Paving flags, slabs and pavers

Frequently asked questions

Are flags and block paving interchangeable?

They look like alternatives in a catalogue and they behave differently under load, which is why the choice is not only aesthetic. Block paving is many small units, so a wheel load spreads through interlock between them and the pavement flexes without cracking — which is why it is used on driveways and why the edge restraint holding the blocks in is a structural component rather than a trim. A large flag carries the load across its own span and relies on continuous support beneath it; unsupported at a corner, it rocks and eventually cracks, and one cracked flag is a replacement rather than a repair. So a surface that will take vehicles usually wants blocks or a thicker flag on a bound bed, and the laying course, the base depth and the edge detail all change with the choice, not just the unit price.

How much extra should be ordered for cutting?

It depends on the pattern and the shape of the area far more than on a flat percentage, and a flat percentage is what usually gets used. A rectangular area laid in a stack or stretcher pattern with units that divide neatly into it can need almost nothing beyond breakages. The same area laid DIAGONALLY generates a cut at every perimeter unit and each cut wastes most of a unit, which is why diagonal and herringbone layouts carry a much larger allowance. Circles, curved edges and a random or mixed-size pattern raise it again, because a cut piece from one position rarely fits another. The other quantity people forget is the laying course and the jointing material, both of which are consumed by the actual depth achieved on site rather than the nominal one.

What does the base under paving actually have to do?

Spread the load and stay put, and most failures are a base problem wearing a surface problem's appearance. Paving units transmit load into the layer beneath them, and that layer has to distribute it over ground soft enough that concentrated load would sink — so the sub-base is compacted granular material at a depth set by the traffic and the subgrade, not a token layer to level things up. Above it the laying course is thin and uniform, because its job is to seat the units, not to take out variation in the base. The classic failure is a thick sand bed used to correct an uneven base: it consolidates unevenly under use, and the surface dips exactly where the bed was deepest. The second is an unrestrained edge, which lets the whole field creep outward until the joints open.