Question 1

What is permeable paving?

Accepted Answer

Permeable paving refers to any paved surface system that allows rainfall to drain through the surface rather than running off as sheet flow. In standard paving (impermeable block paving, concrete, tarmac), all rain that falls on the surface runs off - contributing to surface water drainage loads, flooding risk, and pollution of watercourses (from tyre rubber, oil, and de-icing salts washed off driveways). Permeable paving systems allow rain to infiltrate downward into the sub-base, where it can be stored temporarily, allowed to soak into the subsoil (infiltration), or collected for controlled release. The main types are: open-jointed concrete block paving with permeable aggregate joints; porous concrete or asphalt (where the surface material itself has an open structure); grass reinforcement grids (grass or gravel in a structural plastic grid); and resin-bound paving (where aggregate is bonded in resin but retains void space for drainage).

Question 2

Does permeable block paving require planning permission?

Accepted Answer

In England, converting a front garden to a hard-surfaced driveway (or enlarging an existing one) over 5m2 requires planning permission unless the surface is permeable. DCLG guidance (2008) classifies permeable block paving (laid with open joints and permeable grit filling) as a permitted surface that does not require planning permission. Standard block paving laid with sand-cement joints (impermeable) does require planning permission for a front driveway over 5m2. Permeable block paving systems (such as those conforming to BS EN 1338 or manufactured specifically as permeable products with open joint widths) are tested for infiltration rate and comply with the permitted development rule. The planning authority can be consulted if there is any doubt about the specific product's classification - the infiltration test result (litres per minute per square metre) is the definitive measure.

Question 3

How does permeable block paving work?

Accepted Answer

Permeable block paving works by directing rainwater through the gaps between the blocks rather than off the surface. The blocks themselves are solid concrete but are laid with wider joints (typically 3-7mm) filled with a washed angular grit (not kiln-dried jointing sand, which would wash out). The water passes through the joints into a permeable bedding layer below the blocks, then into an open-graded granular sub-base (typically 100-200mm of clean MOT Type 3 or a larger open-graded aggregate) which acts as a temporary storage reservoir. From the sub-base, water either soaks into the subsoil below (infiltration) or drains at a controlled rate to a nearby soakaway, surface water drain, or watercourse. The sub-base must be designed for the expected rainfall and the subsoil infiltration rate, which varies significantly by geology.

Question 4

What maintenance does permeable paving require?

Accepted Answer

Permeable paving requires more maintenance than standard impermeable paving to sustain its drainage function. The main maintenance task is removing sediment that accumulates in the joints and surface voids, progressively reducing the infiltration rate. For permeable block paving, the joints should be brushed clear of leaves, moss, and fine sediment annually; vacuum sweeping with a specialist sweeper (which draws out sediment from the joints without removing the aggregate) every 2-5 years maintains infiltration performance. Porous asphalt and porous concrete surfaces can be vacuum-swept similarly. Resin-bound paving requires less maintenance as the smooth surface does not trap sediment in the same way, though the underlying porous sub-base can still clog over time. Vegetation growing in joints (from seed blown in) should be removed promptly to prevent root blockage of joints.

Permeable Paving

Just so you know...