How Much Fall Does a Concrete Slab or Footpath Need for Drainage?

Introduction: Fall Is the Difference Between Dry and a Puddle

Ask any experienced concreter what separates a good slab from a bad one and "fall" will come up fast. Fall is the deliberate slope built into a concrete surface so that water runs off it instead of sitting on it. It sounds like a small detail, but it's one of the most important decisions made before a single barrow of concrete is poured — and it's almost impossible to fix once the slab has set.

In Brisbane and South East Queensland, where a single summer storm can dump 100mm of rain in an hour, fall matters even more than it does in drier climates. A shed slab, garage floor, footpath or patio with no fall becomes a shallow pond every time it rains. Get the fall right and the water is gone within minutes of the storm passing. This guide explains how much fall a slab needs, the common fall ratios like 1:100 and 1:60, how the fall is actually set on site, and what goes wrong when there isn't enough.

What "Fall" Actually Means

Fall is simply the amount a surface drops over a given distance. It's usually written as a ratio — like 1:100 — which means the surface drops 1 unit for every 100 units of length. On a 1:100 fall, a slab drops 10mm over every metre. On a steeper 1:60 fall, it drops about 16.7mm per metre.

You'll also hear it described as a percentage or in degrees, but the ratio is what most concreters and drainage guides use. The key point is that fall is directional — the slab is set slightly higher on one side and lower on the other, so gravity carries water toward the low edge, a drain, or an open area of ground where it can soak away.

A slab that looks dead flat to the eye almost never is. A well-built shed slab has a gentle, barely-perceptible slope built into it on purpose. You won't notice standing on it, but water certainly will.

Why Fall Matters So Much in Brisbane

SEQ gets intense, concentrated rainfall. The wet season delivers short, heavy downpours rather than a light steady drizzle, so any flat surface is asked to shed a lot of water in a very short window. Three things make fall non-negotiable here:

Heavy, Fast Rainfall

When 50–100mm falls in under an hour, water needs somewhere to go immediately. A surface with proper fall clears it in real time. A flat surface backs up and ponds, and that water then has time to work its way into joints, cracks, and the base underneath.

Reactive Clay Soils

Large parts of Brisbane, Ipswich and Logan sit on reactive clay that swells when wet and shrinks when dry. Water pooling beside or under a slab keeps the soil saturated, driving that swell-shrink movement that leads to cracking. Falling water away from the slab keeps the surrounding soil more stable. (We cover the cracking side of this in our guide to why concrete cracks.)

Humidity and Slip Risk

Standing water on a garage floor or footpath stays wet longer in Brisbane's humidity, growing algae and moss that make the surface dangerously slippery. Good fall keeps surfaces drying quickly and staying safe underfoot.

Recommended Fall by Surface Type

There's no single "correct" fall — it depends on the surface, its finish, and where the water needs to go. As a general guide, smoother surfaces can drain on a gentler slope, while rougher or broom-finished surfaces need a little more. The table below is a practical starting point for small concrete jobs in SEQ:

Surface Type	Typical Fall Ratio	Drop Per Metre	Notes
Concrete footpath	1:100 to 1:80	10–12.5mm	Cross-fall to one side so water sheds off the edge
Shed slab	1:100 to 1:80	10–12.5mm	Fall away from the shed door and any stored gear
Garage slab (apron / entry)	1:80 to 1:60	12.5–16.7mm	More fall at the entry keeps driving rain out
Patio / outdoor pad	1:100 to 1:80	10–12.5mm	Fall away from the house or building line
Water tank slab / equipment pad	1:100 to 1:60	10–16.7mm	Fall away from the base of the tank or unit
Broom / textured finish (any surface)	1:60 to 1:50	16.7–20mm	Rougher texture holds water — needs steeper fall

As a rule of thumb, 1:100 (10mm per metre) is the gentlest fall you'd want on a smooth external slab, and many concreters treat it as the minimum. For rougher finishes or areas that must stay dry, stepping up to 1:80 or 1:60 gives a safer margin. Too much fall (steeper than about 1:40) starts to feel like a noticeable slope and can make a garage or patio awkward to use — so the aim is enough to drain, not so much you feel it.

How Fall Is Set on Site

Fall isn't something you add to concrete after it's poured — it's built into the formwork and the base before the pour. Here's how it's done on a typical small slab:

a) Establishing Levels

Before anything is dug, the concreter works out the high point and low point of the slab and where the water needs to end up — a garden bed, a drain, a lower part of the yard. On small jobs a string line and a builder's level, spirit level, or laser level are used to set the difference in height across the slab.

b) Setting the Formwork to the Fall

The timber or steel form boards that contain the wet concrete are set to the finished heights — higher on one side, lower on the other. Getting these boards right is where the fall is locked in, so it's worth taking time over. Once the concrete is against the boards, the fall is fixed.

c) Grading the Base to Match

The compacted base under the slab is graded to roughly follow the fall, so the slab stays a consistent thickness across its whole area. If the base were left flat while the top followed the fall, the slab would be thick on one edge and thin on the other. (For more on the layer under the slab, see what base is needed under a concrete slab.)

d) Screeding to the Boards

When the concrete goes in, it's screeded — dragged level — off the form boards. Because the boards are already set to the fall, screeding to them automatically produces the correct slope across the finished surface. From there the slab is floated and finished as normal.

