How the two differ in kind
The two surfaces do the same job underfoot and opposite jobs for water. A permeable system lets rainfall pass through the joints — the blocks themselves are not porous, the JOINTS are, filled with a small clean grit — into a sub-base of open-graded stone that stores it and either infiltrates it into the ground beneath or releases it slowly. An asphalt drive is impermeable by design, so every drop that lands on it runs to wherever the surface is graded, and that has to be somewhere legitimate.
The storage is the part most often overstated. Open-graded sub-base stone holds water in the spaces BETWEEN the stones, which is roughly a third of the layer's bulk volume — so a three-hundred-millimetre sub-base stores about a hundred millimetres of water over its area, not three hundred. A design that treats the layer as solid storage is out by a factor of three, and the calculator here takes the void ratio as an input for exactly that reason.
The second fact is that permeable paving is a maintained surface rather than a laid one. Its infiltration rate when new is enormously higher than anything the design requires, and it falls over years as fines wash into the joints and bind them. That margin is deliberate — it is what buys the years — but it is a depreciating asset, and a system that is never swept eventually behaves like an impermeable one with a rough surface.
The factors that actually differ
Show
| Permeable block paving | Asphalt drive | |
|---|---|---|
| What happens to the rain | Stored in the sub-base voids and infiltrated, or released slowly. The drive stops contributing to a downstream peak. | All of it runs off, to a gully, a channel and a connection — which is a cost and, increasingly, a consent. |
| Build-up | Deeper, and the stone is graded and clean rather than as-dug. Geotextile separation beneath, and the sub-base is structure and storage at once. | Conventional: sub-base, binder course, surface course. Thinner and cheaper. |
| How it fails | Clogging at the joints, gradually and invisibly, until the surface ponds. A maintenance failure rather than a construction one. | Cracking, ravelling and potholes — visible, and repaired by patching, which is also visible. |
| Maintenance | Sweeping or vacuuming on a schedule, and joint grit topped up. Cheap, frequent, and the thing that determines whether it still works in ten years. | Essentially none for years, then a seal or an overlay. |
| Water quality | Genuinely treats. Sediment is caught at the surface and hydrocarbons are broken down biologically in the sub-base — a real benefit on a surface that collects oil drips. | Conveys everything it collects, including the oil, to wherever it discharges. |
| Frost and saturation | Drains, so the sub-base does not sit saturated through a freeze — which is an advantage rather than the vulnerability it sounds like. | Depends on the sub-base draining laterally; a saturated layer under an impermeable surface is where frost damage starts. |
| Repair and alteration | Blocks lift and relay. A service trench can be reinstated invisibly, which asphalt cannot. | Patches show, and a trench reinstatement is permanent evidence of itself. |
| Shape of the cost | Higher to build — depth, graded stone, geotextile, blocks — and it removes or shrinks the drainage installation that the other option needs. | Lower to build, plus drainage, plus whatever consent or attenuation the local regime now requires for a new impermeable surface. |
Which one, and when
Choose permeable block paving when…
- Surface water has nowhere obvious to go, or connecting to a drain is expensive or not permitted.
- The local regime restricts new impermeable paving, which many now do for front gardens.
- The ground will infiltrate at a rate a test can demonstrate.
- Services may be dug up later and reinstating the surface invisibly is worth something.
Choose asphalt drive when…
- There is already a drainage route with capacity, and adding to it is cheap.
- The area is large and the budget is the binding constraint.
- The surface has to take heavy or turning vehicles regularly, where a blocked surface would need a heavier specification.
- Nobody will ever sweep it, which is a real and honest reason to avoid a surface that depends on being swept.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Does the water actually soak away?
- Only if the ground beneath will take it, and that is a measurement rather than an assumption. An infiltration test in a trial pit at the depth the sub-base will sit gives a rate, and the design either works to it or it does not. Where the ground is clay or the water table is high, a permeable surface is still useful but in a different mode: it becomes a TANKED system, lined beneath, storing water in the voids and releasing it slowly through a restricted outlet. That still removes the peak from the downstream system, which is usually what the drainage strategy needs, and it still treats the runoff. What it does not do is disappear the water, and a system sold as infiltration on ground that will not infiltrate is a pond with blocks on top.
- Does it clog, and how bad does that get?
- Yes, gradually, and the design accounts for it by starting enormously over-provided. A newly laid permeable surface infiltrates at a rate far above anything the rainfall requires, and that margin depletes over years as fine material washes into the joints. Sweeping or vacuum sweeping on a schedule removes the surface fines and recovers most of the rate; letting it go means the surface eventually ponds in heavy rain. The things that accelerate it are avoidable: soil washing onto the drive from a bank or a bed, construction traffic during a later project, and topping the joints with anything other than the specified clean grit. A drive that has clogged completely can be restored by removing and replacing the joint material, which is a job rather than a write-off.
- Do I need permission to pave a front garden?
- In a growing number of places, yes if the surface is impermeable and above a certain area — and the reason is precisely this comparison. Paving gardens impermeably transfers rainfall that used to soak in straight into the drainage system, and enough of it at once contributes to surface flooding downstream. The usual regime permits a permeable surface, or an impermeable one that drains to a soakaway or a planted area within the property, without consent, and requires an application for an impermeable surface draining to the highway or the sewer. The rules vary by jurisdiction and the penalty for guessing is retrospective, so it is worth a phone call before the sub-base is ordered.
- Can permeable paving take a heavy vehicle?
- Yes, with the specification designed for it — and the design changes more than the block thickness. The sub-base in a permeable system is doing two jobs at once, storing water and carrying load, and open-graded stone is a perfectly good structural layer when properly specified and compacted. What increases with load is the depth and the grading of that layer, and sometimes a geogrid. The practical caution is turning: vehicles that steer while stationary scuff the joint material out, and heavy turning in one spot is the fastest way to lose the joints. Where a delivery lorry regularly turns on a drive, a reinforced area or a different surface at that spot is a reasonable local answer.
- What happens to oil drips?
- They are trapped and broken down, which is one of the genuine environmental arguments for the surface rather than a marketing one. Hydrocarbons dripping onto a permeable surface are retained near the top of the construction, where the sediment and the microbial population that grows in it are, and they degrade biologically over time. Monitoring studies on these systems consistently find very low hydrocarbon concentrations in the outflow. An impermeable drive does the opposite: it collects the oil and sends it, with everything else, to wherever the drive discharges — which is why highway and car park runoff is a recognised pollution source and why interceptors exist on the impermeable version.
- Which is cheaper over twenty years?
- It depends on two things this site will not invent: whether the asphalt option needs drainage and consent, and whether the permeable one will actually be swept. Built cost favours asphalt clearly. Once a gully, a connection and any attenuation the local regime requires are added to it, the gap narrows and often closes. Over a life, asphalt needs resurfacing or an overlay at some point and permeable paving needs sweeping throughout and eventual joint renewal — and the honest comparison is the annualised one the life-cycle methodology describes rather than a built cost for either. The failure case is specific and worth naming: a permeable drive nobody maintains delivers neither the drainage benefit it was paid for nor the low maintenance of the surface it replaced.