Tarmac Driveway Installation: Sub-Base, Binder Course, Surface Course Depths and Edging Requirements

Quick Answer: A correctly specified domestic tarmac driveway has a 150mm compacted MOT Type 1 sub-base, a 50mm binder course (typically AC 20 dense bin), and a 25–30mm wearing course of AC 6 dense surf or 6mm SMA, edged on three or four sides with concrete-haunched kerb or paving block, and rolled hot to refusal. Standard tarmac is impermeable, so any new front driveway over 5m² triggers planning permission unless drained to a permeable area.

Summary

Tarmac is the cheapest hard surface for a vehicular driveway and remains the standard finish on most rural and semi-rural homes in the UK. The technical name is bituminous macadam (or asphalt concrete in modern BS EN terminology); "tarmac" is a colloquialism that has stuck since the original Tarmacadam Ltd patent of 1902. Modern domestic tarmac uses bitumen, not coal tar.

The big advantage is cost — a 100m² drive can be laid in a day for £4,500–£7,000 — and the simplicity of repair (overlay or pothole patch). The disadvantages are environmental (fossil-derived binder), aesthetic (limited colour options beyond black), and regulatory (the 5m² SuDS rule for front drives).

The skill on a tarmac job is in the temperature management. Bitumen-bound material has a narrow workable temperature window — too cold and it won't compact; too hot and the bitumen runs. Failures usually trace back to the lay-and-roll temperature, base preparation, or edge restraint inadequacy.

Key Facts

Quick Reference Table

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Layer Domestic patio/path Domestic drive Light vehicular
Sub-base 100mm Type 1 150mm Type 1 200mm Type 1
Binder not required 50mm AC 20 60mm AC 20
Surface 25mm AC 6 25–30mm AC 6 30mm AC 10
Edging brick or block concrete-haunched edge KSL kerb
Roller 600kg tandem 1.5T tandem vibratory 3T tandem vibratory
Indicative cost £/m² (2026) £35–£55 £45–£70 £60–£90

Detailed Guidance

Excavation and sub-grade preparation

Excavate to formation level: sub-base depth + binder + surface + 5mm tolerance. For a typical drive that's 230mm. If the sub-grade is soft or clay (CBR <2%), dig 100mm deeper and lay a 1000-grade geotextile on the formation before sub-base.

Compact the formation with a roller — a soft formation telegraphs into the sub-base and shows in the surface profile after a winter.

Sub-base specification

MOT Type 1 to SHW Series 800. Compact in two 75mm layers if total exceeds 150mm — single passes don't densify the bottom of the layer. Check compaction by walking on it: a properly compacted Type 1 layer is firm with no movement underfoot.

Falls and finished levels are set at sub-base stage. The binder course follows the sub-base profile; the surface course follows the binder. Anything you don't get right here will show in the finished surface.

Edge restraints — install before binder course

Set concrete edging on a 100mm bed of C20 concrete (1:2:4) and haunch the back with the same mix. The top of the edging should be 5mm above the intended finished tarmac level — when the surface course rolls, it will key into the edge slightly and the surface will sit just below the edging top, providing containment.

Without proper edge restraint, tarmac edges crumble within 2–3 winters. The bitumen at the edge is unsupported, vehicle wheels overrun the edge, and freeze-thaw breaks off small pieces. Trying to repair tarmac edge damage retrospectively is much harder than installing edges first.

Binder course laying

The binder is 20mm dense bituminous macadam to BS EN 13108-1 (AC 20 dense bin or HRA 30/14). It's the structural layer — it carries the load and protects the sub-base from water ingress.

Material arrives at 130–160°C. Discharge from the lorry direct into a screed or barrowed tip. For domestic-scale jobs without a paver, hand-tipping and raking is normal — the team needs to be experienced because the material cools quickly.

Roll immediately while the material is still hot. Three to five passes with a vibratory roller, working from the edges inward then longitudinally. Check density by feel — a properly compacted binder rings under boot, not thuds.