What Goes Wrong With Poor Fall

A slab with too little fall — or fall running the wrong way — causes problems that only show up after the first heavy rain, when it's too late to change. The common ones are:

Ponding and Standing Water

The most obvious symptom. Water collects in low spots (often called "birdbaths") and sits there for hours or days. On a footpath it's an annoyance; on a shed slab it can mean water creeping toward stored equipment, boxes, or tools.

Moisture Under Sheds and Against Buildings

Water that doesn't drain away pools against the edge of the slab and the base of the shed. Over time this keeps the ground damp, encourages rust on steel shed frames, and can let moisture wick up into whatever is stored inside.

Slippery, Algae-Covered Surfaces

Permanently damp concrete grows algae, moss and mould, especially on the shaded southern side of a building. A green, slimy footpath or patio is a genuine slip hazard — and in Brisbane's humidity it doesn't take long to develop.

Accelerated Cracking and Deterioration

Water sitting on or beside a slab works into joints and any hairline cracks. In SEQ's wet-dry cycle, saturated reactive soil under a slab moves more, which stresses the concrete and can widen cracks over time. Good drainage is one of the simplest ways to help a slab last.

Water Toward the House Instead of Away

The worst outcome is a slab that falls back toward a building. Any patio, path or pad next to a structure should always fall away from it. A surface sloping the wrong way channels rainwater straight to the building line — exactly where you don't want it.

Fall for Different Jobs

Shed Slabs

A shed slab is generally set to fall gently toward the door opening and away from the back and sides, so any water that blows in or is tracked in runs back out rather than pooling inside. A fall of around 1:100 across the slab is typical. If the shed sits on a slope, the slab is usually stepped or set so the natural ground fall works with the drainage rather than against it. See our shed slabs service page for how we handle this.

Garage Slabs

Garages benefit from a bit more fall at the entry — often around 1:80 to 1:60 across the apron — so that driving rain and wash-down water run out the door and don't pool in the middle of the floor. The internal floor still has a gentle fall toward the entry.

Concrete Footpaths

Footpaths usually use a cross-fall — a slope from one long edge to the other, around 1:100 — so water sheds sideways off the path rather than running along its length. Where a path runs beside a building, the cross-fall is set away from the wall. Our concrete footpaths service page has more detail.

Patios and Outdoor Pads

Patios and pads always fall away from the building they adjoin, typically at 1:100 to 1:80. Because these areas are often used for seating or entertaining, the fall is kept gentle enough that furniture sits level and it's comfortable underfoot, while still clearing water.

Water Tank and Equipment Slabs

A pad for a water tank, pump, or air-conditioning unit should fall gently away from the base of the equipment so water doesn't pool around the footing. A slope of around 1:100 is usually enough to keep the base dry.

Common Questions About Slab Fall

Can a concrete slab be perfectly flat?

It shouldn't be. Even an internal shed floor is best given a slight fall toward the door. Any external surface needs fall, because a truly flat slab will always pond somewhere — no surface is poured perfectly level, so the water finds the low spots.

What is the minimum fall for outdoor concrete?

As a practical minimum, 1:100 (10mm per metre) is the gentlest fall generally used on smooth external slabs. Rougher, broom-finished surfaces need more — closer to 1:60 — because the texture holds water.

Can fall be fixed after the slab is poured?

Not easily. Fall is set in the formwork before the pour, so once the concrete has cured it's locked in. Minor pooling can sometimes be helped with a grinder or a thin self-levelling topping that redirects water, but it's far better — and far cheaper — to get the fall right the first time.

Which way should the slab fall?

Always toward where you want the water to end up and away from anything you want to keep dry — buildings, shed contents, and equipment bases. Water should head for a garden bed, a drain, or a lower part of the yard.

Final Thoughts

Fall is one of those details that costs nothing extra to get right and a lot of grief to get wrong. A slab set with a proper slope — even a gentle 1:100 you can't feel underfoot — sheds Brisbane's heavy rain quickly, stays dry and safe, and lasts longer because water isn't constantly working at it. A flat or badly-graded slab ponds, grows algae, keeps the ground damp and slowly degrades.

The good news is that fall is entirely within your control at the planning stage. It comes down to working out where the water should go, setting the formwork to match, grading the base, and screeding cleanly to the boards. Get that right and drainage takes care of itself for the life of the slab.

If you're planning a new shed slab, garage floor, footpath or patio in Brisbane or SEQ and want the fall and drainage done properly, get in touch for a quote. You can also check the pricing guide for a starting-from idea of costs, or read our guide to pouring concrete in the rain for more on working with SEQ's wet weather.