Tarmac vs Block Paving

What Tarmac and Block Paving Actually Are

Tarmac (asphalt) is a hot-laid bituminous composite. It forms a continuous flexible sheet once compacted. Structurally, it behaves as a thin flexible pavement layer that spreads wheel loads laterally before transferring them into the sub-base.

Block paving is a segmented surface system. Individual concrete or clay units are laid on a bedding layer and restrained laterally by edge restraints. Structurally, it behaves as an interlocking lattice that shares load between adjacent units.

Neither surface contributes significant structural capacity by itself. The real strength of both systems comes from the foundation beneath. The surface layer mainly affects how loads are distributed, how water is managed, and how movement becomes visible over time.

Context: Block Paving vs Porcelain for Driveways • Can You Re-Bed Driveway Blocks? • Concrete vs Block Paving

How Each System Carries Load

Tarmac load behaviour

Tarmac behaves as a continuous flexible membrane. Wheel loads are spread over a wider area as they pass through the asphalt layer. This reduces peak stress on the sub-base directly below each tyre.

Because it is continuous, tarmac can tolerate small local voids or slight base irregularities without immediately telegraphing them to the surface. The surface will flex slightly instead of cracking or displacing.

Block paving load behaviour

Block paving behaves as a segmented load-sharing grid. Each block transfers load into the bedding layer and laterally into adjacent blocks. The interlock pattern and jointing sand are critical to this behaviour.

Because blocks are discrete units, any local settlement beneath one area becomes visible as dips, rocking blocks, or joint widening. The system is less forgiving of foundation irregularities, but easier to diagnose and repair when problems appear.

Foundation Depth Differences

Despite common assumptions, tarmac does not allow a thinner foundation. In many cases, it requires the same or greater sub-base depth than block paving.

Typical sub-base ranges (competent ground)

• Light cars only: 100–150 mm
• Cars + vans: 150–200 mm
• Frequent commercial vehicles: 200–300 mm

The reason is simple: surface type does not change soil strength. Only foundation thickness and stiffness do that.

Tarmac may visually hide early movement, but it does not reduce the structural demand placed on the sub-base. Thin bases under tarmac fail just as reliably as thin bases under blocks.

Drainage Behaviour and Water Sensitivity

Tarmac drainage behaviour

Standard tarmac is effectively impermeable. All water must be shed off the surface or intercepted by drainage channels.

If water becomes trapped beneath tarmac, it softens the sub-base, reduces bearing capacity, and accelerates deformation. This is a major cause of rutting and cracking.

Block paving drainage behaviour

Standard block paving is semi-permeable at the joints. Water can infiltrate into the bedding layer and then into the sub-base.

This reduces surface runoff but increases the importance of a free-draining foundation design. Without proper drainage layers or falls, block paving can become a water reservoir sitting on weak soil.

Context: How Much Fall Does a Patio Need? • Patio Drainage Basics • What Are Surface Falls?

How Each Handles Ground Movement

Tarmac and movement

Tarmac tolerates small movements by flexing. Minor settlement may go unnoticed for years.

Once movement exceeds the elastic range of the asphalt, cracks form. These cracks then admit water, which accelerates further structural degradation.

Block paving and movement

Block paving does not hide movement. Settlement becomes visible almost immediately.

This looks worse in the short term, but it allows early intervention before deep structural damage occurs. Blocks can be lifted, the base corrected, and the surface relaid without replacing the entire driveway.

Context: Clay Soil and Driveways • Ground Movement and Driveways • Ground Movement and Patios

Maintenance and Repair Reality

Tarmac maintenance

Tarmac requires periodic resealing to slow oxidation and cracking. Oil contamination permanently softens it.

Local repairs are visible. Colour matching is almost impossible. Structural patching rarely blends seamlessly with old material.

Block paving maintenance

Block paving requires periodic joint sand replenishment. Weed growth is cosmetic, not structural.

Local repairs are invisible when done correctly. Individual blocks can be replaced indefinitely. The surface can be lifted and relevelled decades later.

Common Failure Modes

Typical tarmac failures

• Rutting from softened sub-base.
• Cracking from base movement.
• Edge collapse from poor restraint.
• Water pumping beneath the slab.

Typical block paving failures

• Local settlement and rocking blocks.
• Joint sand loss and surface loosening.
• Edge creep from weak restraints.
• Bedding layer washout.

In both systems, almost all failures trace back to: insufficient sub-base depth, poor drainage, weak edge restraint, or bad compaction.

Context: Signs Your Driveway Was Built Wrong • Can You Re-Bed Driveway Blocks? • Driveway Build-Up Explained

Correct Design Rules

• Design the foundation first, not the surface.
• Scale sub-base depth to axle loads and soil strength.
• Provide mechanical edge restraint for both systems.
• Install drainage to keep the structure dry.
• Compact all layers in thin controlled lifts.
• Overbuild on clay and marginal ground.

Surface choice should be the final decision, not the first. A perfect surface on a weak foundation is a guaranteed future failure.

Related Guides

• Driveway Drainage Basics (Falls, Runoff, and Where the Water Goes)
• Patio Drainage Design (Falls, Interfaces, and Where Water Must Go)
• Patio Ventilation Rules (Why Air Gaps Matter Near Buildings)
• Why Sub-Bases Settle (And How to Prevent Future Patio Movement)
• Why Patio Edging Fails (Creep, Spread, and Unsupported Perimeters)