Tarmacing & Line Marking

Black, flat, down in a day. Tarmac looks like the simplest thing on a groundworks job, and it is the one most often bought on the wrong question. The price per square metre tells you very little. What sits under it, how hot it was when it went down and where the water goes tell you almost everything.

Start with the word. Tarmacadam was stone bound with tar, and almost nothing laid now contains tar. Everybody still calls it tarmac, ourselves included, and no harm in it. What comes off the lorry is asphalt, graded stone coated in bitumen, mixed hot and laid before it cools.

What actually goes down

Machine-laid tarmac is normally two layers. The binder course goes first. Bigger stone, laid thicker, taking the structural work and ironing out the level beneath. The surface course goes on top, finer and tighter, and that is what sheds water, gives grip and takes the wear.

As a rule of thumb a domestic drive runs at something like 50–60mm of binder course under 20–25mm of surface course. A car park or access road wants more, set by the traffic and ground. What should give you pause is one thin lift of black over whatever is there.

The base decides how long it lasts

Tarmac is a flexible surface, which is a polite way of saying it spans nothing. It sits on what is below and copies it. A soft patch left at formation becomes a dip within a couple of winters. A sub-base tipped in as one loose heap turns into ruts along the wheel tracks.

So the money that decides the life of a surface is spent before any black arrives. As a rule of thumb a car drive wants at least 150mm of compacted sub-base, laid in layers, more on clay, usually over a geotextile membrane. Around Gerrards Cross, Beaconsfield and the Chalfonts you can meet gravel at one end of a lane and London Clay at the other, so the build-up is worked out on the plot. There is more on the ground across the villages we cover.

Tarmac does not hide a bad base. It photographs it, and hands you the picture two winters later.

Heat, joints and the roller

Laying asphalt is a race against it cooling. It leaves the plant hot, commonly between about 140°C and 180°C depending on the mix, and can only be worked inside a narrow window.

Falls, edges and the first few weeks

Tarmac is not permeable unless specified as an open-graded mix, so a surface has to shed water: a fall of around 1 in 60 to 1 in 80 on a drive as a rule of thumb, steeper across a car park, nearer 1 in 40, and always away from the house. Where that water goes, and the rules on paving a front garden, are in our drainage guide.

Edges matter more here than on almost any other surface, because tarmac has no edge strength of its own. Left unrestrained, the perimeter frets away a stone at a time. A kerb or edging, bedded and haunched in concrete, holds the shape.

Tarmac gains strength by cooling, not drying, so it is walkable within hours. But ask before you drive on it, because thickness, mix and weather move the answer. Through that first warm spell it marks easily: wheels turned on the spot leave scuffs, and skip legs press in.

A drive is light traffic, where the edges and the detail at thresholds and gates are what people notice. A car park takes standing loads, turning traffic and oil drip, so it wants a deeper build-up and falls that reach the gullies. An access road is heavier again, and one that may be offered to the council for adoption must be built to the highway authority’s specification from the sub-base up. That is a drawing-stage decision. Surfacing sits with the rest of the package.

Line marking, and why setting out comes first

Marking is the cheap end of the work and the part visitors judge the site by. It fails for one of two reasons: set out by eye, or painted too soon.

Setting out means a layout drawing, string lines and measurement. Bays square to each other, aisles wide enough to turn in, the grid checked before any paint goes down. The long-standing bay here is 2.4m by 4.8m with something like a six-metre aisle for ninety-degree parking, though many designers now work wider. Where a scheme has planning consent, its parking standards set the sizes. Hatching does the rest, holding fire routes and loading bays clear.

On new tarmac, wait. A fresh surface carries a film of binder that paint will not key to, so lines lift or bleed. A few weeks of weather is the difference between markings that last and markings you buy twice. Paint is a choice too: acrylics are cheapest and shortest-lived, chlorinated rubber holds colour better, hot-applied thermoplastic is thick and tough, MMA cold plastic harder still.

Two bays deserve care. An accessible bay is commonly 2.4m by 4.8m with a 1.2m hatched access zone alongside, about 3.6m overall, ideally with another 1.2m at the rear, on level ground. Off-street bays on private land are generally not enforceable the way an on-street bay with a traffic order is, and Equality Act 2010 duties are worth proper advice. Electric vehicle bays are a building regulations matter in England. Ask your building control body what Approved Document S requires. On site the point is simpler: ducts and bases go in before the surfacing, never after.

What makes a surface fail early

Nearly always one of a short list:

• A base that was never right. A formation covered over instead of corrected.
• Under-compaction: too few roller passes, or rolling started once it had gone cold.
• Cold joints left unsealed, or joints sitting square in a wheel track.
• No edge restraint, so the perimeter frets away season by season.
• Diesel and oil, which soften bitumen. A standing spill will eat a hole in time.

Tell us the area, the traffic it has to carry and what is under it now. The ground, the levels and the edges settle the price long before the black does. Request a quote and we will come and look.

Access decides how hot material reaches a job, and that is a local question. There is more on each of these.

• Chalfont St Peter
• Chalfont St Giles
• Little Chalfont
• Amersham
• Seer Green