Falls & drainage
Set out falls and levels on site
String line, spirit level, straightedge and laser
Fall determines whether water drains away or collects in puddles. This guide shows how to set out levels and falls with a string line, spirit level, 3 m straightedge and laser, how to convert fall between per mille and cm/m, and what NfA requires as minimum fall for the main surfaces.
Find a fixed reference point (datum)
Choose a fixed reference point — e.g. a door threshold, manhole cover or a set level from the project — from which all other levels are measured. All falls are built relative to this point so the water is directed where it should go.
Determine the direction of fall — away from buildings
Decide where the water is to go (drain, channel, soakaway, lawn), and make sure the fall always runs away from buildings. Fall away from the house is at least 20 ‰, and at least 25 ‰ where there is no sealed paving; with sealed paving and no basement it can be reduced to 15 ‰ (NfA 2023).
Fall from house≥ 20 ‰Without sealed paving≥ 25 ‰Know the minimum fall for your surface
Look up the standard's minimum fall for the specific surface before you set out (NfA): paving blocks/slabs 10 ‰ (1 cm/m), cobbles/setts on gravel 20 ‰, gravel/roads/yards at least 25 ‰, steps and landings 15 ‰, gutters at least 7 ‰ (NfA 2023) and drain pipes at least 3 ‰.
Tiles/paving blocks≥ 10 ‰Cobbles on gravel≥ 20 ‰Gravel/roads≥ 25 ‰Drainage pipe≥ 3 ‰Convert the fall into a height difference
Convert the desired fall into the height difference you need to set out over the run: 10 ‰ = 1 % = 1 cm/m. Height difference = length × fall. Example: 4 m terrace with 15 ‰ → 4 × 0.015 = 0.06 m = 6 cm fall from house to outer edge. Use the calculator »Fall / slope« for the conversion.
Context10 ‰ = 1 % = 1 cm/mRule of thumb: 10 ‰ = 1 % = 1 cm per metre.
Stretch the fall line
Drive in pegs at the fixed point and at the far point, stretch a string line and set it to height according to your calculated height difference. Check the fall of the line with a line level or by measuring the height down to the base at both ends.
Measure the fall with a straightedge and spirit level
For checking over shorter distances: lay a 3 m straightedge with a spirit level, lift the low end until the bubble is level, and measure the distance from the end of the straightedge down to the surface. Fall in ‰ = (height difference in mm) / 3 (the straightedge is 3 m). Check the fall in at least two directions on flat areas.
Formula (3 m straightedge)fall ‰ = h(mm) / 3Use a laser for larger areas
On larger areas, a rotating laser level gives levels directly at each point. Calculate the fall as (level1 − level2) / distance × 1000 ‰. The laser makes it easy to keep a constant fall across the whole area without moving a string line.
Check for hollows
Walk the stretch looking for dips — especially along gutters, which are measured separately over at least 5 m. Persistent water on the surface after rain is the clearest sign of too little or uneven fall. Correct it before the next layer is laid.
Drain pipes are laid in a straight line with an even fall (at least 3 ‰) — kinks and dips in a drain pipe stop the water.
Source
Based on Normer for anlægsgartnerarbejde 1992/2023 (minimum fall, drain pipes) and standard levelling practice; the conversion is pure geometry.
Videos from NDLA
Video guide· Norwegian speech
Nivellerkikkert
Source
NDLA
«Nivellerkikkert» · Photographer: Martin Aune Walther · Editorial: Kjell Brørs
License: CC BY-SA 4.0 · Original at NDLA ↗
Shown unaltered in the original language with the source's own subtitles. The video shows Norwegian practice — the method is often the same, but rules and standards may differ from the Danish ones.