The driveway two sections below mine on the hill drops away from the garage at close to 1 in 4, and every heavy downpour it turns into a shallow river across the last five metres before the kerb. It isn’t bad concrete. The slab is sound, no cracking, no spalling. It was simply laid to carry a car up and down a steep grade without anyone working out where the water landed once it hit that grade, and on a section like this one, that oversight is the whole difference between a driveway you can trust in the wet and one you creep down in first gear with the wipers going.
What counts as steep on a hill section
Kelburn sits on some of the steeper residential land in the country, and a lot of driveways here run at grades between 1 in 8 and 1 in 4 over at least part of their length, sometimes with a levelling-off pad near the garage door to stop a car bottoming out. Anything steeper than about 1 in 6.5 starts to change how the concrete has to be finished and drained, and anything approaching 1 in 4 is at the edge of what most vehicles can manage confidently in the wet without a transition grade easing the angle where the driveway meets the road or the garage floor. A driveway that ignores that transition sends the underside of a car scraping on the way in, and sends water straight down the slope on the way out.
Traction is a finish decision, not an afterthought
A steel-trowelled smooth driveway on a grade over about 1 in 10 is close to a wet-weather hazard once algae or moss gets a foothold on the greywacke clay dust that blows around this part of Wellington. The finishes that actually hold a tyre on a wet slope are a stiff broom finish across the fall of the slab, a light exposed aggregate, or saw-cut grooves run across the direction of travel rather than along it. Grooving across the slope at 20 to 25 mm spacing does more for grip than most people expect, because it breaks the sheet of water into channels instead of letting it form one continuous film under the tyre. On the steepest run of driveway, I would rather see a coarser broom finish than a decorative exposed aggregate that looks good but polishes smooth after years of tyre wear.
Getting the water off before it gets under the slab
The traction finish only deals with water sitting on top of the concrete. The more damaging water is what tracks down the batter beside the driveway and gets under the edge of the slab, because that undermines the base course and starts the slab rocking at the joints. On a hill driveway I want a channel drain or a formed swale at the top edge, catching the batter runoff before it crosses the slab, tied into a piped subsoil drain that runs to the stormwater connection rather than discharging onto the neighbour’s boundary. Cross-falls of about 1 in 40 to 1 in 50 across the width of the slab, draining to that channel, keep the running surface clear without being noticeable underfoot. Skipping this is the most common reason a Wellington hill driveway develops a wet strip down one edge that never quite dries out between storms, at 1200 mm of annual rainfall arriving mostly in short, hard events rather than steady rain.
Where the street end catches people out
The bottom metre or so, where the driveway meets the kerb and the council’s vehicle crossing, has to shed water away from the road rather than adding to what the kerb and channel are already carrying in a downpour. A driveway that has been re-laid without checking the crossing grade against the kerb line can end up ponding at the boundary, which is a maintenance headache and, on a steep street, a skid risk for anyone braking into the driveway from above. Pricing a fresh slab for a steep site like this generally lands somewhere between $180 and $400 per square metre depending on excavation, base depth and how much drainage has to be built in alongside it, and that spread is mostly the drainage, not the concrete itself.
Sorting out the fall, the channel drain and the finish together is a job for whoever is actually forming and pouring the slab, and on the steeper Wellington sections that is worth discussing directly with Wellington concrete driveways contractors before the formwork goes in, since changing the drainage plan after the pour means breaking out concrete rather than adjusting a drawing.
Frequently asked questions
What is the steepest grade a concrete driveway should be poured at? Most councils and engineers treat anything over 1 in 4 as needing a transition grade or a specific design, and anything over 1 in 6.5 as needing a non-slip finish rather than a smooth trowel.
Does exposed aggregate give enough grip on a steep driveway? A fine exposed aggregate helps early on but polishes smoother with years of tyre contact; a broom finish or cross-cut grooving holds its grip for longer on the steepest sections.
Why does my driveway stay wet along one edge after rain has stopped? That is usually batter runoff getting under the slab edge rather than being intercepted by a channel drain, which keeps a strip of base course saturated long after the surface has dried.