Driveway grades and basement ramps, and the levels that fix them

Driveway grades and basement ramps, and the levels that fix them

On a Sydney infill site the basement ramp is one of the first things that should be tested and one of the last things that usually gets drawn. When it fails, it fails late, and it takes a level of parking with it.

The reason is that both ends of the ramp are fixed by someone else. The top is set by the council's kerb and footpath levels at the boundary, which you do not get to move. The bottom is set by the basement slab level, which is driven by ceiling heights, drainage and how deep you can afford to dig.

Everything in between has to satisfy AS 2890.1, and the site either has enough length or it does not.

What the standard controls

Maximum grade. A ramp can be steep, but not arbitrarily so, and long ramps are limited more tightly than short ones.

Change of grade. This is the control that catches designs out. At the top and the bottom, where the ramp joins the flat, the change in slope has to be spread over a transition length rather than happening at a single line. Without it a vehicle grounds out on the crest or scrapes its front on the sag. The standard sets limits for both cases, and they are different, because a car behaves differently going over a summit than dropping into a hollow.

Headroom. The clear height has to be maintained over the full ramp and through the transitions, and services, sprinkler pipework and the underside of the slab above all eat into it. Accessible parking spaces and any van access need more.

Widths and passing. A single width ramp needs sight lines or a signal system so two vehicles do not meet on it.

The check that should happen first

Take the levels at the boundary, take the proposed basement slab level, and measure the horizontal distance available between them.

That single calculation tells you whether the ramp fits. If it does not, the options are to raise the basement, shorten the parking layout to buy ramp length, split the ramp with a mid landing, or reduce a level of parking. All four change the yield and the feasibility, which is why the check belongs at concept rather than at construction certificate stage.

The crossing over the footpath

The vehicle crossing from the kerb to the boundary is council infrastructure and is built to the council's standard drawing. The layback location, the width and the profile are set by them, and they are approved separately from the development consent.

That crossing also has to work with the footpath grade running along the street, which is often not flat. A crossing on a sloping footpath creates cross-fall that the ramp design has to absorb.

Existing services sit in that zone as well, and a pit or a pole in the wrong position moves the crossing, which moves the ramp, which moves the basement layout.

Drainage at the bottom

Water runs down the ramp in a storm. A grated drain at the low point sized for the catchment, with a pump-out system below the basement, is a requirement rather than a detail. Where the ramp is long and steep, an additional grate part way down reduces what reaches the bottom.

Getting that wrong floods a basement, and it is a civil engineering item that is easiest to resolve while the levels are still being set.

The sequence that works

Levels first, then layout. Survey the boundary and the street, set the basement level provisionally, test the ramp geometry, then design the parking around what is left.

Doing it in that order costs a day at concept. Doing it the other way around has cost projects an entire parking level after documentation was complete.

We run this as part of civil engineering on residential and mixed use developments across Sydney, alongside the stormwater design that has to work with the same set of levels.