Lay a Curved Cotswold Buff Path into a MOT Type 1 Sub-Base at a 1-in-80 Fall

Set the line before the slabs come out

Leave the Cotswold Buff sandstone in the crate until the two datum points are fixed. On a 9m run at 1-in-80, the finished surface has to drop 112.5mm from the high point to the low point. Pin the string line at both ends, set the low end 112.5mm lower, then take the intermediate levels from that reference. On a bend, the fall is still measured along the centreline of the path. The outer edge travels farther than the inner edge while sharing the same overall drop.

That difference catches out plenty of curved paths. Using the outer arc for the level marks makes the inner arc steeper than 1-in-80, which encourages water to sheet towards the inside of the curve. Work from the developed length of the centreline, put intermediate pegs in at 1.5m centres, and check them with a spirit level and a gauge block cut to 18.75mm, the drop over 1.5m. Cotswold Buff is a pale calcareous sandstone, so standing water and dirt tracking show up more quickly than they do on a riven grey. A slight backfall can be visible within a season.

Build the MOT Type 1 to keep the fall intact

MOT Type 1 to the Specification for Highway Works clause 803 is a graded limestone or granite material crushed from 40mm down to dust. The fines matter because, at the right moisture, they lock the coarse aggregate during compaction.

Spread it in layers no thicker than 100mm before compacting. A single 200mm tip may look firm after the plate has crossed it, yet the lower band can stay loose and settle later. When that happens, the carefully set 1-in-80 fall gains a low spot and the path starts to pond.

As a working figure, allow roughly six passes per layer with a 2-tonne plate compactor. The plain site test is visual and tactile: after the final pass, the plate stops making the surface rebound and a boot heel leaves no print. The moisture wants to be damp enough to bind, with no pumping of fines to the surface.

For a 9m curved path at 900mm wide, a 150mm consolidated sub-base takes close to 1.9 cubic metres of Type 1 after allowing for about 20 percent loss from loose volume to compacted volume. Ordering 2.3 loose cubic metres gives a usable margin. On the curve, trim the sub-base to the finished paving fall instead of leaving the correction to the mortar bed. The laying course can then stay in the 30 to 40mm band, including at the inside of the bend where it is easy to shave it too thin.

Make the curve without a radial saw bench

Rectangular calibrated slabs can form a gentle sweep with tapered joints before cutting becomes unavoidable. On a 4m radius, widening the joints from 8mm on the inner arc to 14mm on the outer arc can absorb the curve across four or five courses. Once the radius tightens past 3m, the joint taper stops hiding the geometry and the slabs need cutting.

A 115mm angle grinder with a continuous-rim diamond blade will score the top face, and a straight cut can be snapped over a batten. Curved work needs a wet full-depth cut so the pale Buff face does not burn or glaze. Dry cutting calcareous sandstone throws silica dust and can scorch the surface, leaving a grey bloom that will not scrub away. Mark the radius with a trammel made from a screw at the arc centre, a batten, and a pencil set at the required distance. Score every unit against that line before committing to the cut. A bad cut in a 600 by 300mm Cotswold Buff unit is costly, and batch colour variation means a later single replacement rarely blends with the surrounding stone.

Efflorescence on a pale sandstone face

Cotswold Buff and most Indian sandstones can develop a white salt bloom in the first months after laying. This is efflorescence: soluble salts in the stone and bedding mortar move to the surface as water evaporates, then crystallise. Darker stone can disguise the deposit, while Buff tends to show it as a chalky film across the path, so the installer often gets the blame.

Early acid treatment is the damaging move. First efflorescence usually clears over three to six months of weathering because the salt reservoir is finite. Rain dissolves and disperses the salts after the initial migration has run its course.

If the bloom remains after a full wetting and drying cycle, use a dedicated Indian sandstone efflorescence treatment based on a buffered acid. Apply it to a pre-wetted surface and rinse thoroughly so the crystalline deposit lifts without etching the stone.

Neat hydrochloric acid, sometimes sold as brick cleaner, will burn the calcareous surface and open the pores. More open pores then draw in more salt and staining. Bedding also affects the problem: a full mortar bed at a 4-to-1 sharp sand to cement ratio, laid wet and not soaking, carries far less soluble salt than a sloppy over-wet mix. Keep bedding mortar off the visible arris as well, because smearing the face during laying creates another salt source unrelated to the stone itself.

Choose jointing by the actual joint width

For tapered curve joints running from 8mm to 14mm, a two-component polyurethane-bound grout such as Rompox Easy sits in the useful range. Joints below about 5mm are too tight for a two-component grout such as Rompox to compact properly. A brush-in slurry starts to slump and stain the arris once the joint is above about 3mm.

Work in sections small enough to finish inside the pot life. For a two-part resin that window is short, often 45 to 60 minutes once the components are mixed. Wet the joints, pour the mixed grout, drive it down with a stiff broom and a rubber squeegee held at a shallow angle, then clean the residue from the face while the resin is still mobile. On pale Buff, cured resin residue leaves a permanent sheen that later scrubbing will not remove.

Temperature changes the job. Below about 5C the resin will not cure, and above 25C the pot life collapses. A two-component product copes better with foot traffic and freeze-thaw movement than a straight cement slurry, which cracks and lets water track into the bedding. At a threshold or other hard edge, a narrower joint still has to give the resin enough key, since a 3mm doorway joint is the first weak spot for a wide-joint product.

Put a drain where the fall sends the water

At the low end of a 1-in-80 path, the water shed by the whole surface arrives in one place. On a curve, that place is often the inside of the bend against a wall or lawn edge. An ACO HexDrain channel across the low end deals with that discharge when it is bedded in concrete and its grating is set 3mm below the finished paving, allowing water to run in instead of skipping across. The outlet then goes to a soakaway or a surface-water connection.

A permeable resin-bound surface over the bottom two or three metres takes a different approach, letting water pass through the surface into a free-draining sub-base. That only works when the sub-base below it is actually permeable. MOT Type 1, with its binding fines, is not. A permeable path needs Type 3 open-graded sub-base beneath the resin-bound zone.

Combining a bound Buff path on Type 1 with a resin-bound apron over the same Type 1 creates the shape of a drain without the drainage function. With a linear channel at the bottom, the build remains impermeable and the HexDrain carries the discharge. A resin-bound apron over Type 1 still behaves like an impermeable low point with no outlet.