Surface course laying

The surface course is 25–30mm AC 6 dense surf, 6mm SMA (Stone Mastic Asphalt), or 10mm dense surf for slightly coarser texture. SMA is more durable and increasingly common for higher-spec drives — it costs about 10–15% more than dense surf.

The surface course goes down within 24–48 hours of the binder, before any traffic. If the binder is left exposed for longer, it must be cleaned and tack-coated before surface course application.

Lay and roll while hot. The wearing course runs about 130–150°C as delivered; needs to be compacted before falling below 90°C. In cool/windy conditions, lay smaller bays so the team can keep up with the roller.

Edging interface and detailing

The surface course should finish 0–3mm below the top of the edging — flush or marginally below. Tarmac proud of the edging gets snagged by snow ploughs, wheel barrows and shopping trolleys, and the edge fails first.

Around manhole covers and gully gratings, the same level rule applies — feature levels finish flush with surface course. Cut the surface course back from features by 50mm, lay haunching mortar around the cover/gulley, then patch in tarmac to bring level. Direct laying of tarmac onto un-prepared cover frames produces ridges that fail within a year.

Falls and drainage

Set falls at 1:60 minimum (about 17mm per metre) toward whatever the drainage destination is. Standard impermeable tarmac on a front drive over 5m² needs:

See the SuDS regulations article for the full compliance route options.

Porous asphalt — the SuDS-compliant alternative

Porous asphalt (PA, also called open-graded asphalt) uses a low-fines aggregate gradation that creates 18–25% air voids in the mix, allowing surface water to drain through. It's laid in a single course of 50–60mm over a permeable Type 3 sub-base.

PA performance considerations:

For a domestic front drive over 5m² where the homeowner doesn't want planning permission and doesn't have a permeable area, porous asphalt is often the cleanest answer.

Coloured tarmac

Standard tarmac is black (bitumen colour). Pigmented binder produces red, green, or buff finishes. Costs are roughly 30–50% higher than standard. Colour stability is moderate — pigments fade slightly within 5–7 years. The colour comes from the binder + a coloured aggregate; if you want strong colour retention, specify a coloured aggregate too.

A cheaper alternative for an aesthetic upgrade is to lay standard black tarmac and apply a sealer-coat with pigment after 2–3 years (to allow the bitumen to weather).

Frequently Asked Questions

Why does my new tarmac drive have soft patches?

Two common causes: (1) the binder or surface course was rolled below 90°C and didn't compact properly — that area will rut under wheel load; (2) bitumen pooling from over-application or sub-base contamination. Soft patches that develop only in summer often relate to thin spots where the bitumen-aggregate ratio has gone high.

Can I drive on it the same day?

Yes, after about 4–6 hours when the surface has cooled. Avoid sharp turns or stationary spinning wheels for the first 7 days while the surface stiffens fully. Hot weather extends the curing time — a drive laid in 28°C ambient may be tacky for 24+ hours.

How do I look after a tarmac drive?

Annual jet wash (low pressure) to clear leaves and grit. Spot-treat oil drips with a degreaser within a week of spillage — bitumen is dissolved by hydrocarbon oils. Re-seal with a polymer-modified sealer at 5–7 years if appearance matters; structurally a sound drive doesn't need sealing.

When does it need replacement vs overlay?

If the surface is alligator-cracked, has rutted by more than 25mm, or the binder is exposed across more than 20% of the area, full replacement is needed. If cracks are linear and limited and the structural layer is sound, an overlay (after planing 25mm off) extends life by 10–15 years at half the cost of replacement.

Is tarmac suitable for steep drives?

Up to 1:6 (17%) gradient, yes. Above that, rolling becomes difficult and braking vehicles can shear the surface. For very steep drives (1:5 or steeper), specify a higher SMA grade with polymer-modified bitumen, or switch to imprinted concrete which has more grip.

Regulations & Standards