: read a surface, spin it 180 degrees end for end, read again. If the bubble jumps, the vial is off and every fall you set today will be wrong by that amount, compounding across ten metres into something you will feel underfoot.

Bedding Marshalls Fairstone on a Full Mortar Bed

Fairstone sandstone flags are riven, so the underside thickness varies plate to plate. That kills any dot-and-dab ambition straight away. These want a full wet bed of sandstone patio bedding mortar, mixed around 4:1 sharp sand to cement with just enough water to hold a squeezed shape without slumping. Lay a bed thick enough to bury the deepest flag with 25mm to spare, usually 40mm to 50mm of mortar, and screed it to the string.

Each flag beds down with a rubber mallet, tapping the high corners until the top face kisses the string line and the fall reads true on the level. Butter a slurry primer onto the back of every flag first. Sandstone is thirsty and pulls moisture out of the bed before it grabs, so the primer, a slurry of SBR and cement painted on the underside, gives a bond that survives frost heave. Skip it and you get hollow flags that ring when you tap them a season later, then lift at the corners.

Joints on Fairstone run 8mm to 10mm. Keep them consistent with spacers or an eye you trust, because a wandering joint width is the first thing a client notices from the patio doors. Point them once the bed has gone off, not while it is green, or the pointing drags the flags out of line.

Porcelain Wants a Different Back

Porcelain slabs have a dense, glassy back that bonds to almost nothing on its own. A porcelain slab priming slurry, brushed on and left tacky, is the only thing standing between a 20mm porcelain plank and a rocking slab within a fortnight. No primer, no bond, every time.

Polymeric Jointing and the Edge That Holds It All

Once the flags are down and the fall checks out end to end, the joints get filled and the edges get locked. These two jobs decide whether the patio still looks laid in five years or spreads out sideways like a dropped deck of cards.

Polymeric jointing is a graded kiln-dried sand mixed with a polymer binder that sets firm when watered in. It goes down dry, swept diagonally across the joints so it packs full without bridging the slab faces, then compacted with a soft brush and topped up. The watering matters more than anything. A fine rose on the can, several light passes, never a jet that blasts the sand back out. The polymer needs to wet through the full joint depth and cure, which on Fairstone at 10mm wide means the sand has to reach 40mm down. A surface skin over an empty joint cracks out by spring.

The fall you set at the start now earns its keep. Water runs off the slab faces and into the joints on any patio, and polymeric sand resists washout far better than a plain sand-cement slurry, but only if the fall carries the bulk of the water off the surface before it sits. Ponding kills polymeric joints faster than anything. That is the whole reason for the 7mm per metre in the first place, so the joints never sit under standing water long enough to soften.

Edges are the other half. A patio bedded on mortar with unrestrained edges will creep, and the joints open as it goes. Block paving edge restraint, a haunched concrete kerb or a purpose-made restraint strip pinned into the sub-base, holds the perimeter courses from spreading. On a mortar-bedded flag patio the haunching mortar itself often does the job, brought up against the outer flags at 45 degrees and kept 25mm below the top face so it stays hidden. On the low edge, where all 70mm of fall dumps the runoff, the restraint doubles as the last line before the water leaves. Get a channel or a gravel margin in there or the runoff undercuts the edge flags within a couple of wet winters.

Where a Resin Path Meets the Patio

A resin bound gravel path tying into the patio edge is a different beast to bed. Resin bound needs a solid open-textured base, usually a permeable macadam, laid to its own fall so surface water drains through the aggregate rather than sheeting off. Where the path abuts the low edge of the patio, the two falls have to agree or you get a trap line that collects grit.

Set the resin base 6mm below the finished patio edge and let the resin build up to meet it flush. The join wants an aluminium edging trim pinned along the patio flags so the resin has something clean to butt against, otherwise the outermost 50mm of resin ravels and picks loose underfoot. The fall on the path can run flatter than the patio, 1 in 80 is fine on permeable resin because the water goes down not across, but it must never fall back toward the patio edge.

So the question left hanging over the whole job: does the 70mm of surface fall carry enough water off fast enough for your soil and your rainfall, or does the low edge need a drainage channel taking the runoff away before it ever reaches the resin path?