How do I set a fall or a gradient accurately?

The packing-block trick that turns an ordinary spirit level into a gradient gauge, how to convert between 1-in-X and millimetres per metre, and how to hold a consistent fall over a long run.

Difficulty: intermediate
Time: 30 min
Read: 5 min

Short answer

Put a block of known thickness under the downhill end of a level. When the bubble reads centre, the surface is falling by the block's thickness over the level's length. A 20mm block under a one-metre level is a fall of 1 in 50 — and because a gradient is a ratio, the same setting checks the fall anywhere along the run.

Anything that has to shed water needs a fall, and eyeballing it produces surfaces that pond in the middle or drain the wrong way. The trick that trades use needs no special tool: a piece of scrap of a known thickness under one end of a level converts it into a gauge for one specific gradient, and you carry that gauge down the whole job.

What you'll need

Spirit level, ideally 1m or longer
A block or offcut of a known thickness
Straight batten, for long runs (optional)
Pegs and line, for outdoor setting out (optional)
Tape measure

Setting a fall with a spirit level and a packing block

Step by step

Find out what fall the job actually needs.The number is set by what the surface is for, not by preference, and it is worth checking the requirement for your specific application rather than assuming. Too little and water sits; considerably too much looks wrong and, on some surfaces, causes its own problems.
Convert the requirement into millimetres per metre.A gradient of 1 in 40 is 25mm per metre. 1 in 50 is 20mm. 1 in 60 is about 16.7mm, and 1 in 80 is 12.5mm. Millimetres per metre is the form you can actually set a block to.
Cut a packing block to that thickness.For a 1 metre level and a 1 in 50 fall, cut a 20mm block. For a 2 metre level, double it to 40mm. Cut it accurately, because the block is now your gauge and its error is the job's error.
Set the block under the downhill end.Level on the surface, block under the low end. When the bubble reads centre, the surface has exactly the fall you wanted. Put the block under the wrong end and you have set the same slope the wrong way, which is a surprisingly easy mistake to make and a very obvious one afterwards.
Fix the block to the level if you are doing a lot.Tape or glue the block permanently onto the end of a cheap level and you have a dedicated fall gauge that cannot be set up wrongly. Write the gradient on it in marker.
Use a straight batten for long runs.Put the level on a 3 metre batten and scale the block accordingly — 60mm for a 1 in 50 over 3 metres. Fewer repositions means less accumulated error, exactly as with levelling.
Set the two end points first, then work between them.Establish the finished height at the top and the bottom of the run, mark both, then run a taut line between them. Every intermediate point is checked against that line rather than against a fresh gradient measurement, which is the same datum principle as marking out.
Check the fall in both directions.A path or a patio usually needs a fall one way and to be level across its width, or a slight cross-fall. Check both axes, because a surface with the right longitudinal fall and an unintended cross-fall drains sideways into whatever is beside it.
Never fall towards a building.Any hard surface next to a house falls away from it, and the finished surface must stay well below the damp proof course. This is the one rule that is not a matter of degree.

Tips

A gradient is a ratio, so it is independent of length. You can check a 1 in 50 fall with a 300mm level and a 6mm block, or a 3 metre batten and a 60mm block, and both are the same slope.
Sighting along a line, and along the finished surface, catches dips and humps that a level walked along the run will step straight over.
For a run that has to fall consistently, set string lines to the finished level at the correct fall and work to the lines, rather than checking each area with a level as you go.
Some spirit levels have an adjustable or a second offset vial specifically for common drainage falls. If you do a lot of this, it is worth having.

Common mistakes

Putting the block under the uphill end — It sets the fall in exactly the wrong direction, and everything downstream of that point is wrong. Physically marking the block end 'LOW' avoids it.
Setting the fall by eye and checking later — A slope of 1 in 60 is not visible to the eye against a background that is itself not level. It has to be set with a reference from the start.
Measuring the fall with a short level over a long run — The errors accumulate exactly as they do when levelling, and small dips get stepped over. Use the longest straightedge you can and set string lines for the run.
Getting a hard surface too close to the damp proof course — Water splashes and drives up the wall above the protective barrier, causing damp inside. The finished level matters as much as the fall.

Questions people ask

What does 1 in 80 actually mean?

The surface drops one unit for every eighty units of horizontal distance — 12.5mm per metre, or 125mm over ten metres. Expressed as a percentage it is 1.25%. All three are the same number in different clothes.

How do I set a fall with a laser level?

A self-levelling laser gives a level plane, so it gives you the datum rather than the slope. Use it to establish the finished height at the top and bottom of the run — measuring down a different distance at each — and then work between those two points with lines.

Can I set a fall on a floor inside a building?

Yes, and it is done for wet rooms and shower areas, where the fall runs to a drain. The technique is the same; the difference is that the fall is usually created in the screed or with a preformed tray rather than in the substrate.